Jensen K., Wirth N. PASCAL User Manual and Report.pdf

Lecture Notes in Computer Science Edited by G. Goos and J. Hartmanis

18

Kathleen Jensen Niklaus Wirth

PASCAL User Manual and Report

Editorial Board: P. Brinch Hansen - D. Gries C. Moler • G. SeegmQIler • N. Wirth Ms.,Kathleen Jensen Prof. Dr. Niklaus Wirth Institut fLir Informatik ETH Zerich Clausiusstra6e 55 C H - 8 0 0 6 Z0rich

Library of Congress Cataloging in Publication Data

Jensen, Kathleen, 1949PASCAL: user manual and report. (Lecture notes in computer science, v. 18) Bibliography: p. 1. PASCAL (Computer program language). I. Wirth, Niklaus, joint author. II. Title. IIIo Series. QA76.73 .P35J46 001.6'424 74-16327

AMS Subject Classifications (1970): 68-02, 68 A 0 5 CR Subject Classifications (1974): 4.2, 4.22

ISBN 3-540-06950-X Springer-Verlag Berlin • Heidelberg • New York ISBN 0-387-06950-X Springer-Verlag New York • Heidelberg • Berlin This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Verlag Berlin • Heidelberg 1974. Printed in Germany.

PREFACE

This manual is directed to those who h a v e p r e v i o u s l y acquired some programming s k i l l . The i n t e n t i o n is to p r o v i d e a m e a n s of learning Pascal without outside guidance. It is based on The ~rparammino Lanauaae Eascal (~evised ~eaort) Ill--the basic definition of Pascal and concise reference manual for t h e experienced Pascal programmer.

The linear structure of a book is by no means ideal for introducing a language, whether it be a formal or natural one. Nevertheless, it is recommended to follow the given organization, paying particular attention to the example programs, and then t o reread those sections causing dlfficultes. O n e m a y w i s h , h o w e v e r , t o r e f e r e n c e c h a p t e r 12 if t r o u b l e s a r i s e concerning the input and output conventions of the programs. The manual was prepared as e file on a c o m p u t e r , that is . as a sequence of characters of a single type font. This is very convenient for the p u r p o s e s of u p d a t i n g : u n f o r t u n a t e l Y ~ it is sometimes a bit awkward to read. T h e r e a d e r is a s k e d to be indulgent with the absence of s u b - and s u p e r s c r i p t s (e.g. m r a i s e d t o t h e p o w e r n is d e n o t e d by m * * n ) . Chapters 0--12 define the language Pascal and serve as a standard for both the implementor and the programmer. The implementer must regard the task of recognizing Standard Pascal as the minmum requirement of his system, while the programmer who intends his programs to be transferable from one installation to another s h o u l d u s e o n l y f e a t u r e s d e s c r i b e d as Standard P a s c a l . On the o t h e r h a n d , any i m p l e m e n t a t i o n may (and u s u a l l y d o e s ) go b e y o n d t h e m i n i m u m . C h a p t e r s 13 a n d 14 d o c u m e n t t h e i m p l e m e n t a t i o n of P a s c a l on t h e CDC 6 0 0 0 m a c h i n e . C h a p t e r 13 describes the a d d i t i o n a l f e a t u r e s of t h e l a n g u a g e P A S C A L 6 0 0 0 , w h e r e a s c h a p t e r 14 is d e v o t e d to t h e u s e of t h e c o m p i l e r and t h e system under the operating system SCOPE.

The efforts o f m a n y go i n t o this manual, e n d we e s p e c i a l l y thank the members of the Institut fuer Informatik, ETH Z u r i c h . end John Larmouth, Rudy Schild, Olivier Lecerme, and Pierre Desjardins for their criticism, suggestions, and encouragement. Our implementation of Pascsl--which made this manual both possible and necessary--is the work of Urs A m m a n n , a i d e d by Helmut Sandmayr.

June

1974

Kathleen densen Niklaus Wirth ETH Zurich Switzerland

Table

of Contents

U S E R M A N U A L by K. Jensen and N. Wirth O.

Introduction

I. N o t a t i o n

and

. . . . . . . . . . . . . . . . . . . . . . . .

3

Vocabulary . . . . . . . . . . . . . . . . . . .

9

2. T h e A. B. C. D.

Concept The type The type The type T h e type

of D a t a Boolean integer real . char .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . . . . . . . ..... . . . . . . . . . .

3. T h e A. B. C. D. E. F.

Program Heading and the Declaration Part . . . . . . . . The program heading . . . . . . . . . . . . . . . . . . . The label declaration part . . . . . . . . . . . . . . . . The constant definition part . . . . . . . . . . . . . . . The type definition part . . . . . . . . . . . . . . . . . The variable declaration part . . . . . . . . . . . . . . The procedure and function declaration part . . . . . . .

12 12 13 14 14 16 16 16 16 18 18 19

4. T h e C o n c e p t of A c t i o n . . . . . . . . . . . . . . . . . . . . . A. T h e a s s i g n m e n t s t a t e m e n t . . . . . . . . . . . . . . . . . B. T h e c o m p o u n d s t a t e m e n t . . . . . . . . . . . . . . . . . . . C. R e p e t i t i v e statements . . . . . . . . . . . . . . . . . . C.I T h e w h i l e s t a t e m e n t . . . . . . . . . . . . . . . . . C.2 The repeat statement . . . . . . . . . . . . . . . . . C.3 The for statement . . . . . . . . . . . . . . . . . . D. C o n d i t i o n a l statements . . . . . . . . . . . . . . . . . . D.I T h e if s t a t e m e n t . . . . . . . . . . . . . . . . . . . D.2 The case statement . . . . . . . . . . . . . . . . . . E. T h e g o t o s t a t e m e n t . . . . . . . . . . . . . . . . . . . .

2O 20 21 22 22 23 23 26 26 31 31

5. S c a l a r a n d S u b r a n g e T y p e s . . . . . . . . . . . . . . . . . . A. S c a l a r t y p e s . . . . . . . . . . . . . . . . . . . . . . . B. S u b r a n g e t y p e s . . . . . . . . . . . . . . . . . . . . . .

34 34 35

6.

Structured

7. R e c o r d A. T h e 8.

Set

Types

in G e n e r a l

-- T h e

Array

in Particular

.

Types . . . . . . . . . . . . . . . . . . . . . . . . with statement . . . . . . . . . . . . . . . . . . . .

Types . . . . . . . . . . . . . . . . . . . . . . . . . .

9. ~ i l e T y p e s . . . . . . . . . . . . . . . . . . . . . . . . . A. T e x t f i l e s . . . . . . . . . . . . . . . . . . . . . . . . B. T h e s t a n d a r d f i l e s " i n p u t " a n d " o u t p u t " . . . . . . . . . 10.

Pointer

11.

Procedures and A. P r o c e d u r e s . B. F u n c t i o n s . C. R e m a r k s . .

Types . . . . . . . . . . . . . . . . . . . . . . . .

12.

Input and Output A. T h e p r o c e d u r e B. T h e p r o c e d u r e

13.

PASCAL 6000-3.4 . . . . . . . . . . . . . . . . . . . . . . . A. E x t e n s i o n s to t h e l a n g u a g e P a s c a l . . . . . . . . . . . . A.I S e g m e n t e d f i l e s . . . . . . . . . . . . . . . . . . .

36 42 47 5O 55 57 59 62

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67 67 78 82

. . . . . . . . . . . . . . . . . . . . . . read . . . . . . . . . . . . . . . . . . . . write . . . . . . . . . . . . . . . . . . .

84 84 86

Functions . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . .

. . . .

. . . .

. . . .

88 88 88

VI

A.2 External procedures . . . . . . . . . . . . . . . . . Specifications left undefined in the preceding chapters B.I T h e p r o g r a m h e a d i n g a n d e x t e r n a l f i l e s . . . . . . . . B.2 Representation of files . . . . . . . . . . . . . . . B.3 The standard types . . . . . . . . . . . . . . . . . . B.4 The standard procedure write . . . . . . . . . . . . . C. R e s t r i c t i o n s . . . . . . . . . . . . . . . . . . . . . . . D. A d d i t i o n a l p r e d e f i n e d t y p e s , p r o c e d u r e s , a n d f u n c t i o n s . D.I A d d i t i o n a l p r e d e f i n e d t y p e s . . . . . . . . . . . . . D.2 Additional predefined procedures and functions.. B.

14.

How to Use the PASCAL 6000-3.4 System . . . . . A. C o n t r o l s t a t e m e n t s . . . . . . . . . . . . . . B. C o m p i l e r o p t i o n s . . . . . . . . . . . . . . . C. E r r o r m e s s a g e s . . . . . . . . . . . . . . . . C.I C o m p i l e r . . . . . . . . . . . . . . . . . C.2 Run-time . . . . . . . . . . . . . . . . . References Appendix Appendix Appendix Appendix Appendix Appendix

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . . . . . . . . . . . . . . . . . Standard Procedures S u m m a r y of O p e r a t o r s Tables . . . . . . . . Syntax . . . . . . . . Error Number Summary Programming Examples

A B C D E F

and . . . . . . . . . .

Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

. . . . . .

. . . . . .

.

. . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

9O 91 91 92 93 96 97 97 97 98 I00 100 100 I O2 102 I02 I04 I05 I O8 I09 110 119 122 126

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . .

by N. Wirth Preface

to

the

1. I n t r o d u c t i o n 2.

Summary

3. N o t a t i o n ,

Revised

Report

of the

language . . . . . . . . . . . . . . . . . . .

terminology,

4.

Identifiers,

5.

Constant

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . .

Numbers

definitions

and and

vocabulary . . . . . . . . . . . .

Strings

. . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . .

133 136 137 140 140 141

. . . .

. . . .

. . . .

. . . .

. . . .

142 142 143 145

7. D e c l a r a t i o n s a n d d e n o t a t i o n s of v a r i a b l e s . . . . . 7.1. E n t i r e v a r i a b l e s . . . . . . . . . . . . . . . . 7.2. C o m p o n e n t v a r i a b l e s . . . . . . . . . . . . . 7.3. R e f e r e n c e d v a r i a b l e s . . . . . . . . . . . . . .

. . . .

. . . .

. . . .

. . . . .

146 147 147 148

6. D a t a 6.1. 6.2. 6.3.

type definitions . Simple types . . . Structured types . Pointer types . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

8.

Expressions . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. O p e r a t o r s . . . . . . . . . . . . . . . . . . . . . . . 8.2. F u n c t i o n d e s i g n a t o r s . . . . . . . . . . . . . . . . . .

148 149 151

9.

Statements . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. S i m p l e s t a t e m e n t s . . . . . . . . . . . . . . . . . . . 9.2. S t r u c t u r e d s t a t e m e n t s . . . . . . . . . . . . . . . . .

151 152 153

10.

Procedure declarations . . . . . . . . . . . . . . . . . . . . 10.1.Standard procedures . . . . . . . . . . . . . . . . . .

158 160

11.

Function declarations . . . . . . . . . . . . . . . . . . . . 11.1.Standard functions . . . . . . . . . . . . . . . . . . .

163

162

VII

12. Input and Output 13. P r o g r a m s

. . . . . . . . . . . . . . . . . . . . .

164 167

. . . . . . . . . . . . . . . . . . . . . . . . .

14. A s t a n d a r d for i m p l e m e n t a t i o n and p r o g r a m

interchange.

15. I n d e x . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

168 169

0 ~NTRODUCTION

Much

of

grasp of e

the

following

text

of computer terminology program. The purpose

assumes

the

reader

and a "feeling" of this section

for is

has

a minimal

the structure to spark that

intuition.

{ program assuming find the sterling, 1. 2 . . . . ~rocram

0.1 annual inflation rates o f 9 , 8 , a n d 10 p e r c e n t . factor by which the frank, dollar, pound m a r k , or g u i l d e r will have been devalued in n years.} inflation(output);

#onst n = 10 ; var i : integer; wl.w2~oW3 becin i := O; w l := 1.0; r_@~.~at i := i + I ; w I

:=

w I

*

: real; w 2 := 1.0;

w3

:=

1.0;

1.07;

w2 w3

:= w2 * 1.08; := W3 * 1 . 1 0 ; w r i t e l n (i .w l , w 2 . w 3 ) i=n end.

1 2 3 4 5 6

1.090000000000e+00 1.144900000000e+00 1.225043000000e+00 1.310796010000e+00 1.402551730700e+00 1.500730351849e+00

1.080000000000e+00 1.166400000000e+00 1.2597t2000000e+00 1.360488960000e+00 1.469328076UOOe+00 1.586874322944e+00

1.100000000000e+00 1.210000000000e+00 1.331000000000e+00 1.464100000000e+00 1.610510000000e+00 1.771561000000e+00

7

1.bObTU1476478e+O0

1.713824268779e+00

1.948717100000e+00

9 10

1.718186179832e+00 1.838459212420e+00 1.96915t359290e+00

1.850930210282e+00 1.999004627104e+00 2.158924997293e+00

2.14358~810000e+00 2.357947691000e+00 2.593742460t00e+00

An &3ooritbj~ or computer program consists of t w o e s s e n t i a l parts, a description of ~ctions which ere to be performed, and a description o f t h e ~@_~&o w h i c h is m a n i p u l a t e d by t h e s e a c t i o n s .

Actions described The

are described by so-called

program

is

by so-called ~tatements, ~cl~ra~L~zns end ~L~finitions.

divided

into

a

~eadin~

and

end

a body,

data

is

called

b3ock. The heading gives the program s name a n d l i s t s parameters. (These ere (file) variables and represent arguments and results of the computation. See chapter 13.) file "output" is e compulsory parameter. The b l o c k consists six sections, where any except the lest may be e m p t y . I n required order they ere:

a

its the The of the



The first section l i s t s a l l l a b e l s d e f i n e d in t h i s b l o c k . The second section defines synonyms for constants: i.e. it introduces i d e n t i f i e r s t h a t m a y l a t e r be u s e d in p l a c e of t h o s e constants. The third contains type definitions: end the fourth. variable definitions. The fifth section defines subordinate program parts (i.e. procedures and functions). The s t a t e m e n t part specifies t h e a c t i o n s to be t a k e n . The above program outline is more precisely e x p r e s s e d in a syntax ~ a p r a m . Starting a t t h e diagram named p r o g r a m , a p a t h through the diagram defines a syntactically correct proqram. Each box r e f e r e n c e s a d i a g r a m by t h a t n a m e . w h i c h is t h e n u s e d to define its meaning. Terminal symbols (those actually written in a P a s c a l p r o g r a m ) a r e in r o u n d e d e n c l o s u r e s . (See a p p e n d i x D for t h e f u l l s y n t a x d i a g r a m of P a s c a l . )

program

block ~1 .... i~.di,l,~o~ I-

9

j ~ I ~~G-o~ ~-~PROCEDUII~~--I

Figure O.a

p..... ter list I

Syntax diagrams defining the general structure of a program

"i

An alternative formulation of a syntax is the traditional Backus-Naur ~orm. where syntactic constructs a r e d e n o t e d by English words enclosed between the angular brackets < a n d >. These words are suggestive of the n a t u r e or m e a n i n g of t h e construct. A sequence of constructs (I or more elements) enclosed by the mete-brackets { and } imply their repetition zero or m o r e t i m e s . (For t h e B N F of P a s c a l . see appendix D . ) As an example, the construct of f i g u r e O.a is d e f i n e d by the following formulas, called "productions": ::= < p r o g r a m heading> . ::~ ~ r o m r a ~ ( { . } ) ; ::= < i d e n t i f i e r >

Each procedure (function) has a structure similar to a program; i.e. each consists of a heading and a block. Hence. procedures may be declared (nested) within other procedures. Labels, constant synonyms, type, variable, and procedure declarations are local to the procedure in w h i c h t h e y a r e d e c l a r e d , That i s , their identifiers have significance only within the program text which constitutes the procedure declaration a n d w h i c h is c a l l e d the ~eooe of these identifiers. Since procedures m a y be n e s t e d , so may scopes. Objects which are declared in t h e m a i n p r o g r a m , i.e. not local to some procedrue, are called ~Iobal and have significance throughout the entire program. Since blocks m a y be n e s t e d w i t h i n o t h e r b l o c k s by p r o c e d u r e and function declarations, o n e is a b l e to a s s i g n a l e v e l of n e s t i n g to each. If the outermost program-defined block (e.g. the main program) is called level O, t h e n a b l o c k d e f i n e d within this block w o u l d be of l e v e l I; in g e n e r a l , a block defined in l e v e l i would be of level (i+I). Figure O.b i l l u s t r a t e s a block structure,

where

QF--q

I

Figure

O.b

Block

structure

level level level level

0 I 2 3

: = : :

M P, A, B

Q R,

S

In t e r m s of t h i s f o r m u l a t i o n , t h e s c o p e or r a n g e of v a l i d i t y of an identifier x is the entire b l o c k in w h i c h x is d e f i n e d , i n c l u d i n g t h o s e b l o c k s d e f i n e d in t h e s a m e b l o c k as x. (For t h i s example, note that all identifiers m u s t be d i s t i n c t . S e c t i o n 3.e discusses the case where identifiers are not necessarily distinct,) objects

defined

in b l o c k

are

accessible

in

blocks

M

M ,P ,A .B ,Q .R ,S

P A B

P ,A ,B A.B B

Q

Q ,R ,S

R S

R S

For programmers acquainted with ALGOL, PL/I, or FORTRAN, it may prove helpful to glance at Pascal in t e r m s of t h e s e o t h e r languages. For this purpose, we list the following characteristics of P a s c a l : 1. 2.

3. 4.

5. 6.

7.

8. 9.

10.

Declaration of variables is mandatory, Certain key words (e.g. ~eain, ~nd, ~eoeat) ere "reserved" and cannot be used as identifiers. In this manual they are underlined. The semicolon (;) is c o n s i d e r e d as a s t a t e m e n t s e p a r a t o r , not a s t a t e m e n t t e r m i n a t o r (as e . g . in PL /I). The standard data types are those of whole and real numbers, the logical values, and the (printable) characters. The b a s i c d a t a s t r u c t u r i n g facilities include the array, the record (corresponding to COBOL's and PL / I ' s "structure"). the set, and the (sequential) file. These structures can be combined end nested to form arrays of sets, files of records, etc. D a t a may b e allocated dynamically and accessed via pointers. These pointers allow the full generality of l l s t p r o c e s s i n g . T h e m e is a f a c i l i t y to d e c l a r e n e w , b a s i c d a t a t y p e s w i t h symbolic constants. The let data s t r u c t u r e offers facilities s i m i l a r to t h e P L / I "bit s t r i n g " . Arrays may be of arbitrary dimension with arbitrary bounds; the array bounds are constant. (i.e. There are no dynamic arrays.) As in FORTRAN. ALGOL, and PL/I, there is a go to statement. Labels are unsigned integers and m u s t be declared. The compound statement is that of ALGOL, and corresponds t o t h e DO g r o u p i n P L / I . The facilities o f t h e ALGOL s w i t c h and the computed go to o f FORTRAN a r e r e p r e s e n t e d by the case statement.

The

for

statement,

corresponding

F O R T R A N . may o n l y have steps is executed only as long variable lles within the controlled s t a t e m e n t m a y not

to

the

DO

loop of

of 1 (~g) or -1 (~ownto) and as t h e v a l u e o f t h e c o n t r o l limits. Consequently. the be e x e c u t e d at a l l .

11. 12. 13. t4. 15.

16.

There are no conditional expressions a n d no m u l t i p l e assignments. Procedures e n d f u n c t i o n s m a y be c a l l e d r e c u r s i v e l y . T h e r e is no " o w n " a t t r i b u t e for v a r i a b l e s (as in A L G O L ) . Parameters are called either by value or by reference: there i s no c a l l by name. The "block structure" differs from that o f ALGOL a n d P L / I insofar as there are no anonymous blocks, i.e. each block is given a name, and thereby i s made i n t o a procedure. All objects--constants, variables, etc.--must be declared ~e£o~_~ they are referenced. The following two exceptions ere however allowed: I) the type identifier in a pointer type definition (chaptem 10) 2) procedure and function calls when there is a forward reference (chapter 11).

Upon f i r s t c o n t a c t w i t h P a s c a l , m a n y t e n d to b e m o a n the a b s e n c e of certain "favorite features", Examples include an exponentiation operator, concatenation of strings, dynamic arrays, arithmetic operations on B o o l e a n v a l u e s , a u t o m a t i c t y p e conversions, and default declarations. These were not oversights, but deliberate omissions. In some cases their presence would be primarily an invitation to inefficient programming solutions; in o t h e r s , it w a s felt t h a t t h e y w o u l d be Contrary to the aim of clarity and reliability a n d "good programming style". Finally, a rigorous selection among the immense variety of programming facilities available had to be made in order to keep the compiler relatively compact and efficient--efficient and economical for both the user who writes o n l y s m a l l p r o g r a m s u s i n g f e w c o n s t r u c t s of t h e l a n g u a g e a n d t h e u s e r w h o w r i t e s l a r g e p r o g r a m s a n d t e n d s to m a k e u s e of the full language.

I

__NTAT~.~.~.!~T.J~A N D

The basic wocabularv consists letters, digits, and special operators and delimiters:

VOCABULARY

of b a s i c symbols.

symbols classified into The ~ . ~ ~ymbols are

+ -

: "

( )

and Drrav

and _f..il~

ail aot

/

<>

]

case

function

~

• ,

<: >: >

} % ..

~iv ~a downto 91se

iZ in /~l mod

£rocedure ~r_Qar.am ~ Keoea~

~hen

~ar ahila ~ith

~9~d-delim~ (or reserved words) ere normally underlined in the hand-wrltten program to e m p h a s i z e t h e i r i n t e r p r e t a t i o n as single symbols with fixed meaning. The orogrammer m a y not u s e these words in a context other than that e x p l i c i t in t h e d e f i n i t i o n of P a s c a l : in p a r t i c u l a r , t h e s e w o r d s m a y not be u s e d as identifiers. They are written as a s e q u e n c e of l e t t e r s (without surrounding escape characters). The

construct: {
sequence

of s y m b o l s

not

containing

"} ">}

m a y be i n s e r t e d b e t w e e n a n y t w o i d e n t i f i e r s , n u m b e r s , or s p e c i a l symbols. It is called a ~ o m m e n £ a n d m a y be r e m o v e d from t h e program text without altering its m e a n i n g . T h e s y m b o l s { a n d } do not occur o t h e r w i s e in t h e l a n g u a g e , a n d w h e n a p p e a r i n g in syntactic descriptions, they denote meta-symbols l i k e I and ::=. (On systems where the curly brackets are unavailable, the c h a r a c t e r p a i r s (* and *) a r e u s e d in t h e i r p l a c e . ) ~de~if~ers are names denoting constants, types, variables. Procedures. and functions. They must begin with a letter, which may be followed by any combination and number of letters and d i g i t s . A l t h o u g h an i d e n t i f i e r m a y be v e r y l o n g . i m p l e m e n t a t i o n s may impose a limit as to how m a n y of t h e s e c h a r a c t e r s a r e significant. Implementations of Standard Pascal will always recognise the first ~ characters of an identifier as significant. That is. identifiers denoting distinct objects s h o u l d d i f f e r in t h e i r f i r s t U c h a r a c t e r s .

10

~

Figure

l

1.a

e

t

t

e

r

~

Identifier

examples of legal identifiers: sum root3 pi h4g x thisisaverylongbutneverthelesslegalidentifier thisissverylongbutprobablythesameidentifierasabove illegal 3rd

identifiers: array

level.4

root-3

Certain i d e n t i f i e r s , called ~ e n d a r d i d e n t i f i e r s , ere predefined (e.g. sin, cos). In contrast to the w o r d - d e l i m l t e r s (e.g. QrraM), one is not r e s t r i c t e d to this d e f i n i t i o n and may elect to redefine any standard identifier, as they are assumed to be declared in a hypothetical block s u r r o u n d i n g the entire program block. Decimal n o t a t i o n is used for ~ u m b e r s . The letter E preceding the scale factor is pronounced as "times 10 to the power of". The syntax of u n s i g n e d numbers is s u m m a r i z e d in figure 1.b.

Figure

1.b

Unsigned

number

Note that if the number c o n t a i n s digit must precede and succeed occur in a number.

unsigned 3

numbers : 03 6272844

0.6

a decimal point, the point. Also.

5E-8

49.22E +08

at least one no comma may

IE 10

incorrectly written numbers: 3,487.159 XII .6

El0

5.E-16

Blanks. ends of lines, and comments ere considered as semara~rs. An a r b i t r a r y n u m b e r of s e p a r a t o r s may occur between any two consecutive Pascal symbols with the following exception: no separators may o c c u r w i t h i n i d e n t i f i e r s , n u m b e r s , or s p e c i a l symbols. H o w e v e r , at l e a s t o n e s e p a r a t o r m u s t o c c u r b e t w e e n a n y p a i r of c o n s e c u t i v e identifiers, n u m b e r s , or w o r d s y m b o l s .

Sequences of celled ~nos. the quote mark

characters enclosed To i n c l u d e a q u o t e twice.

examples of strings: "a' ";" "3 ° * t h i s s t r i n g has

by mark

single quote marks are in s s t r i n g , o n e w r i t e s

'begln" "don'°t " 33 c h a r a c t e r s "

2 --THE _C~.~,CF,,.P_-TBE_ D A T A

Data is the general expression d e s c r i b i n g a l l t h a t is o p e r a t e d on by t h e c o m p u t e r . At t h e h a r d w a r e a n d m a c h i n e c o d e l e v e l s , a l l data are represented as sequences of binary digits (bits). Higher level languages allow one to u s e a b s t r a c t i o n s a n d to ignore t h e d e t a i l s of r e p r e s e n t a t i o n - - l a r g e l y by d e v e l o p i n g t h e C o n c e p t of d a t a ~ v o e . A data type defines t h e s e t of v a l u e s a v a r i a b l e m a y a s s u m e . Every variable occurring in a p r o g r a m m u s t be a s s o c i a t e d w i t h One and only one type. A l t h o u g h data t y p e s in P a s c a l can be quite sophisticated, each must be ultimately built from unstructured types. An unstructured t y p e is e i t h e r d e f i n e d by the programmer, a n d t h e n c a l l e d a d e c l a r e d s c a l a r t y p e . or o n e of the four standard scalar types--integer, r e a l , B o o l e a n . or char. A scalar type is characterized by the s e t of its d i s t i n c t values, u p o n w h i c h a l i n e a r o r d e r i n g is d e f i n e d . The v a l u e s a r e denoted by identifiers in the definition of t h e t y p e (see chapter 5).

A. T h e

type

Boolean

A Boolean value is one of t h e the predefined identifiers false The following applied to Operators.)

and or no__t

logical Boolean

logical logical logical

operators operands:

logical truth and true. yield e (Appendix

values

denoted

Boolean value B summarizes

by

when all

conjunction disjunction negetion

Each of the relational operators (=, <>. <=, <, >, >=, ~) yields a Boolean value. Furthermore. the type Boolean is defined such that false < true. Hence, it is possible to define each of the 16 B o o l e a n operations using the above logical and relational operators. For example, if p and q are Boolean values, one can express implication equivalence e x c l u s i v e OR

as as as

p <= q p = q p <> q

13

Standard Boolean functions--i.e, standard functions which yield a Boolean result--ere: (Appendix A summarizes all s t a n d a r d functions.)

odd(x) e o l n (f) eof(f)

B.

The

t r u e if t h e i n t e g e r x is odd, f a l s e o t h e r w i s e e n d of a l i n e , e x p l a i n e d in c h a p t e r 9 e n d of f i l e . e x p l a i n e d in c h a p t e r 9

type i n t e g e r of

A value of type integer is an element implementation-defined s u b s e t of w h o l e n u m b e r s . The following arithmetic operators a p p l i e d to i n t e g e r o p e r a n d s : *

rag_d_ + -

multiply divide and truncate a mw3t b : a - ((a ~ add subtract

(i.e. v a l u e b)*b)

The relational operators =, r e s u l t w h e n a p p l i e d to i n t e g e r Four

important

abs (x) sqr(x) trunc(x)

round(x)

standard

yield

is

integer

not

when

rounded)

<>. <, <=. >=. > y i e l d a B o o l e a n opersnds. <> d e n o t e s i n e q u a l i t y .

functions

yielding

sqr yield of type

integer

results

are:

an integer result only when their i n t e g e r . If i is a v a r i a b l e of t y p e

the " n e x t " i n t e g e r , a n d the preceding integer m o r e c l e a r l y e x p r e s s e d by i-I

There exists an implementation-dependent maxint, if a a n d b are i n t e g e r e x p r e s s i o n s ,

the

to

be c o r r e c t l y

implemented

expressions

standard identifier the operation:

b

is g u a r a n t e e d

value

t h e r e s u l t is t h e a b s o l u t e v a l u e of x . t h e r e s u l t is x s q u a r e d . x is a r e a l v a l u e : t h e r e s u l t is its w h o l e p a r t . (The fractional part is discarded. Hence trunc(J.7)=3 and trunc(-J.7)=-3) x is a real value; the result is t h e r o u n d e d integer, round(x) m e a n s for x > = 0 t r u n c ( x + 0 . 5 ) , and for x < 0 t r u n c ( x - 0 . 5 )

Notes: abs and argument is also integer, then succ(1) yields pred(i) yields This is, h o w e v e r , i+1 and

a go

an

the

when:

14

a b s (a o@. b ) abs (a) abs (b)

C . The

type

<: <: <:

maxint maxint maxint

, , and

real

A v a l u e of t y p e r e a l implementation-defined

is

an e l e m e n t of subset of r e a l

As l o n g as at l e a s t o n e of t h e real (the other possibly being operators yield a real value: * / + -

multiply divide (both operands the result is add subtrcct

Standard result:

functions

abs (x) sqr (x) Standard result:

when

the numbers.

operands is of t y p e of type integer) the

may be always

accepting

integers, real)

a reel

following

but

argument

yield

a real

absolute value x squared

functions

sin (x) cos (x) amctan (x) ln(x) e x p (x) sqrt (x)

with

trigonometric

real

or

integer

argument

and

meal

functions

natural logarithm exponential function square root

Warninq: although r e a l is i n c l u d e d as a s c a l a r t y p e , it c a n n o t a l w a y s be u s e d in t h e s a m e c o n t e x t as t h e o t h e r s c a l a r t y p e s . I n Particular. the functions pred and succ cannot take real arguments, a n d v a l u e s of t y p e m e a l c a n n o t be u s e d w h e n i n d e x i n g arrays, n o r in c o n t r o l l i n g for statements, nor for defining the base type of a set.

D.

The

type

char

A value of t y p e c h a r is an e l e m e n t of a f i n i t e a n d o r d e r e d s e t of characters. Every computer s y s t e m d e f i n e s s u c h a s e t for t h e purpose of c o m m u n i c a t i o n . These characters are then available on the input and output equipment. Unfortunately. there does not

15

exist one standard character set; the elements and their ordering dependent.

therefore, the definition of is strictly implementation

The following minimal assumptions hold independent of t h e u n d e r l y i n g impementation:

The c h a r a c t e r set includes 1. t h e alphabetically ordered A...Z 2. the numerically ordered and digits 0...9 3. t h e b l a n k c h a r a c t e r .

A character enclosed c o n s t a n t of t h i s t y p e .

examples: '~" (To represent

'G"

set

"°°'

an apostrophe,

one

the

capital

contiguous

in a p o s t r o p h e s

'3 °

of

for

set

(single

type

Latin of

letters

the

quotes)

char,

decimal

denotes

a

°X ° writes

it

twice.)

The two standard f u n c t i o n s ~r.~[ a n d ~ h r a l l o w t h e m a p p i n g o f t h e g i v e n c h a r a c t e r s e t o n t o a s u b s e t of n a t u r a l n u m b e r s - - c a l l e d the ordinal numbers of the character set--and vice verse; ord end chr e r e c a l l e d ~ r a n s f e r ~ u n c t i o n s . ord(c)

chr(i)

is the ordinal number of the character c in t h e underlying ordered c h a r a c t e r s e t . (also s e e s e c t i o n 5.A) is the character value with the ordinal number i.

One sees immediately i.e. chr(ord(c)) = c

Furthermore, ci < e 2

the

that

ord

-and-

ordering iff

and

chr

are

ord(chr(i))

of a given character ord(cl) < ord(c2)

inverse

functions.

= i

set

is

defined

by

This definition can be extended to each of t h e r e l a t i o n a l operators: =. <>. <, <=, >=. >. If R d e n o t e s o n e of t h e s e operators, then ci

When type

R e2

iff

ord(cl)

R ord(c2)

t h e a r g u m e n t of t h e s t a n d a r d functions c h a r , t h e f u n c t i o n s c a n be d e f i n e d as:

pred(c) succ(c)

= =

pred

and

succ

is

of

c h r (ord (c)-I) chr (ord (c)+I)

Note: The predecessor ( s u c c e s s o r ) of a c h a r a c t e r is d e p e n d e n t upon the underlying c h a r a c t e r s e t a n d is u n d e f i n e d if one d o e s not e x i s t .

TH~ PR0~RAM H E A ~ N G

3 AN_E THE DECLARAT~]~N EAR_T_

Every program consists of a heading and a block. The block contains a declaration part, in which all objects local to the program are defined, and a statement part. which specifies the actions to be executed upon these objects. ::= < p r o g r a m heading> ::: < l a b e l d e c l a r a t i o n part>

A.

Program

part>

heading

The heading gives the program a n a m e (not o t h e r w i s e significant inside the program) a n d l i s t s its p a r a m e t e r s , through which the program communicates with the environment (see c h a p t e r 13.B.I).
B.

Label

heading>

::= & ~ l i g ~ i m < i d e n t i f i e r > { ,

declaration

(
identifier>

part

Any statement in a program may be m a r k e d by p r e f i x i n g the statement with a label followed by a c o l o n (making possible a reference by a goto statement). However, t h e l a b e l m u s t be defined in t h e j a b e l ~ c l a r ~ & i ~ A &~i~ before its u s e . T h e s y m b o l label heads this part, which has the general form: label



A l a b e l is d e f i n e d most 4 digits. example: i~el

C.

}; to

be

an

unsigned

integer,

end

consists

of

at

~,1~;

Constant

A ~&tao~ a constant. part, which f,Ji~&~

{,

definition

part

~efinition introduces an i d e n t i f i e r as a s y n o n y m for The symbol ~ introduces the constant definition has t h e g e n e r a l form:



= ;

{

= ;}

17

where a (possibly

constant signed),

is either" or a s t r i n g .

a

number,

a constant

identifier

The use of c o n s t a n t i d e n t i f i e r s g e n e r a l l y m a k e s a p r o g r a m m o r e readable and acts as a c o n v e n i e n t d o c u m e n t a t i o n a i d . It a l s o allows the programmer to group machine or example dependent quantities at the beginning of the p r o g r a m w h e r e t h e y can be easily noted and/or changed. (Thereby aiding the portability and m o d u l a r i t y of t h e p r o g r a m . ) As an

{

example,

program example

~roaram #onst

consider

3.1 of constant convert

addin

the

following

definition

pert

program:

}

(output);

= 32;

mulby

=

1.8;

low

= O;

high

sesarator = * °; var degree : low..high; J~eain writeln (separator): f o r d e g r e e := low J&o h i g h d o beQi~ write(degree°'c',round(degree*mulby if odd(degree) then writeln end : writeln ; writeln (separator) and.

0c 2c 4c 6c 8c I0c 12c 14c 16c IBC 20C 22c 24c 26c 28c 30c 32c 34c 36c 38c

32f 36f 39f 43f 46f 50f 54f 57f 61f 64f 68f 72f 75f 79f 82f 86f 90f 93f 97f 100f

Ic 3c 5c 7c 9c 11c 13c 15c 17e 19c 21c 23c 25c 27c 29c 31c 33c 35c 37c 39c

34f 37f 41f 45f 48f 52f 55f 59f 63f 66f 70f 73f 77f 81f 84f 88f 91f 95f 99f I02f

= 39;

+ addin),'f')"

18

D.

Type

definition

part

A data type in P a s c a l m a y be e i t h e r d i r e c t l y described in t h e variable declaration or referenced by a ~ypQ ~dentifier. Provided are not only several standard type identifiers, but also e mechanism, t h e ~Z#,~ ~ e f i n i t i o o , for creating new types. The symbol ~.@ introduces a program part containing type definitions, The definition itself determines a set of values and associates an i d e n t i f i e r with the set. The g e n e r a l form is: t~oe



Examples chapters.

E.

Variable

of

type

= : definitions

declaration

Every variable ~ar!ab!e ~ ~ a n y u s e of t h e

[ ere

found

in

the

subsequent

part

occurring in a s t a t e m e n t declaration . This variable.

A variable declaration associates en with a new variable by s i m p l y listing its type. The symbol ~&~ heads the The general f o r m is: {

= :}

{, < i d e n t i f i e r > } { , }

example : v~ rootl,root2.root3: count,i: integer ; found : Boolean; filler : chari

m u s t be d e c l a r e d in a must textually Precede

identifier and a data type the identifier followed by variable declaration part.

: : : i}

reali

This identifier/type association is v a l i d t h r o u g h o u t the entire block containing the declaration, unless the identifier is redefined in a subordinate block. Suppose a b l o c k B is n e s t e d within block A. (i.e. declared w i t h i n t h e s c o p e of a n d h e n c e subordinate t o A . as in f i g u r e 0 . b ) It is p o s s i b l e t o d e c l a r e an identifier in B that is already declared in A . T h i s has t h e effect of associating that identifier with a variable local to B--not available to A--which may be o f a n y t y p e . T h e l a t t e r definition is then valid throughout the s c o p e o f Bo u n l e s s redeclared in a block subordinate to B . It is n e t a l l o w e d to declare a single identifier more than once within the same level and scope. Hence the following is a l w a y s i n c o r r e c t . ~J~ a : integer; a : real;

19

F.

Procedure

and

function

declaration

part

Every procedure or function must be defined (or a n n o u n c e d ) before its use. Procedure end function declarations are treated in chapter 11. P r o c e d u r e s are subroutines end are activated by procedure statements. Functions are subroutines that yield a result value, and therefore can be u s e d as c o n s t i t u e n t s of

expressions.

4 THE

~QNCEPT

OE A C T I O N

Essential to a computer program is action. That is, a program must do something with its data--even if that action is the choice of doing nothing! ~ ~ describe these actions. Statements are either &imole (e.g. the assignement statement) or structured.

A.

The

assignment

statement

The most fundamental of s t a t e m e n t s is t h e ~ s s i ~ d 3 ~ wtatement. It specifies that a newly computed value be assigned to a Variable. T h e f o r m o f an a s s i g n m e n t is:

:= < e x p r e s s i o n >

w h e r e := is t h e & s s i c n m e n t ~ p e r a t o r . relational operator =. The statement current value of a is replaced with ~ecomes 5".

not t o be c o n f u s e d w i t h t h e "a := 5" is pronounced "the the value 5", or simply, "a

The n e w v a l u e is o b t a i n e d by e v a l u a t i n g an ~ x m r e s s i o n consisting of constant or variable operands, operators, and function designators, (A f u n c t i o n designator specifies the activation of a function. Standard functions are listed in Appendix A; user defined functions are explained in chapter 1 1 . ) An e x p r e s s i o n is e rule for calculating e value where the conventional rules of left to right evaluation and ~Eerator erecedence are observed. The operator ~ot (applied to a Boolean operand) has the highest precedence, followed by the multiplying operators (*. /, ~iv, ~_#J~, ~U~), then the adding operators (+, -, ~), and of lowest precedence, the relational operators (=, <>, <, <=. >=, >. &~). Any expression enclosed within parentheses is evaluated independent of preceding or succeeding operators. examp les : 2 * 3-4 * 5 15 ~ 4 * 4 B0/5/3 4/2 *3 sqrt (sqr(3)+11*5) The syntax The reader

of is

= = = =

(2*3) (15 ~iv (80/5)/3 (4/2)*3

(4*5) 4)*4

Appendix D reflects the recommended to reference

= -14 = 12 = 5.333 = 6.000 = 8.000

exact rules of precedence. it whenever in doubt.

Boolean expressions have the property thst their value known before the entire expression has been evaluated. for example, that x=O. Then (x>O) is

~

already

may b e Assume

(x
to

be

false

after

computation

of

the

first

21

factor, and the second need not be e v a l u a t e d . T h e r u l e s of Pascal neither require nor forbid the evaluation of t h e s e c o n d part in such cases. This means that the programmer must assure that the second f a c t o r is w e l l - d e f i n e d , independent of t h e v a l u e of the first factor. Hence. if o n e a s s u m e s that the array a has an index ranging f r o m I t o 10. t h e n t h e f o l l o w i n g example is in error~ x

:=

O:

~eoeat (Note

a[11] .)

x

that

:=

x+1

if

no

~ntil a[i]

=

(x>lO)

~

O,

program

the

(a[x]=O) will

refer

to

an

element

Direct assignment is p o s s i b l e to variables of any type, except files. However. the variable (or the function) and the expression m u s t be of i d e n t i c a l type. with the exception t h a t if the type of the variable is r e a l , t h e t y p e o f t h e e x p r e s s i o n may be integer. (If a subrange t y p e is i n v o l v e d , its associated scalar type determines the validity of the assignment; see section 5 . B .)

examples of root I root 1 root3 found

assignments : := p i * x / y := - r o o t I := ( r o o t l + r o o t 2 ) * ( 1 . 0 := y>z count := c o u n t + I degree := d e g r e e + 10 sqrpr := s q r ( p r ) y := s i n ( x ) + cos(y )

B.

The

compound

+ y)

statement

The ~pmpound ~tatement specifies t h a t its c o m p o n e n t statements be executed in the same sequence as they are written. The symbols ~egin a n d ~ n d a c t as s t a t e m e n t brackets. Note that the "body" of a Program has t h e f o r m o f a c o m p o u n d statement.

{ program 4.1 the compound ~ro~ram

statement

beginend(output);

~j~ sum : integer; ~e~in s u m := 3 + 5 ; w r i t e l n (sum , - s u m )

e_o~. B

-8

}

22

Pascal uses the semicolon to ~eoarete statements, not to terminate statements; i.e. the semicolon is N O T p a r t of t h e statement. The explicit rules regarding semicolons are reflected in the syntax of Appendix D . If one h a d w r i t t e n a s e m i c o l o n a f t e r t h e s e c o n d s t a t e m e n t , t h e n an ~ m o t y ~ t a t e m e n t ( i m p l y i n g no action) would have been assumed between the semicolon and the symbol ~nd. This does no harm, for an e m p t y s t a t e m e n t is allowable at this point. Misplaced semicolons can. h o w e v e r , cause troubles--note t h e e x a m p l e in s e c t i o n 4.D .

C. R e p e t i t i v e statements

Reoetltive ~tatmments specify that certain statements be repeatedly executed. If the number of repetitions is k n o w n beforehand (before the repetitions are b e g u n ) , t h e for s t a t e m e n t is usually the appropriate c o n s t r u c t to e x p r e s s t h e s i t u a t i o n ; o t h e r w i s e , t h e r e p e a t or w h i l e s t a t e m e n t s h o u l d be u s e d .

C .1 T h e The

while

while while

statement

statement

has



the

~g

form:



The expression controlling the repetition must Boolean. It is e v a l u a t e d b e f o r e e a c h i t e r a t i o n , so t a k e n t o k e e p t h e e x p r e s s i o n as s i m p l e es p o s s i b l e .

{ program 4.2 compute h(n) Qroeram

=

I +

1/2 +

egwhile(input,

I/3

+

o..

+ I/n

be care

of t y p e must be

}

output):

varn ~ealn

: integer; h : real; read(n); write(n); h :: O; while n>O ~ ~p.~ h := h + I/n; nnd; writeln(h) end.

10

n

:= n-1

2.928968253968e+00

The statement executed statement in the above becomes false. If its

by the while statement (a c o m p o u n d c a s e ) is r e p e a t e d u n t i l t h e e x p r e s s i o n value is f a l s e at t h e b e g i n n i n g ° t h e

23

statement

C.2

The

is

not

repeat

The repeat

has



The sequence is executed the Boolean iteration.

{ program 4.3 compute h(n)

at

all.

statement

statement

reoeat

orooram

executed

the

form:

{ ; }

until



of at

statements between the symbols ~eat and until least once. Repeated execution is c o n t r o l l e d by expression, which is evaluated after every

=

I +

I/2

egrepeat(input,

+

I/3

+

...

+

I/n

}

output);

vat n : integer ; h : real; beoin read(n); write(n); h := O; repeat h := h + I/n; n

:= n - 1

n=O; writeln

(h)

am~. 10

The above happens if correct for

2.928968253968e+00

program performs correctly n<=O. The while-verslon a l l n. i n c l u d i n g n=O.

Note that it is a sequence of statement executes; a bracketing redundant (but n o t i n c o r r e c t ) .

C.3

The

for

of

for n>O, Consider what the same program is

statements that pair ~e~!~...~nd

the repeat w o u l d be

statement

The for statement indicates that a statement be r e p e a t e d l y executed while a progression of values is assigned to t h e &pn~ zariable of t h e f o r s t a t e m e n t . It h a s t h e g e n e r a l form: for


variable>

:= < i n i t i a l value> do < s t a t e m e n t >

to



variable>

:= < i n i t i a l value> do < s t a t e m e n t >

downto

value>

(or)
value>

24

{

program 4.4 compute h(n)

=

1 +

1/2

+

1/3

+

...

+

1/n

}

o r o o r ~ eGfor(input, output); yam i,n : integer; h : real; beain r e a d (n) ; w r i t e (n) ; h := O; i := n d Q ~ n t o I d ~ h := writeln(h) end.

10

+

1/i;

2.928968253968e+00

{ program 4.5 compute the cosine using cos(x) = 1 -x*'2/(2"I) ~rQoram

h

cosine

(input,

the expansion: + x*'4/(4"3"2"I)

-

...

}

output);

const vat

eps = Ie-14; x , s x , s , t :real; i .k ,n :integer ; b#ain read(n); for i := I t o n ~L~ beoin read(x); t := I; k :-- O; s := I; s x ~DJ~ abs(t) > eps*abs(s) do ~e~D k :~ k + 2; t := -t*sx/(k*(k-1)); :=

S

end

:~ s q r ( x ) ;

s+t

;

writeln

(x ,s ,k d i v

2)

end end.

1.534622222233e-01 3.333333333333e-01 5.000000000000e-01 1.000000000000e+O0 3.141592653590e+00

9.882477647614e-01 9.449569463147e-01 8.V75825618904e-01 5.403023058681e-01 -1.000000000000e+O0

5 6 ? 9 14

The control variable, the initial value, and the final value must be of t h e s a m e s c a l a r t y p e (excluding type real), and must not be altered by the for statement. The initial and final values are evaluated o n l y o n c e . If in t h e c a s e of ~ (~nw~to) the initial value is g r e a t e r (less) than the final value, the for statement is not e x e c u t e d . The final value of the control variable is left undefined upon normal exit from the for statement.

25

A

for

statement

for is

v

of t h e

:= el & g

equivalent

to

e2 ~g the

j~ e1<=e2 &hen ~eain v := el; {at

and

is

this

a

for

~or

v

el

equivalent

As

v

statement

v

is

the

statements:

succ(v);

S:

...;

v

:=

S;

...;

v

:= e2;

form:

e2 ~ g

S

statement:

v

a final

consider

S; is

v

:=

pred(v);

summing

S

undefined}

the

following

program.

{ program 4.6 compute I - I/2 + 1 / 3 - . . . + I / 9 9 9 9 - 1/10000 1) l e f t to r i g h t , in s u c c e s s i o n 2) l e f t to r i g h t , a l l p o s a n d n e g t e r m s , 3) r i g h t t o l e f t in s u c c e s s i o n 4) r i g h t t o l e f t , a l l p o s a n d n a g t e r m s , ~roqram

e2;

undefined}

if el>=e2 then beain v := el; ea~ {at this point,

example

of

:=

of t h e

#ownto to

S

sequence

S;

point,

:=

form:

4 ways. then

subtract

then

subtract}

(output);

v_ar

s 1.s 2p ,s 2n ,s 3,s4p ,s 4n .lrp .lrn ,rlp ,rln : real; i : integer ; _be.qin s 1 := 0; s2p := O; s 2 n := O; s 3 := O; s 4 p := O; s 4 n fgj& i := I J2o 5 0 0 0 ~L~ begin l r p := I / ( 2 - i - I ) ; { pos terms, left to right } l r n := I / ( 2 - i ) ; { n e g t e r m s , left to right} r l p := I / ( I 0 0 0 1 - 2 - i ) ; { p o s t e r m s , r i g h t to l e f t } r l n := I / ( I 0 0 0 2 - 2 - i ) ; { n e g t e r m s , r i g h t to l e f t } sl := s l + lrp - l r n : s 2 p := s 2 p + I r p ; s 2 n := s 2 n + I r n : s 3 := s 3 + r l p - r l n ; s 4 p := s 4 p + r l p ; s 4 n := s 4 n + r l n gnU; w r i t e l n (s I ,s 2p ~s 2n ) ; writeln(s3,s4p-s4n) end.

~.930991830595e-01 6.930971830599e-01

6.930971830612e-01 6.930991830601e-01

:=

O;

26

Why

do t h e

four

D. Conditional

"identical"

sums

differ?

statements

A ~qgo~L~t±onal ~tatement an if or case statement, selects a single statement of its component statements for execution. The if ~Lt~tement specifies that a statement be executed only if a certain condition (Boolean expression) is true. If it is false, then either no statement or the statement following the symbol e l s ~ is e x e c u t e d .

D .I The

if s t a t e m e n t

The t w o

forms

if

for an

if s t a t e m e n t

are:



~hen





~hen



(or) if

else



The expression b e t w e e n t h e s y m b o l s if a n d ~ h e n must be of t y p e Boolean. Note that the first form may be regarded as an abbreviation of the second when the alternative statement is the empty statement. Caution: there is never a semicolon before an else1 Hence, the text: if

p J&heI3 . ~ e a i n

is i n c o r r e c t . if

p then;

$1;

Perhaps ~#ain

Here. the statement between the then statement following

S2;

even

$1;

S2;

$3 end;

more

else

$4

deceptive

is t h e

text:

$3 ~nd

c o n t r o l l e d by the if is t h e e m p t y s t a t e m e n t . and the semicolon; hence, the compound the if s t a t e m e n t will a l w a y s be e x e c u t e d .

The s y n t a c t i c a m b i g u i t y a r i s i n g f r o m the c o n s t r u c t : if < e x p r e s s i o n - l > J~Q2.J3 i f < e x p r e s s i o n - 2 > ~ h e n < s t a t e m e n t - l > rise is r e s o l v e d by i n t e r p r e t i n g t h e c o n s t r u c t as e q u i v a l e n t iL ~hen ~eain if ~hen else

The r e a d e r is f u r t h e r c a u t i o n e d t h a t e c a r e l e s s l y statement can be very c o s t l y . Take t h e e x a m p l e n-mutually exclusive conditions, c1.,.Cno each

to

f o r m u l a t e d if w h e r e one has instigating a

27

distinct action, si. Let P ( c i ) be t h e true, and say that P(ci)>=P(cj) for i < j . s e q u e n c e o f if c l a u s e s is : if

probability of ci b e i n g Then the most efficient

ci t h e n sl e l s e i f c2 S b e n s 2 else ... else

The fulfillment of statement completes remaining tests.

if

c(n-1)

~hen

s(n-1)

e!se

a condition and the execution of the if statement, thereby bypassing

If " f o u n d " is a v a r i a b l e of t y p e B o o l e a n , a n o t h e r of t h e if s t a t e m e n t can be i l l u s t r a t e d by: if e = b A much

J~b~.J3 f o u n d

simpler

found

:= a = b

sn

:= t r u e

statement

is:

:lse

found

:= f a l s e

frequent

its the

abuse

28

{ program 4.7 write roman numerals ~ro~ram

}

roman (output):

~ar x oy : integer ; bgqio y := I; ~j~p~&t_ x := y: write(x," w_hile x > = 1 0 0 0 do bw_qin write(°m°): if x>=500 then bea~j3 w r i t e ( ' d ") ;

"): x

:= x - l O 0 0

~n~:

x

:= x - 5 0 0

end :

x

:= x - l O 0

end:

x

:= x - 5 0

~nd:

x

:= x - t 0

eZld;

x

:= x - 5

end;

x

:= x - 1

end;

w_h_Ale x>=lO0 d o if

b~gin write('c°): x > = 5 0 t, hen beEi_.on w r i t e ( ' l ' ) :

w_hi!e x>=10 d~ be_~i~n

write('x'):

if x > : 5 ~hsn beoln x >= be~i[z writeln;

~h_~le

write('v'); 1 ~l write('i'): y : : 2*y

until y>5000 and.

I 2 4 U 16 32 64 128 256 512 1024 2048 4096

i ii iiii viii xvi xxxii lxiiii cxxviii cclvi dxii mxxiiii mmxxxxviii mmmmlxxxxvi

N o t i c e a g a i n t h a t it is o n l y o n e s t a t e m e n t t h a t is c o n t r o l l e d by an if c l a u s e . Therefore, when more than one action is i n t e n d e d , a compound statement is n e c e s s a r y .

The next program r a i s e s a r e e l v a l u e x t o t h e p o w e r y. w h e r e y is a non-negative integer. A simpler, and evidently correct version is obtained by o m i t t i n g the inner while statement: the result z is t h e n o b t a i n e d through y multiplications by x. N o t e the loop invariant: z*(u**e)=x**y. The inner while statement leaves z and u**e invariant, and obviously improves the efficiency of t h e a l g o r i t h m .

29

{ program 4.B exponentiation #j~oera~

with

natural

exponent

exponentiatiom(input,

}

output);

v~/~ e , y : i n t e g e r ; u , x , z : r e a l ; #eQin read(x,y); write(x oy)' Z := 1; u :: X ; e := y; Wb~

e>O

b ~

{z*u**e

do

while ~_~ b~_~in

en~. ; := e - l ;

e a~; writeln(z)

= x**y,

z {z

e>O}

odd(e) do e :-- e d i v

2:

u

:= s q r ( u )

:= u * z

= x*~y}

2.000000000000e+O0

7

I .2UOOOOOOOOOOe+02

The following program plots a reel-valued function f ( x ) by letting the X~xis run vertically and then printing an a s t e r i s k in positions corresponding to t h e c o o r d i n a t e s . The position of the asterisk is o b t a i n e d by c o m p u t i n g y=f(x), multiplying by a scale factor s, rounding the product to t h e n e x t i n t e g e r , a n d then adding a constant h end letting the asterisk be p r e c e d e d by that many blank spaces.

30

{

orogram 4.9 graphic representation of e f(x) = exp(-x) * sin(2*pi*x)

&re,ram

function }

graph 1(cutout) ;

const

d = 0.0625: {3/16o 16 l i n e s for i n t e r v a l [x,x+1]} s = 31: {31 c h a r a c t e r widths for i n t e r v a l [y,y+1]} h = 34: { c h a r a c t e r position of x-axis} c = 6.28318: {2*pi} lim = 32: var XoY : reel: i,n : integer: beain x := O; f_.gor_ i := I J2~ l i m d o b~eqin y := e x p ( - x ) * s i n ( c * x ) ; n := r o u n d ( s ' y ) + h: r_epe_a~ w r i t e ( ° "); n := n - 1 until n=O: writeln('**) : X

:=

x+d

end_ en_dd.

95

31

O.2

T he

case

statement

The case statement c o n s i s t s of an e x p r e s s i o n (the s e l e c t o r ) a n d a llst of s t a t e m e n t s , e a c h b e i n g l a b e l l e d by a c o n s t a n t of t h e type of t h e s e l e c t o r . T h e s e l e c t o r t y p e m u s t b9 a s c a l a r t y p e , excluding the type real. The case statement selects for execution that statement whose l a b e l is e q u a l t o t h e c u r r e n t v a l u e of t h e s e l e c t o r ; if no s u c h l a b e l is l i s t e d , t h e e f f e c t is undefined. Upon completion of t h e s e l e c t e d s t a t e m e n t , control goes t o t h e e n d of t h e c a s e s t a t e m e n t . T h e f o r m is: cas~ ~Z
label'list>

: ; :

end

examples: (assume var gase i of O: x := O; I: x := s i n ( x ) ; 2: x := c o s ( x ) ; 3: x := e x p ( x ) ; 4: x := I n ( x ) end

i:

i n t e g e r ; ch: c h a r ; ) c a s e ch of ' a ' , ' g ' . ' d ' : ch := s u c c ( e h ) ; "z°,*e°: ch := p r e d ( c h ) ; "f" °S°: {null case} end

Notes: "Case labels" are ~ o r d i n a r y l a b e l (see s e c t i o n 4 . E ) and c a n n o t be r e f e r e n c e d by a g o t o s t a t e m e n t . T h e i r o r d e r i n g is arbitrary; however, labels m u s t be u n i q u e w i t h i n a g i v e n c c s e statement. Although the efficiency of the case statement d e p e n d s on t h e implementation, the general rule is to u s e it w h e n o n e has several mutually exclusive statements with similar probability of s e l e c t i o n .

E.

The

goto

statement

A qoto ~tatement is a s i m p l e s t a t e m e n t indicating that further processing s h o u l d c o n t i n u e at a n o t h e r p a r t of t h e p r o g r a m t e x t , n a m e l y at t h e p l a c e of t h e l a b e l . ~ot~



Each label (an u n s i g n e d i n t e g e r t h a t is at m o s t 4 d i g i t s ) m u s t appear in a label declaration p r i o r to its o c c u r r e n c e in t h e program body. The s c o p e of a l a b e l L d e c l a r e d in a b l o c k A is the entire text of block A. That is, ~ i ~ s t a t e m e n t in t h e statement oart of A may be p r e f i x e d w i t h L : . T h e n any o t h e r statement w i t h i n t h e ~ b o l e of b l o c k A m a y r e f e r e n c e L in a g o t o statement.

32

example

(program

label

I:

fragment):

{block

A}

* o ,

g r o c e d u r e B; { b l o c k B} l a b e l 3; :eoin 3: w r i t e l n ( ' e r r o r ' ) ; . , °

got o 3

end ; { b l o c k ~in

{block

B}

A}

I: w r l t e l n ( ° t e s t f a i l s ' ) {a ~'~oto 3" is not a l l o w e d sod

in b l o c k

A}

Warning: The effect of jumps from outside of a s t r u c t u r e d statement into that statement is not defined . Hence. the following examples are i n c o r r e c t . (Note t h a t c o m p i l e r s do not n e c e s s a r i l y i n d i c a t e an e r r o r . ) examples : a)

for

i:= I ~ 10 do _beoin S I; 3:S2 and ;

b)

if P then woto

3;

o

if c)

a'~bed~ 3: S

oroc@dure beoin

P

...

3: S eo~ ; b=_q~i~z ... ~oto 3 end.

A goto statement should be r e s e r v e d for u n u s u a l or u n c o m m o n s i t u a t i o n s w h e r e the n a t u r a l s t r u c t u r e of an a l g o r i t h m has to be broken. A good rule is to a v o i d the u s e of j u m p s to e x p r e s s regular i t e r a t i o n s and c o n d i t i o n a l e x e c u t i o n of s t a t e m e n t s , for such jumps destroy the reflection of the structure of computation in the t e x t u a l ( s t a t i c ) s t r u c t u r e of t h e p r o g r a m . Moreover, the lack of correspondence between textual and computational (static and dynamic) structure is extremely detrimental to the c l a r i t y of t h e p r o g r a m and m a k e s the task of

33

verification much more difficult. The Pascal program is o f t e n an i n d i c e t i o n not yet learned "to think" in P a s c a l

construct

in

other

programming

lenguages).

presence that the (as t h i s

o £ g o t o ° s in a programmer has is a n e c e s s a r y

SCALAR

A.

Scalar

AND

5 SUBRANG~

TYPE~

types

The basic definition identifiers

data types in Pascal indicates an o r d e r e d set which denote the values.


identifier>

example: type color

=

are the ~calar ~YPeS. Their of values by enumerating the

(

{.

}

) ;

=

(white,red,blue,yellow.purple.green, orange,black); (male,female);

sex = day = (mon,tues.wed,thur,fri,sat,sun); operators = (plus.minus,times,divide): illegal ~voe

example: workday = (mon,tues,wed,thur,fri.sat): free = (sat,sun): the type of sat is ambiguous)

(for

The reader defined as: type This true

is

already

Boolean

=

automatically and specifies

acquainted

(false,

with

the

standard

type

Boolean

true);

implies the standard that false
identifiers

false

and

The relational operators on all scalar types types. The order is constants are listed.

=, <>, <, <=. >=, and >. are applicable provided both comparands are of the same determined by t h e s e q u e n c e in which the

Standard

arguments

functions

succ(×)

e.g.

pred (x)

ord ( x ) The ordinal ord(pred(x))

Assuming Boolean, following

+

with

succ(blue) pred(blue) ord(blue)

number I.

of t h e

that c and cl and sl...sn are meaningful

of

scalar

= yellow = red = 2 first

constant

are of type color are arbitrary statements:

types

the the the

are:

successor of x predecessor of x ordinal n u m b e r of x

listed

is

(above), statements,

O,

ord(x)

b is of then

=

type the

35

for

c

while if

:= b l a c k

(cl<>c)

c>white

#ownto

and

then

c

red

b do

~

sl

sl

:= p r e d ( c )

case c of red .blue ,yellow: s I; p u r p l e : s2: g r e e n , o r a n g e : s3; white.black: s4 and

B.

Subrange

types

A t y p e may b e d e f i n e d as a a~bran~e of any other already defined scalar type--called its ~ss#_~iated ~calar ~voa. The definition o£ a subrange simply indicates the least and the largest constant value in the subrange, w h e r e t h e l o w e r b o u n d m u s t be less then the upper bound. A subrange of t h e t y p e r e a l in ~#_~ allowed. i&t#_~ < t y p e

identifier>

=

., < c o n s t a n t >

;

Semantically, a subrange type is an appropriate substitution for the associated scclar type in all definitions. Furthermore. it is the associated scalar type which determines the validity of all operations involving values of subrange types. For example, given the declaration; vat

a:

1..10;

The associated assignments a

:= b;

c

b:

0..30;

scalar

:= b;

b:=

type

c:20..30; for

a.

b,

and

e is

integer.

Hence

the

c;

are all valid statements, although their execution may sometimes be infeasible. The phrase "or s u b r a n g e thereof" is t h e r e f o r e assumed to be implied throughout t h i s t e x t a n d is not a l w a y s mentioned (as it is in t h e R e v i s e d Report.)

example : type days = (mon,tues ,wed,thur,fri,sat.sun): workd = mon..fri: {subrange of days} index = 0..63; {subrange of inteqer} letter = a .. z ; { s u b r a n g e of c h a r }

{scalar

type}

Subrange types provide the means for a more explanatory statement of the problem. To the implementor they also suggest an opportunity to conserve memory space and to introduce validity checks upon assignment at run-time. (For an example with subrange types, see program 6.3.)

STRUCTURED TYPES

IN

6 GN~NJ~_~AL--THE

ARRAY --IN PA_~T~CDLA,B,

Scaler and subrange types are unstructured types, The other types in Pascal are &~Wcture~ ~vees. As s t r u c t u r e d statements were compositions of other statements, structured types are compositions of o t h e r t y p e s . It is t h e t y p e ( s ) of the ~ommo~nt& and--most importantly--the structuring method that characterize a structured type, An option available to e a c h of t h e s t r u c t u r i n g methods is an indication of t h e p r e f e r r e d internal data representation. A type definition prefixed with the symbol ~_~ed signals the compiler to economize storage requirements, even at the e x p e n s e of additional execution time and a possible expansion of t h e c o d e . due to the necessary packing and unpacking operations. It is t h e user's responsibility to realize if he w a n t s t h i s t r a d e of efficiency for space, (The a c t u a l e f f e c t s upon efficiency and savings in s t o r a g e space are implementation dependent, and may. in f a c t , be n i l . )

The array

type

An array type consists of e f i x e d n u m b e r of c o m p o n e n t s (defined when the array is i n t r o d u c e d ) w h e r e a l l a r e of t h e s a m e t y p e , called the ~mooneOJ2 or ~se ~voe. Each component can be explicitly denoted and directly accessed by t h e n a m e of t h e array variable followed by the so-called io_~ex in square brackets. Indices are computable; t h e i r t y p e is c a l l e d t h e & B ~ . ~ , Furthermore, the time required to select (access) a component does not depend upon the value of the selector (index); hence the array is termed a £_Qndom-~cces& &~uctur~. The the

definition of an a r r a y s p e c i f i e s i n d e x t y p e . The g e n e r a l f o r m is: ~&e

A

= array[T1]

both

the

component

of

variable

declarations

memory : ~.~L~[O..max] sick : ~r_~[days] £~ (Of c o u r s e these identifiers.)

and

o_f T2;

where A is a n e w t y p e i d e n t i f i e r ; TI is t h e i n d e x scalar type (where typos integer and real are index types); a n d T 2 is a n y t y p e .

examples

type

examples

-and-

~L integer Boolean assume

the

sample

t y p e a n d is a not allowable

assignments

memory[i+j] := x sick[men] := t r u e definition

of

the

auxiliary

37

{ program find the mrooram

part 6.1 largest and

minmax(input,

smallest

number

in a g i v e n

output);

const n = 20; vat i.u .v.min.max : integer; a : arrav[1..n] of integer; beoin [ a s s u m e t h a t at t h i s p o i n t in t h e p r o g r a m , contains the values: 35 6 8 94 7 88 -5 -3 - 6 3 0 -2 74 88 52 4 3 5 4} rain := a[ while i < beoiQ u _if u > v

I] ; m a x := rain; i := 2; n do := a]~i] ; v := a [ i + 1 ] ; then beqin ~ u > m a x J.hen max := u ; i f v < m i n t h e n rain := v en_ d e l s e be_~i__nn ~ v>max then m a x : = v: if u < m i n t h e n rain := u en~ : i := i + 2 e_O~: if i = n t h e n i f a [ n ] >max ~Lh~o m a x :: a [ n ] _ei~_~ i ~ e [ n ] <~nin t h e n rain := a [ n ] ; writeIn(max,min) erld .

94

-6

list

array a 12 3 5 9

}

38

[ program 0.2 extend program ~rocr~

4.9

to

print

x-axls

}

graph2(output);

const

d = 0.0625: { 1 / 1 6 , 16 l i n e s for interval s = 31; |31 character widths for interval maxl = 67; {max length of line} hl = 34; {character position of x-axls} c = 6.2U318; |2*pi} lim = 32; vat x.y : real, i.n : integer: a : ~rr~l~[O..maxl] of char" b@ain x := 0 ; f o r i := 0 t o m a x l d o a [ i ] := " ", j~ i := I t o l l m ~-G beef in

a[h]

:=

[x.x+1]~ [y.y+1]}

° : ° :

y := e x p ( - x ) * s i n ( c * x ) : a[n] := " * ' ; writeln(a); a[n] := " "; x := x

n

:= r o u n d ( s ' y )

+ d

en~ end.

!

+ hl;

39

(Consider how one would extend program 6.2 to print more one function--both with and without t h e u s e of an a r r a y . )

Since T2 may be of a n y t y p e , structured. In particular, if original array A is said to declamation of a multidimentional vat

M

: grrav[a..b]

than

the components of a r r a y s m a y b e T 2 is a g a i n an a r r a y , t h e n t h e be m u l t i d i m e n s i o n ~ . Hence. the a r r a y M c a n be s o f o r m u l a t e d :

of a r r e v [ c . . d ]

of

T;

(of t y p e

T)

and M [ i ] [j] then of M.

denotes

the

jth

component

For multidimensional arrays, convenient abbreviations: va____rr M

: ~r_~av[a..b,c.,d]

~

it

is

of

the

customary

ith

component

to

make

the

T;

and M[i.j]

We may regard M component (in t h e i t h r o w o f NI).

as jth

a matrix column) of

and the

say ith

that M[i.j] is t h e j t h component of M (of t h e

T h i s is not l i m i t e d to t w o d i m e n s i o n s , f o r T c a n a g a i n be structured t y p e . In g e n e r a l the (abbreviated) f o r m is: ~vo~ &r_~2~&~[ If O index o~dimensional, expressions .

types and

are a

= [ ,
type>}] the is

gL


a

type>

is said array by n denoted

;

t o be index

40

{ program 6.3 matrix multiplication EroRr~j~

1

matrixmul(input,

output):

const vat i s a b c beain fan ~

m = 4; p : 3; n : 2; : 1.,m; j : 1..n; k : 1..p; : integer ; : arr_@~t[1..m/1..p] of integer; : ~r_~[1..p °1..n] o f i n t e g e r ; : ~rrav[1..m,1.,n] of integer; {assign ~nitial values t o a a n d b} i "= I t o m d o fan k := I t o p d o beoio read(s); write(s); eli,k] := s end : writeln end : writeln ; ~ k := 1 ~j& p ~ ~eoin f~L~ J := I • o n d o bf,~ read(s): write(s); b[k°j] := s end : writeln end : writ eln ; {multiply e * b} ? o r i := 1 k s m d a O~t~l f a n J := I t a n do b~_qUia s := O: f o r k := I t o p d o s := s + a [ i , k ] * b [ k ° j ] c[i~,O] := s ; w r i t e ( s ) and : writeln n___dd e ; writeln #_O_~ •

~trinc&

I -2 1 -I

2 0 0 2

-I -2 2

3 2 I

I 6 I -9

10 -4 4 -2

were

defined

;

3 2 I -3

earlier

as

sequences

of

characters

enclosed

41

in single quote marks (chapter I). S t r i n g s c o n s i s t i n g of e single character are the c o n s t a n t s of t h e s t a n d a r d t y p e c h a r (chapter 2); those of n characters (n>l), are d e f i n e d as c o n s t a n t s of t h e t y p e d e f i n e d by: oacked

~r~[

Assignment types.

1..n] ~

(:=)

The

char

is p o s s i b l e

relational

between

operators

operands =,

<>,

of ~ d e n t i c a l <,

<=,

array

and >= a r e

applicable on operends of identical packed character arrays, where the underlying character set determines the ordering,

Access to individual components of packed arrays is o f t e n c o s t l y , a n d t h e p r o g r a m m e r is a d v i s e d to p a c k or u n p a c k e p e e k e d array in e single operation. This is possible through the standard

variable

procedures

~j~rey[m..n] and

where

end

unpack.

Letting

A be an a r r a y

o~ T

Z be a v a r i a b l e Eacked

pack

of t y p e

~rrav[u..v]

(n-m)

>=

of t y p e

~L T

(v-u), t h e n

p a e k (A ,i .Z )

means

~

j := u ~ v do Z[J] := A[j--u+i]

u n p a c k ( Z ,A ,i )

means

~or

j := u ~ £ v ~ A[j--u+i] := Z[j]

and

where J denotes the p r o g r a m .

an a u x i l i a r y

variable

not

occurring

elsewhere

in

7 RE&gAE TYPES

The record types are perhaps the most flexible of data constructs. Conceptually, a record type is a template for a structure whose parts may h a v e q u i t e distinct characteristics. For example, assume one wishes to record information about a person. Known are the name, the social security number, sex. date of birth, number of dependents, and marital status. Furthermore, if the person is married or widowed, the date of the (last) marriaqe is given; if divorced, one knows the date of the (most recent) divorce and whether this is the first divorce or not; and if single, given is whether an independent residency is established. All of this information can be expressed in a single "record". More formally, a record is a s t r u c t u r e consisting of a f i x e d number of components, celled field&. Unlike the array. components are not constrained to be of identical type and c a n n o t be d i r e c t l y indexed. A type definition specifies for e a c h component its t y p e a n d an i d e n t i f i e r , t h e 2ijild i # @ n t i f i e r , to denote it. The scope of a field identifier is t h e s m a l l e s t record in which it is defined. In o r d e r t h a t t h e t y p e o f a selected component be evident from the program text (without executing the program), the record selector consists of constant field identifiers rather than a computable value. To take a simple example, assume one wishes to compute with complex numbers of the form a+bi. where a and b are real numbers and i is the square root of - 1 . T h e m e i s no s t a n d a r d type "complex", However, the programmer can easily define a record type to represent complex numbers. This record would need two fields, both of type real. for the real and imaginary parts. The syntax necessary to express this is: ::= ~ e c o r ~ ~nd ::= < f i x e d p a r t > I ; ::= < r e c o r d s e c t i o n > {; < r e c o r d s e c t i o n > } ::= < f i e l d i d e n t i f i e r > { , } I

Applying these declaration: ~Yoe vat

complex x

rules,

one

= ~ecord ~nd ; : complex;

can

state

re.im

the

following

definition

l

:

and

: real

where complex is e t y p e i d e n t i f i e r ° r e a n d im fields, a n d x is a v a r i a b l e of t y p e c o m p l e x . a r e c o r d m o d e up o f t w o c o m p o n e n t s or f i e l d s .

are identifiers Consequently. x

of is

43

Likewise, date

a toy toy

or

a variable :

representing

a date

can

be

defined

as:

~ecord

mo:(jan.feb,mar,apr,may,june. july ,aug ,sept ,oct ,nov ,dec ) ; day: 1..31; year : integer

as : = ~ecord

kind

: (ball ,too ,boat ,doll ,blocks game , m o d e l , b o o k ) ; cost : real; received: date; enjoyed: (alot,some,alittle.none); b r o k e n ,lost : B o o l e a n

a homework

assignment

assignment

= ~eeord

as: subject

:(history.language,lit, math,psych.science); assigned: date; grade: 0..4: weight: 1..10

end

To reference a record component, the followed by a point, and the respective example, the following assigns 5+3i to x: x,re :: 5; x.im :: 3

name o f t h e r e c o r d field identifier.

is For

If the record is itself nested within another structure, the naming of the record variable reflects this structure. For example, assume one wishes to record the most recent small pox vaccination for each member in t h e f a m i l y . A p o s s i b i l i t y is t o define the m e m b e r s as a s c a l a r , a n d t h e n t h e d a t e s in an a r r a y of r e c o r d s : family= ~L~d~ v a c c i n e :

An u p d a t e

(father,mother.childl,child2,child3); g~[family] £L date:

might

then

vaccine[child3] vaccine[child3] vaccine[child3] Note:

the

type

be recorded

as:

.mo := a p r ; .day := 23; .year := 1 9 7 3

"date"

also

i~icludes,

for

instance,

e

31st

April.

44

{

program operations

&re,ram

7.1 on

complex

complex

numbers

(output);

const fac = 4; tree complex = record re.im vat x,y : complex; n : integer ; be~in x.re := 2; x .Ira := 7; y.re := 6 ; y.im : = 3; for n := 1 &o 4 d~

writeln("

{x

x

=

:

integer

",x.re:3,x.im:3,

end;

'

y

=

".y.re:3,y.im:3);

+ y}

writeln(

{x

}

° sum

=

",x.re x .im

+ y.re:3, + y.im:3);

* y}

writeln

(" p r o d u c t

writ eln : x .re := x . r e

=

+ fac;

",x . r e * y . r e x.re*y.im x .im

:= x . i m

- x . i m * y .ira:3, + x.im*y.re:3); - fac;

end end.

x = 2 sum = product

7

y

=

6

3

x = 6 3 y = sum = 12 6 product = 27 36

6

3

x = 10 - 1 sum = 16 product =

6

3

6

3

b =

10 -9

48

y 2 63

= 24

x = 14 - 5 y = sum = 20 -2 product = 9 9 12

The syntax for a record type also makes provisions for ~art. implying that a record type may be specified as of several ~#j~_~. This means that different although said to be of the same type. may assume which differ in a certain manner. The differences may a different number and different types of components. Each variant declarations by one or case clause

a ~arisnt consisting variables. structures consist of

is charaeterised by a list. in parentheses, of of its pertinent comoonents. E a c h l i s t is l a b e l l e d more labels~, and the set of lists is p r e c e d e d by a specifying the data type of these labels (i.e. the

45

type according exsmple, assume type Then

to w h i c h t h e the existence

maritalstatus

one

t~De

can

=

describe

variants of a

(married.

persons

by

are

widowed, data

discriminated).

divorced,

As

an

sinRle)

of t h e

person = ~ecord #ase maritalstatus ~ married: ( < f i e l d s of m a r r i e d p e r s o n s o n l y > ) ; single: ( < f i e l d s of s i n g l e p e r s o n s o n l y > ) ;

;

end

Usually, currently r e c o r d is ms

e component ( f i e l d ) of t h e r e c o r d valid variant. For example, the l i k e l y to c o n t a i n a c o m m o n f i e l d

i t s e l f i n d i c a t e s its above defined person

: maritelstetus

This frequent situation can be abbreviated by declaration of the discriminating component--the f i e l d - - i n t h e c a s e c l a u s e i t s e l f , i.e. by w r i t i n g case

ms:

The s y n t a x
maritalstatus

defining

the

including so-called

aL

variant

part

is:

: := c a s e < t a g f i e l d > < t y p e i d e n t i f i e r > { ; } ::= : ( < f i e l d l i s t > ) I : : = { , } : : = ::= : I

It is before

Dart>

helpful defining

the ~a~

to "outline" it as a v a r i a n t

the information about record structure.

I , Person A. n a m e (last, f i r s t ) B, s o c i a l s e c u r i t y n u m b e r ( i n t e g e r ) C . s e x (male, f e m a l e ) D. d a t e of b i r t h ( m o n t h , day. y e a r ) E . n u m b e r of d e p e n d e n t s (integer) F. marital status if m a r r i e d , w i d o w e d a. d a t e of m a r r i a g e (month. day, year) if d i v o r c e d a. d a t e of d i v o r c e (month. day, year) b. f i r s t d i v o r c e (false, true) if s i n g I e a. i n d e p e n d e n t r e s i d e n c y (false,true)

DJE

a person.

46

Figure 7.a with different

is

a corresponding attributes.

woodyard

picture

of

two

"sample"

people

A)

edward i i lliH

845680539

B) iiiiii i

c)

male

~ug i

30

i1941

1

t

D)

E)

H,,,,, ,,

single

F)

true Figure

A record ~ype

defining

7.a

Two

"person"

sample

can

people

now

be

formulated

as:

alfa = ~acked ~r_~,~[1..lO] ~L char; status = (married,widowed,divorced,single); date = ~eeord mo : ( J a n . f e b . m a r , a p r , m a y , j u n , july.aug.sept.oet,nov,dec); day : I..31;

person

year : integer and; = ~eeord name : E e c o ~ end;

first.last:

alfa

ss : i n t e g e r ; sex : (male ,female): birth : date; depdts : integer; c a s e ms : s t a t u s ~ married~widowed : (mdete: date); divorced : (ddate: date; firstd: Boolean); single : (indepdt : Boolean)

end;

{person}

I. A l l field names must be distlnct--even if t h e y o c c u r in different variants. 2. If t h e f i e l d l i s t f o r a l a b e l L is e m p t y , t h e f o r m is: L : () 3. A field list can have only one variant p e r t a n d it m u s t succeed the fixed pert(s). (However, a variant part may itself contain variants. Hence, it is p o s s i b l e to h a v e n e s t e d variants.)

47

Referencing a record component is e s s e n t i a l l y a simple linear reconstruction of the outline. As e x a m p l e , assume a variable p of t y p e p e r s o n a n d " c r e a t e " t h e f i r s t of t h e m o d e l p e o p l e . p.name.last := " w o o d y a r d p.name.first := " e d w a r d p.ss := 8 4 5 6 8 0 5 3 9 ; p .sex := m a l e : p.birth.mo := a u g : p.birth.day := 30; p .birth.year := 1941; p.depdts : = 1; p .ms := s i n g l e ;

p.indepdt

A.

The

with

:= t r u e

statement

The above notation can be a b i t t e d i o u s , and the user may wish to abbreviate it using the ~ith ~tatement. The with clause effectively opens the scope containing the field identifiers of the specified record variable, so that the field identifiers may o c c u r as v a r i a b l e identifiers. (Thereby providing an o p p o r t u n i t y for the compiler to optimize the qualified statement.) The general f o r m is: ~tik__h < r e c o r d

variable>

{.


variable>}

do



Within the component statement of t h e w i t h s t a t e m e n t one denotes a field of a record variable by d e s i g n a t i n g o n l y its f i e l d identifier (without preceding it w i t h t h e n o t a t i o n of t h e e n t i r e record variable). The with statement of a s s i g n m e n t s :

below

is

~ith p.name.birth ~L~ ~J~l l a s t := " w o o d y a r d "; first := " e d w a r d "; ss := 8 4 5 6 8 0 5 3 9 ; s e x := m a l e ; mo := a u g ; d a y := 30; year := 1941; depdts := I; ms := s i n g l e ; indepdt := t r u e end {with}

equivalent

to

the

preceding

series

48

Likewise. vat

currentdate . .

: date:

°

_with c u r r e n t d a t e do i ~ too=dec t h e n b e u i n mo := j a n :

en~ ~ I s e mo is

equivalent var

:= y e a r + 1

:= s u c c ( m o )

to

currentdate if

year

: date;

currentdate.mo=dec then b ~ eurrentdate.mo := j a n : currentdate.year := e u r r e n t d a t e . y e a r + 1

grid else And the earlier :

currentdate.mo following

:= s u c c ( c u r r e n t d a t e . m o )

accomplishes

with vaccine[child3] b e Q i ~ mo := a p t :

d~ day

:= 23:

the

year

vaccine

:=

update

exampled

1973

grid

No assignments elements of the w__ith r

do

may be m a d e by t h e record variable list.

contain

any

variables

with a[i] dn b_eoin ... i := i+1 end is _nw~ a l l o w e d . form : w_ith r I, r 2 . . . . . . is

to

any

S;

for

S

r must not example :

The

qualified statement That is, given:

equivalent

r n do

to

k i t h r I do ~ith r 2 w~h

rn

do

S

S

subject

to

change

by

49

Whereas :

is

yam

a a

NOT

allowed,

y_~

a b

IS

: array[2..8] : 2..8; for

the

of

integer:

definition

: integer ; : ~eoo~_~[ a: r e a l ; end ;

allowed,

for

distinguishable

the

from

variable b is Furthermore, within

a is

ambiguous,

b : Boolean

notation

the

of

real

for

the

"'b.a '°.

integer

Likewise.

distinguishable from the qualified statement

the S of

wi~h b ~ g S both

"b"

and

"b.b"

reference

the

Boolean

"b.b",

a

is

the

Boolean

easily

record "b.b*

8

--THE ~ET T~PE~

A s e t t y p e d e f i n e s t h e s e t of v a l u e s t h a t is t h e p o w e r s e t of its base type, i.e. the s e t of a l l s u b s e t s of values of t h e b a s e type. including t h e e m p t y s e t . T h e b a s e t y p e m u s t be s s c a l a r or subrange type. ~y_#_~ < i d e n t i f i e r >

The base type implementations sets, which can

= &~,JL ~L~ < b a s e

type>;

of a set must be e scalar of Pascal may define limits be quite small (e.g. the number

type; however, for the size of bits in a word).

Sets are built up from their elements by s e t c o n s t r u c t o r s (denoted by in the syntax). They consist of the enumeration of the set elements, i . e . of e x p r e s s i o n s of t h e b a s e type. which are separated by c o m m a s a n d e n c l o s e d by s e t b r a c k e t s [ and ] . Accordingly. [] d e n o t e s the empty set, ::= [ < e l e m e n t llst> ] ::= < e l e m e n t > {, < e l e m e n t > } ::= < e x p r e s s i o n > I

I .. < e x p r e s s i o n >

The form m..n denotes the set of all elements such that m<=i<=n. If m > n , [ m . . n ] d e n o t e s t h e Examples

of

[13] [ i + j .i-J] ['A'..'Z"

set

The following structure:

Relational

±a

<>

type

:

"0"..'9"] operators

union intersection set difference of A t h a t a r e

+

=

constructors

i of the base empty set.

operators

are

applicable

on

all

(e.g. A-B denotes the set not also eIements of B . )

applicable

to

set

operands

objects

of

all

with

set

elements

are:

t e s t on ( i n ) e q u a l i t y t e s t on s e t i n c l u s i o n set membership. The first operand is a s c a l a r t y p e , the s e c o n d is of its a s s o c i a t e d set type; the result is true w h e n t h e f i r s t is an e l e m e n t of the second. otherwise false.

51

examples tyoe vat

of

primary = (red . y e l l o w .blue); color = ~ £J~ p r i m a r y ; hue1o'hue2 : color:

ch: char: chsetl,chset2: var

Set more

-and -

declarations

opeode add

~8_~ ~ Z

"a'..'z';

: &at g~ 0..7; : Boolean:

operations complicated

assignments hue1 hue2

:= [red] ; h u e 2 := []7 := h u e 2 + [ s u c c ( r e d )]

chsetl chset2 add

are relatively fast and can tests. A simpler test for:

:= := :=

[ "d'.'a'°'g'] [ "a'..°z°]-[ch] [2",3]

be u s e d

~d~ ( c h = ' a ' ) o n ( e h = ° b ' ) o ~ ( e h = ' c ' ) o ~ ( c h = ' d ° ) g ~ ( c h = ' z is : i_f£ ch i n [ "a°..'d °,'z "] &hen s

{ program example program

8.1 of set

operations

setop(output

;

<= o p c o d e

to

")

eliminate

~hen

s

}

)

type

d a y s = (m , t ,w , t h . f r , s a , s u ) ; w e e k = se__t_t o f d a y s ; varr wk,work,free : week; d : days ; ~roce~w~ check(s : week): {procedures introduced ~ d : days; ~eoin write(" "); f o ~ d := m & ~ su ~ i ~ d i ~ s & h e n w r i t e ( ° x ") ~ I s e w r i t e ( ' o ' ) ; writeln end; { c h e c k } beoin w o r k := [] ; f r e e := [] ; wk := [ m . . s u ] : d := sa; f r e e := [d] + f r e e +[su] ; check (free) ; w o r k := w k - f r e e ; check(work); /j[ f r e e <= w k t h e n w r i t e ( ' o ° ) 7 ~/[ w k >= w o r k t h e n w r i t e ( ' k ' ) ; i f D o t (work >= f r e e ) the[l w r i t e ( " j a c k °): i ~ [sa] <= w o r k ~ h e n w r i t e ( ' forget it'); writeln end.

O0000XX xxxxxoe ok j a c k

in

chapter

11}

52

On

program

development

Prooramming--in the sense of designing and formulating algorithms--is in general a complicated process requiring the mastery of numerous details and specific techniques. O n l y in exceptional c a s e s w i l l t h e r e be a s i n g l e g o o d s o l u t i o n . Usually, so many solutions e x i s t t h a t t h e c h o i c e of an o p t i m a l p r o g r a m requires a thorough analysis not only of the available algorithms and computers but also of the w a y in w h i c h t h e program will most frequently be u s e d . Consequently, the construction of an a l g o r i t h m should c o n s i s t of a sequence of deliberations, investigations, and design decisions. In the early stages, attention is b e s t c o n c e n t r a t e d on the global problems, and the first draft of a solution may pay little attention to details. As the design process progresses, the problem can be split into subproblems, and gradually more consideration given to the details of p r o b l e m smecification a n d to t h e c h a r a c t e r i s t i c s of the availmb~e tools. The terms ~ i s e ~efin_~ment [2] a n d ~ f ~ ~ m m i n ~ [4] are associated with this approach. The remainder of t h i s c h a p t e r illustrates the development of an algorithm by rewording (to be c o n s i s t e n t with Pascal notation) an example C .A.R. Hoare presents in ~ r u ~ ~rog_r_~in~ [4."Notes on O a t a S t r u c t u r i n g " ] . The assignment is to g e n e r a t e the prime numbers range 2..n, where n>=2. After a comparison algorithms, t h a t of E r a t o a t h e n e s " s i e v e is c h o s e n simplicity (no m u l t i p l i c a t i o n s or d i v i s i o n s ) . The

first I. 2. 3. 4. 5.

formulation

is

f a l l i n g in t h e of t h e v a r i o u s b e c a u s e of i t s

verbal.

Put a l l t h e n u m b e r s Select and remove sieve. Include this number Step through the number. I f t h e s i e v e is not

between 2 and the smallest

n into number

in t h e sieve,

"primes" removing

empty,

repeat

all

steps

t h e "'s~eve" remainino in

multiples

of

the

this

2--5.

Although initialization of variables is t h e f i r s t s t e m in t h e execution of a p r o g r a m , it is o f t e n t h e l a s t in t h e d e v e l o p m e n t process. Full comprehension of t h e a l g o r i t h m is a p r e r e q u i s i t e for making the proper initializations; updating of these initializations with each program modification is n e c e s s a r y t o keep the program runnino. (Unfortunately. updating is not a l w a y s sufficient!) Hoare chooses a set t y p e w i t h e l e m e n t s 2 , . n to r e p r e s e n t both the sieve and the primes. The following is a s l i g h t v a r i a t i o n of the program s k e t c h he p r e s e n t s .

53

const n = 10000: va~ sieve,primes : set next ,j : integer ;

of

2..n;

{ initialize} sieve := [2..n] : p r i m e s := [] : n e x t ~eoeat {find next prime} while Dot(next in sieve) do n e x t primes := p r i m e s + [next] : j := n e x t ; ~hile j < = n d~l { e l i m i n a t e } beoin sieve := s i e v e - [j] ; j

:= 2; := s u c c ( n e x t ) :

:=

J + next

and until

sieve=[]

As an exercise Hoare makes the assignment to rewrite program, so that the sets only represent the odd numbers. following is one proposal. Note the close correlation with first solution.

cons~

n = 5000; sieve,primes next ,j,c :

the The the

{n' = n di~A 2} : set o~ 2..n; integer ;

DJ&W~LJ3 { i n i t i a l i z e } sieve := [2..n] : p r i m e s := [] ; n e x t r_em~at { f i n d n e x t p r i m e } ~ h i l e DD_t(next in s i e v e ) d o n e x t primes := p r i m e s + [next] ; c := 2 * n e x t - I; {c = n e w p r i m e } j := n e x t ; mile j < = n ~LQ { e l i m i n a t e } ~eoin sieve := s i e v e - [J] ; j

:= 2: := s u c c ( n e x t ) ;

:=

j+c

and unt$~

sieve=[]

end .

It is desirable that all basic set operations ere relatively fast. Many implementations restrict the maximum s i z e of s e t s according to their " w o r d l e n g t h ~. so t h a t e a c h e l e m e n t of t h e b a s e s e t is r e p r e s e n t e d b y o n e b i t (0 m e a n i n g absence, I meaning presence). Most implementations would therefore not accept a set with 1Oo000 elements. These considerations lead to an adjustment in t h e d a t a r e p r e s e n t a t i o n , as s h o w n in p r o g r a m U.2. A large set can be represented as an array of smaller sets such that each "fits" into one word (implementation dependent). The following program uses the second sketch as an abstract model of the algorithm. The sieve and the primes are redefined as arrays

of sets: next undeveloped.

is

defined

as

a

record.

The

output

is

left

54

{ program 8.2 generate the primes between 3..I0000 using a sieve containing o d d i n t e g e r s in t h i s r a n g e . } oroQram

primes (output);

const

wdlenoth = 59; { i m p l e m e n t a t i o n dependent} m a x b i t = 58; w = 634; {w : n div w d l e n g t h div 2} sieve ,primes : ~rrav[O..w] ~J~ : e t ~ f O . . m a x b i t next : rec0rd word,bit :inteoer end ; j , k . t ,e : i n t e g e r ; empty : boolean; beoin {initialize} ZDJZ t

:-- 0 ~j~ w

beein sieve[O] next.bit

;

do

sieve[t] := [ O . o m a x b i t ] ; p r i m e s [ t ] := := s i e v e [ O ] - [0] ; n e x t . w o r d := O; := I; e m p t y := f a l s e ;

[] e n d ;

~ h next do r_~3J~ { find next prime } ~h~ n o t ( b i t in s i e v e [ w o r d ] ) d o bit := s u c e ( b i t ) ; primes[word] := p r i m e s [ w o r d ] + [bit] ; c := 2 * b i t + 1; j := b i t ; k := w o r d ; ~b.ile k < = w do { e l i m i n a t e } b~liil s i e v e [ k ] := s i e v e [ k ] - [j] ; k := k + w o r d * 2 ; j := j + c: while j>maxbit do begin k := k + 1 ; j := J - w d l e n g t h end end ; if sieve[word]=[] then be__qin e m p t y := t r u e ; bit := 0 --end ; while empty ~ (word
and up~il and.

empty ; {ends

with}

9

EZLE T~_EE~

In m a n y w a y s t h e s i m p l e s t s t r u c t u r i n g m e t h o d is t h e s e q u e n c e , In the data processing profession t h e g e n e r a l l y a c c e p t e d t e r m to d e s c r i b e a s e q u e n c e is a ~_~ouential ~ i l e . P a s c a l u s e s s i m p l y t h e word LJ,~J: to specify a structure consisting of a s e q u e n c e of components--all of w h i c h a r e of t h e s a m e t y p e . A natural ordering of the components is d e f i n e d t h r o u g h t h e sequence, and at any i n s t a n c e o n l y one c o m p o n e n t is d i r e c t l y accessible. The other components are a c c e s s i b l e by p r o g r e s s i n g sequentially t h r o u g h t h e f i l e . T h e n u m b e r of c o m p o n e n t s , called the I en at~h of the file, is not fixed by the file type definition, T h i s is a c h a r a c t e r i s t i c which clearly distinguishes t h e f i l e f r o m t h e a r r a y . A f i l e w i t h no c o m p o n e n t s is s a i d to be _em_~. type



: ~i1~

~

;

The declaration of every file variable f automatically introduces a buffer wariablg, d e n o t e d by ft, of t h e c o m p o n e n t type. It can be considered as a ~ i n d o w t h r o u g h w h i c h o n e c a n either inspect (read) e x i s t i n g c o m p o n e n t s or a p p e n d (write) n e w components, and which is automatically m o v e d by c e r t a i n f i l e operators. The s e q u e n t i a l p r o c e s s i n g a n d t h e e x i s t e n c e of a b u f f e r v a r i a b l e suggest t h a t f i l e s may be a s s o c i a t e d with ~condarv ~to=a~e and ~erioheral~. Exactly how the components are allocated is implementation dependent, but w e a s s u m e t h a t o n l y s o m e of t h e components are present in p r i m a r y s t o r e at a n y o n e t i m e , a n d o n l y t h e c o m p o n e n t i n d i c a t e d by ft is d i r e c t l y a c c e s s i b l e . When the standard otherwise

window ft is moved beyond the gnd gf Boolean function eof(f) returns the f a l s e . The b a s i c f i l e - h a n d l i n g operators

a ~ile value are:

f, t h e true,

r e s e t (f)

resets the f i l e w i n d o w to t h e b e g i n n i n g for t h e purpose of r e a d i n G , i.e. a s s i g n s to ft t h e v a l u e of the f i r s t e l e m e n t of f. e o f ( f ) b e c o m e s f a l s e if f is not e m p t y ; o t h e r w i s e , ft is not d e f i n e d , and eof(f) remains true.

r e w r i t e (f)

precedes the rewriting of t h e f i l e f. The c u r r e n t value of f is replaced with the empty file. eof(f) becomes true, and a new file may be written.

get(f)

advances the file window to the next component; i.e. assigns t h e v a l u e of t h i s c o m p o n e n t to t h e buffer variable ft . If no n e x t c o m p o n e n t e x i s t s , then eof(f) becomes true. and the resulting value of ft is not d e f i n e d . T h e e f f e c t of g e t ( f ) is

56

defined only execution.

put(f)

examples

if

eof(f)

is

false

prior

to

its

appends the value of the buffer variable ft t o the file f. T h e e f f e c t is d e f i n e d o n l y if p r i o r to execution the predicate eof(f) is t r u e . e o f ( f ) r e m a i n s t r u e , a n d ft b e c o m e s u n d e f i n e d ,

of

declarations

-and-

statements

with

yam d a t a : ~J~J~ ~L~ i n t e g e r ; a : integer;

a := s q r ( d a t a ~ get (data)

~ar

clubt := p : put (club)

club : ~ile p : person;

Program

parts

I. R e e d

a

file

~L~ p e r s o n ;

files );

: of r e a l

numbers

and

compute

their

sum

S.

{ program part c o m p u t e sum } S := O; reset(f); w_j31]e n o t e o f (f) d ~ be~ S := S + ft ; g e t ( f )

en_g 2,

The following program fragment operates ordered sequences of i n t e g e r s fl,f2 ..... fm end g l g2 . . . . . gn s u c h t h a t f ( i + 1 ) >= fi and g(i+1) a n d _merges t h e m i n t o o n e o r d e r e d file h(k+1) >= h ( k ) f o r k = 1,2 . . . . . It

uses the following variables: endfg : Boolean; fog,h : ~ile ~ integer

on

two

>= gi, f o r a l l h such that (re+n-l).

files

i.j

of

57

{ program part merge f and g into

h }

bea~3 reset(f): reset(g); rewrite(h); endfg := s o l ( f ) or eof(g); ~hile D o t e n d f g do bw~in if ft
not ht (g)

eof(g) do := g t ; p u t ( h ) ;

not eof(f) do h t := f t ; p u t ( h ) ; (f)

_end

--end

Files may be local to a p r o g r a m (or l o c a l t o a p r o c e d u r e ) or they may already exist outside the program. The latter are called e~ternal ~les. External files e r e p a s s e d as p a r a m e t e r s in the program heading (see chapter 13) i n t o the program.

A.

Textfiles

Files whose Accordingly, type

text

components the standard =

~ile

D~

are type

characters are t e x t is d e f i n e d

called ~extfil~. as f o l l o w s :

char;

Texts are usually subdivided into l ~ . A straight-forward method of indicating the separation of two consecutive lines is by using control characters. For instance, in the ASCII character set the two characters ~ (carriage return) and l~ (line feed) are used to mark the end of a line. H o w e v e r . many computer installations use a character set devoid of such control characters; this implies that other methods for indicating t h e end o f a l i n e m u s t be e m p l o y e d . We may consider the t y p e t e x t as b e i n g d e f i n e d over the base type char (containing printable characters only) extended by a (hypothetical) line separator character. This control character cannot be assigned to v a r i a b l e s of t y p e c h a r , b u t c a n be b o t h

58

recognized operators:

and

generated

of

the

textfile

skip to the beginning textfile x (xt b e c o m e s next line)

eoln(x)

a Boolean function indicating whether t h e e n d of the current line in the textfile x has b e e n reached. (If t r u e . xt c o r r e s p o n d s to the position of a line separator, b u t xt is a b l a n k . )

write

line

special

readln(x)

abbreviated

current

following

terminate

a textfile notation

the

the

writeln(x)

If f is abbreviated operators°

of the the first

textfile next line character

x of of

the the

a n d ch a c h a r a c t e r variable, the following may be u s e d in p l a c e of t h e g e n e r a l f i l e

form

(f .eh )

read (f .ch )

The following program demonstrate some typical I.

by

expanded

form

ft

: = ch;

ch

:= ft : g e t (f)

schemata operations

p u t (f)

use the above conventions performed on t e x t f i l e s .

to

Writing a text y. Assume that P(c) computes a (next) character and assigns it t o p a r a m e t e r c. I f t h e c u r r e n t l i n e is to be terminated, a Boolean variable p is s e t to t r u e ; a n d if t h e t e x t is t o be t e r m i n a t e d , q is s e t t o t r u e .

rewrite(y); ~eeeat £eo eat P(c); wntil p ; wr it e l n (y) until q

2.

w r i t e (y~.'h)

Reading a text x. Assume that Q(c) denotes the processing of a (next) character c. R denotes an action to be executed upon encountering the end of a line. r e s e t (x) ; while opt eof(x) do be~in wb_~ ~ eoln(x) do b#_q/J3 r e a d (x .'c ) ; Q ( c ) end ; R ; readln(x) ~nd

59

3.

Copying a text s t r u c t u r e of x.

x

to

a text

y, w h i l e

preserving

the

line

reset(x); rewrite(y); w h i l e #3&J2 e o f ( x ) d o b e q i n {copy a line} while not eoln(x) do beain read(x,c); write(y,c) end ; readln(x); writeln(y) end

B . The s t a n d a r d

files

"input"

and

"output"

The textfiles "input" and "output" usually represent the standard I/O media of a computer installation (such as the card reader and the line printer). Hence, they are the principal communication line between the computer and its human user.

Because these two files are used very frequently, considered as "default v a l u e s " in t e x t f i l e o p e r a t i o n s t e x t f i l e f is not e x p l i c i t e l y i n d i c a t e d . That is is e q u i v a l e n t

write

read(ch)

read (input,ch)

writeln

writeln (output)

r eadln

r e a d l n (input)

eof

e a r (input)

eoln

eoln (input)

Note: The s t a n d a r d f u n c t i o n s to the f i l e i n p u t (output).

~se&

Accordingly, for the case where x the first two of the program follows: (assume ~ar sh: char) a text

on

file

are the

to

write(ch)

Writing

they when

"output";

~leo eat ~ e o e a ~ P (ch); w r i t e ( c h ) unti~, p ; writeln uDtil q

(output

,eh)

(rewrite)

must

not

is "input" schemata

and y can be

be a p p l i e d

is "output" expressed as

60

Reading

a text

from

file

"input":

wh~ no#- e o f d o b~,q~& { p r o c e s s a line} w h i l £ no#_ e o l n d o beain read(ch); Q (oh) ond ; R ; readln end

Further extensions convenient handling described in c h a p t e r

of the procedures of legible input 12.

write and

and read (for output date)

the are

The next two examples of programs show the use of the textfiles input and output, (Consider what changes would be necessary if only get and put° not read and write° are to be used.)

{ program orosr~d~

9.1 fcount

--

frequency

count

of

letters

in

input

file

}

(input .output);

~ar

ch: char ; count : ~r_~av[ "a'..'z "] of integer: letter : set of "a'..°z'; beRin letter : = [ °a ° . . ' z ' ] ; f o e c h : = "a ° J&£ "z ° d o c o u n t [ c h ] := wh~ not eof do D#qia ~hile no& eoln do b~g~3 read (ch) : write(ch) ; if ch &n letter J2h£.J3 c o u n t [ c h ] end ; wrtteln; readln

O;

:=

count[ch]+l

e~ end.

In some installations when a textfile is s e n t t o a o r i n t e r , the first character of each line is used as a p r i n t e r control character; i.e. this first character is n o t o r i n t e o , but i n s t e a d interpreted as controlling the paper feed mechanism of t h e printer. The following conventions a r e in w i d e u s e : blank

: :

r I "

:

:

feed one llne space before printing feed double space before printing s k i p to t o p of n e x t p a g e b e f o r e p r i n t i n g no l i n e f e e d ( o v e r p r i n t )

61

The following llne, resulting

{ program oroeram

9.2 insert

program inserts a b l a n k at t h e b e g i n n i n # in n o r m a l s i n g l e space printing.

--insert

leading

blank

of

each

}

(input ,output):

vat ch: char; begin @D~i~J& D o t e o f d o beoin write(" '); ~ h ~ l e n o t e o l n do be~in r e a d (ch) : w r i t e ( c h ) eoj~ ; wrlteln ; readln end .

If read and parameter, the and output are the Paramenter

write are used without indication of a file default convention specifies that the files input assumed: in w h i c h e a s e , t h e y m u s ~ be m e n t i o n e d in l i s t of t h e p r o g r a m heading.

10 POINTER

~YPES

A ~ _ ~ ~#riable (staticly allocated) is o n e t h a t is d e c l a r e d in a program and subsequently d e n o t e d by its i d e n t i f i e r . It is called static, for it e x i s t s ( i . e . m e m o r y is a l l o c a t e d for it) during the entire execution of the block to which it is l o c a l . A variable may, on the other hand, be generated dynamically (without any correlation to the static structure of the program) by the procedure new. Such a variable is c o n s e q u e n t l y called a

Dynamic variables do not occur in an explicit variable declaration and cannot be r e f e r e n c e d directly by i d e n t i f i e r s . Instead, generation of a dynamic variable introduces a ~oi~_E~ Value (which is nothing other than the storage address of t h e newly allocated variable). Hence, a pointer t y m e P c o n s i s t s of an unbounded s e t of v a l u e s p o i n t i n g to elements of a given type T. P is t h e n s a i d to be b o u n d to T . T h e v a l u e n i l is a l w a y s an element o f P a n d p o i n t s t o no e l e m e n t at a l l , tvo~



If p is reference

a to

a

= ~

pointer variable


identifier>;

variable of type

bound T, and

to a type pf denotes

T. then p is that variable.

a

Pointers are a simple tool for the construction of complicated and flexible data structures. If the type T is a record structure that contains one or more fields of type fT. then structures equivalent to arbitrary f i n i t e g r a p h s m a y be b u i l t . where the T's represent the nodes, and the pointers are the edges. As an example, consider the construction of a "data bank""for a given ~roup of people. Assume the persons are represented by records as defined in chapter 7. One may then form a chain or linked list of such records by adding a field of a pointer type as shown below.

~

link person

= fperson; = ~ecord . . ,

next

: link:

~nd:

A

linked

list

of

n persons

can

be

represented

as

in

figure

10.a.

63

first Figure

A variable element of

10.a

Linked

list

of type llnk. the list. The link

called of t h e

If we assume that the file numbers, then the following construct the above chain. Va~, f i r s t ,

first for_

p : link;

i:

"first" points te t h e last person is n i l .

"input" cede

contains n social could have been

first

security used to

integer;

:= n i l : i

:=

I ~o

n do

be~zln mead(s); new(p); Pt .next := f i r s t ; pt .ss := s ; first := p a~

For purposes of access, ene intreduces anether variable, say pt. of type l i n k a n d a l l o w s it t o m o v e f r e e l y t h r o u g h t h e l i s t . To demenstrate selection, assume there is a person with social security number equal to n and access this person. Th e s t r a t e g y is te advance pt via link until the desired member is located: pt := ~j~le

first; ptt.ss

<>

n do

pt

:= p t t . n e x t

In words this s a y s , "'Let pt p o i n t t o t h e f i r s t e l e m e n t . While the social security number of the member pointed to (referenced) by pt d o e s n e t e q u a l n, a d v a n c e pt to t h e v a r i a b l e indicated by the link (also a pointer variable) of t h e r e c o r d w h i c h pt currently references." N o t e in p a s s i n g that f i r s t t .nextt .next accesses the third person. Note that this simple search statement w o r k s o n l y , if o n e is s u r e t h a t t h e r e is at l e a s t o n e p e r s o n w i t h s e c u r i t y n u m b e r n on the list. But is this realistic? A check against failing te recognize t h e e n d of t h e l l s t is t h e r e f o r e mandatory. One might first try the following solution: pt := f i r s t ; wh~l& (pt <> But recall referenced

nil)

section in the

and

(ptt .ss

4.A. second

<>

If pt factor

n) d O

pt

:= p t t .next

= nil, the variable ptt , of t h e t e r m i n a t i o n condition,

64

does which

(1)

not ~ x i s t at a l l . T h e f o l l o w i n g treat this situation correctly: pt

:= f i r s t ;

~hile ~[ (2)

b

two

possible

solutions

:= t r u e ;

(pt <> nil) P t t .ss

are

and

b ~

= n &h#~3 b

pt := f i r s t ; ~ h i l e pt <> ~ i l d ~ b ~ if p t f .ss = n ~ pt := pt~ .next

:= f a l s e

goto

else

pt

:= p t t .next

13;

end To p o s e a n o t h e r problem, say one wishes to add the sample to the bank. First a s p a c e i n memory m u s t be a l l o c a t e d , reference created by m e a n s o f t h e s t a n d a r d procedure Dew. new ( p )

allocates a new variable v and assigns the pointer reference of v to the pointer variable p. If t h e t y p e of v is e record type with variants, then new(p) allocates enough storage to accommodate all variants. The form

n e w ( p ot 1 . . . . .

Warning: of new, program allowed;

person and a

tn)

can be used to a l l o c a t e a variable of the appropriate size for the variant with tag field values equal to the constants t 1...tn. The tag field values must be listed contiguously a n d in t h e order of their declaration. Any trailing t a g f i e l d s m a y be o m i t t e d . T h i s ~W~,~& ~ o t imply assignment to the tag fields.

if a record variable pt is c r e a t e d by t h e s e c o n d f o r m then this variable m u s t not c h a n g e its v a r i a n t during execution. Assignment to the entire variable is n o t however one can assign to the components of pf.

I f n e w p is a v a r i a b l e o f t y p e l i n k (as d e f i n e d a b o v e ) , new(newp) creates a new variable of t y p e p e r s o n e n d a s s i g n s the reference to newp. The value of the new variable is u n d e f i n e d upon allocation. A c c e s s is v i a t h e p o i n t e r . examples: newp~ . s s

:=

newpf

paul

:=

The procedure either the form d i s p o s e (p) or"

845680539

assignes assigns

~isoose

is

the

a social the

record

"inverse"

security paul

of

i1~

number to

newpf

and

may h a v e

65

d l s p o s e ( p ,t I .... in) to which form of new was u s e d to c r e a t e the v a r i a b l e to by p. D i s p o s e t h e n " e r a s e s " t h e v a r i a b l e r e f e r e n c e d

relative pointed by p.

In t h e next step the new m e m b e r , r e f e r e n c e d by t h e p o i n t e r n e w p , must be i n s e r t e d after t h e m e m b e r r e f e r e n c e d by pt. See f i g u r e 10.b.

newp

Figure

Insertion

is a s i m p l e

newpt .next pt~

Figure

.next

10.e

::

10.b

matter

Before

of

changing

:= p i t . n e x t : newp

illustrates

the

result.

newp ~ I p n e e w #

Figure

I0.c

After

the

pointers:

66

Deletion of accomplished ptf .next

the member in t h e s i n g l e

following the instruction;

auxiliary

pointer

pt

is

:= p t t . n e x t t . n e x t

It is often p r a c t i c a l t o p r o c e s s a list u s i n g 2 p o i n t e r s - - o n e following t h e o t h e r . In t h e c a s e of d e l e t i o n , it is t h e n l i k e l y that one pointer--say p l - - p r e c e d e s t h e m e m b e r to be d e l e t e o , a n d p2 p o i n t s t o t h a t m e m b e r . D e l e t i o n can t h e n be e x p r e s s e d in t h e single instruction:

p It ,next

: = p2t . n e x t

One is, h o w e v e r , w a r n e d t h a t d e l e t i o n s in t h i s most installations, r e s u l t in t h e l o s s of u s a b l e p o s s i b l e r e m e d y is t h e f o l l o w i n g :

manner (free)

w i l l . in store. A

p l t . n e x t := p 2 t . n e x t ; d i s p o s e (p 2 ) This provides the implementor with the opportunity to m a r k t h e store defining the variable referenced by p 2 as f r e e . What a c t u a l l y is done may g r e a t l y v a r y a m o n g i n s t a l l a t i o n s ; elaborate "garbage collections" may be implemented, or the dispose

instruction

may s i m p l y

be i g n o r e d .

Li_~ ~liQcation is the most efficient representation for inserting and deleting e l e m e n t s . A r r a y s r e q u i r e s h i f t i n g down (up) of e v e r y e l e m e n t b e l o w t h e i n d e x in t h e c a s e of i n s e r t i o n (deletion). and files require complete rewriting. For an example involving a tree l i s t . r e f e r to c h a p t e r 11 ( p r o g r a m

A word

to

the

structure 11.5).

instead

of

a linear

wise

Pascal provides a w i d e v a r i e t y of date s t r u c t u r e s . It is left to the programmer to e v a l u a t e his p r o b l e m in d e t a i l s u f f i c i e n t to d e t e r m i n e t h e s t r u c t u r e b e s t s u i t e d to e x p r e s s t h e s i t u a t i o n and to evaluate the algorithm. As indicated by t h e "data b a n k " example, linked allocation is e s p e c i a l l y n i c e for i n s e r t i o n a n d d e l e t i o n . If. h o w e v e r , t h e s e o p e r a t i o n s happen infrequently, but instead efficient a c c e s s is m a n d a t o r y , t h e n t h e r e p r e s e n t a t i o n of t h e data as an a r r a y of r e c o r d s is u s u a l l y m o r e a p p r o p r i a t e .

PROCE~RUS

11 AND f U N C T I O N S

As one grows in the art of computer programming, one constructs programs in a sequence of refinement ~tes&o At each step the Programmer breaks his task into a number of subtaskso thereby defining a number of partial programs. Although it is possible to camouflage this structure, this is undesirable. The concegt of the ~K~_~edure (or &wbroutine) allows the display o£ the subtasks as explicit subprograms.

A,

Procedures

The &roce~ur~ ~eclaration serves to d e f i n e a p r o g r a m p a r t a n d to associate it w i t h an i d e n t i f i e r , s o t h a t it c a n b e a c t i v a t e d by a orocedu~A statement. The declaration h a s t h e s a m e f o r m as a program, e x c e p t it is i n t r o d u c e d by a & r _ ~ z e d u r e b e a d i D _ q i n s t e a d of a p r o g r a m heading. Recall the program part that found the minimum and maximum values in a list of integers. As an extension, say that increments of j 1 . . j n a r e a d d e d t o a[ I] . . . a [ n ] , t h e n m i n a n d m a x are again computed. The resulting program, which employs a Procedure to d e t e r m i n e min and max, follows.

68

{ program 11.1 extend program program

6.1

}

minmax2(input.output);

c o n s t n = 20; vat a : ~rrav[1..n] ~j[ i n t e g e r ; i,j : i n t e g e r ; ~rocedure minmax; va~ i :1..n; u,v,min.max :integer; ~e~in rain := a[ I] : m a x := rain; i := 2; wh i ] . ~ iv ~h~n b~aio i f u > m a x th~d3 m a x := u ; i f v < m i n t h e n rain := v e~l~ @ l s e h ~ ~ v > m a X t/3F,/1 m a x := v; ~C[ u < m i n tll@Xl rain := u and : i :: i + 2 and ; i f i=n theo i f a [ n ] >max t h e n m a x := a [ n ] e l s e i f a[n]
{read

for

i

array}

:=

I ~a

n

do

be~i__~n r e a d ( a [ i ] )" w r i t e ( a [ i ] :3) e~u3 : writeln ; minmax , f o r i := I t o n do bw_~in r e a d ( j ) ; a [ i ] := a[i] +j" write(a[i] eo_~ : wrlteln ; minmax an~.

-1

-3

44 40

Although I.

4 -6

7

7 15 -7

8 54 23 - 5 79

3

9

9

9 -6

45

79

79

3

1

1

9 88

7 43

12

17 - 7

48

59

39

9

?

7 12

simple, The

:3)

15 - 4 88

this

simplest ~rocedure

program

form

illustrates

of the

PROCEDURE

:

many

points:

HEADING,

namely:

5

69

2.

LOCAL VARIABLES. Local variables i, u, v , m i n , only within the scope variables h a v e no e f f e c t o£ m i n m a x .

to procedure minmax are the a n d m a x . T h e s e m a y be r e f e r e n c e d of minmax; assignments to these on t h e p r o g r a m outside the scope

3.

GLOBAL VARIABLES. Global variables are a, i. and j: They may b e r e f e r e n c e d throughout the program. (e.g. The first assignment in m i n m a x is m i n := a [ i ] .)

4.

NAME PRECEDENCE. Note that i is the name for both a global and a local variable. These a r e not t h e s a m e variable! A procedure may reference any variable global to it, or it may choose to redefine t h e n a m e . If a variable n a m e is r e d e f i n e d , the new name/tyoe association is then valid f o r t h e s c o p e of t h e d e f i n i n g procedure, and the global variable of t h a t n a m e ( u n l e s s p a s s e d as a parameter) is no longer available within the procedure scope. Assignment to t h e l o c a l i ( e . g . i := i + 2 ) h a s no effect upon t h e g l o b a l i; a n d s i n c e i d e n o t e s the local variable', the global variable i is effectively inaccessible. It is a good programming practice to declare every identifier w h i c h is n o t r e f e r e n c e d outside the procedure. as s t r i c l y l o c a l to t h a t p r o c e d u r e . N o t o n l y is t h i s g o o d documentation, but it a l s o p r o v i d e s added security. For example, i c o u l d h a v e b e e n l e f t as a g l o b a l v a r i a b l e ; but then a later extension to the program which called procedure mlnmax within a loop controlled by i w o u l d cause incorrect computation.

5.

The PROCEDURE "minmax" in the

STATEMENT. In this example main program activates the

the statement, procedure.

Examining the last example in m o r e d e t a i l , o n e n o t e s t h a t m i n m a x is called twice. By formulating the program part as a procedure--i.e, by not explicitly writing this program part twice--the programmer conserves not o n l y his t y p i n g t i m e , b u t also space in m e m o r y . T h e s t a t i c c o d e is s t o r e d only once, and space defining local variables is activated only during the execution of the procedure.

One should not hesitate, however, from formulating an action as a procedure--even when called only once--if doing so enhances the readability. Defining development s t e p s as p r o c e d u r e s makes a more communicable and verifiable program. Often necessary with the decomposition of a problem into subroutines is the introduction of n e w v a r i a b l e s to represent the arguments and the results of t h e s u b r o u t i n e s . The purpose of such variables s h o u l d be c l e a r f r o m t h e p r o g r a m text. The following program extends m i n i m u m a n d m a x i m u m v a l u e of an

the above example to compute the a r r a y in a m o r e g e n e r a l sense.

70

{ program 11.2 extend program groqram

11.1

}

minmax3(input

,output)

const n = 20; & y o e l i s t = aF_~av[ 1..n] yam a . b : list ; i.j,minl,min2,maxl.max2

of

;

integer; :

integer;

~ocedure m i n m a x ( v _ a r g : l i s t ; liar j,k : i n t e g e r ) ; v a ~ i : 1..n; u ,v : i n t e g e r ; beain j := g[ I] ; k := j; i := 2; ~hile i v ~_hen beajd3 if u > k J~h#O k := u ; if v < j J 2 ~ J := V end e ~ ~.e_q.~.n i f v > k ..i~hen k := v ; .i.f" u < j & h e n j : = u .~ o.#. ;

i := i + 2 end : if i =n ~b#d& i f g [ n ] > k t h e n k := g[n] e ~ & e if g[n]
-1

-3

4 -6

?

U 54

23 79

-5

45

43

3 ~ -U 2

I 34

-8 45 34

4.4 40

2 15 -7

9 U8

15 - 4 ~d8

I

3

9 8b

9

9 -6

4

34 53

3

7 43 9.5

12

45

79

79

3

1

1

5

d -I

3 -2

-4

6

6

6

7

17 - 7

4U 72

75

9

'7

7

12

71

In p r o g r a m heading:

11.2

procedure

The formal parameter declaration procedure.

one

encounters



(

the

second

form

of

the

procedure

{ ; }

parameter section lists followed by its type. part, which introduces

);

the n a m e of each f o r m a l It is followed by the the o b j e c t s l o c a l to t h e

The labels in the label definition part and all identifiers introduced in t h e f o r m a l p a r a m e t e r part, the constant definition part, the type definition part, the variable, procedure, or function declaration parts are i~cal to the procedure declaration w h i c h is c a l l e d t h e ~ c o o e of t h e s e o b j e c t s . T h e y a r e not known o u t s i d e t h e i r s c o p e . In t h e c a s e of l o c a l v a r i a b l e s , their values are undefined at t h e b e g i n n i n g of t h e s t a t e m e n t part. Parameters provide a substitution mechanism that allows a process to be r e p e a t e d w i t h a v a r i a t i o n of its a r g u m e n t s . (e.g. minmex is called t w i c e t o s c a n a r r a y a a n d o n c e to s c a n a r r a y b.) One n o t e s a correspondence between the procedure heeding and the procedure statement. The latter contains a list of ~ctu~J, &arame~@~r_&, which are s u b s t i t u t e d for t h e c o r r e s p o n d i n g ~ormal ~arameters that are d e f i n e d in t h e p r o c e d u r e declaration. The correspondence is established by the positioning of the parameters in t h e l i s t s of a c t u a l and f o r m a l p a r a m e t e r s . There exist four kinds of parameters: so-called value parameters, variable parameters, procedure parameters (the a c t u a l p a r a m e t e r is a procedure identifier), and function parameters (the a c t u a l parameter is a f u n c t i o n i d e n t i f i e r ) . Program 11.2 shows the case of the E#j~bl~ ~arameter. The actual parcmeter ~ust ~ ~ E~riable; the corresponding formal parameter m u s t be p r e c e d e d by t h e s y m b o l E#32 a n d r e p r e s e n t s this actual variable during t h e e n t i r e e x e c u t i o n of t h e p r o c e d u r e . Furthermore, if x 1 . . x n e r e t h e a c t u a l v a r i a b l e s that correspond to the formal variable parameters v l . . v n , t h e n x l..xn s h o u l d be ~latincj~ v a r i a b l e s . All address calculations a r e d o n e at t h e t i m e of t h e c a l l , H e n c e . if a v a r i a b l e is a c o m p o n e n t o f an a r r a y , expression is e v a l u a t e d w h e n t h e p r o c e d u r e is c a l l e d . To d e s c r i b e arrow for Oarameter parameter. performed parameter with j and When

procedure its i n d e x

the memory allocation pictorially, o n e c o u l d d r a w en each variable parameter f r o m t h e n a m e of t h e f o r m a l to the memory l o c a t i o n of t h e c o r r e s p o n d i n g actual Any operation involving the formal parameter is t h e n directly upon the actual parameter. Whenever the represents a ~ e s u l t of t h e p r o c e d u r e - - a s is t h e c a s e k a b o v e - - i t m u s t be d e f i n e d as a v a r i a b l e p a r a m e t e r .

no s y m b o l

heads

the

parameter

section,

the

parameter(s)

of

72

t h i s s e c t i o n a r e s a i d to be M~llw_~ ~i@J~eJr~J&~/~(s). In t h i s case t h e actual parameter Wjistha an ~KD/~/~JJ3 (of w h i c h a v a r i a b l e is a simple case). The corresponding formal parameter represents a local variable in the called procedure. As its i n i t i a l v a l u e , this variable receives t h e c u r r e n t v a l u e of t h e c o r r e s p o n d i n g actual parameter (i.e. t h e v a l u e of t h e e x p r e s s i o n at the t i m e of the procedure call). The procedure may t h e n c h a n g e t h e v a l u e of this variable by assigning to it; t h i s c a n n o t , h o w e v e r , affect the value of the actual parameter. Hence. a value parameter can never represent a r e s u l t of a c o m p u t a t i o n , The difference in is shown in program

{ program procedure ~roora~

the effects 11.3.

11.3 Parameters

parameters

of

value

and

variable

parameters

}

(output);

va~ a,b : integer; ~roced~r_e h(x : integer; ~/~ beqin x := x + 1 ; y := y+1; writeln(x ,y) end ; ~e~in e := 0; b := 0; h(a.b); writeln(a,b) e~u~,

1

1

0

1

y : integer);

In program 11.2 none of the values in array g are altered; i.e. g is not a result. Consequently g could have been defined as a value parameter without affecting the end result. To understand why this was not done. it is helpful to look at the implementation. A procedure c a l l a l l o c a t e s a n e w a r e a for e a c h v a l u e p a r a m e t e r ; this represents the local v a r i a b l e . T h e c u r r e n t v a l u e of t h e actual parameter is " c o p i e d " i n t o this l o c a t i o n ; exit from t h e procedure simply releases this storage. If a parameter is not used to transfer a result of the procedure, a value parameter is generally preferred. The referencing is then quicker, and one is protected against mistakenly altering the data. However in the case where a parameter is of a structured type (e.g. an array), one should be cautious, for the copying operation is relatively expensive, and the amount of storage needed to allocate t h e c o p y may b e l a r g e . Because referencing of each element in the array occurs only once, it is desirable to define the parameter as a variable parameter.

73

One

may

change

the d i m e n s i o n

of the

array

simply

by r e d e f i n i n g

n. To make the program applicable for an array of reals, needs only to change the type and variable definitions: statements are not dependent upon integer data.

one the

The use of the procedure identifier w i t h i n the text of the procedure itself implies Cenur@_~ve e x e c u t i o n of the p r o c e d u r e . Problems whose definition is n a t u r a l l y r e c u r s i v e , often lend themselves to r e c u r s i v e s o l u t i o n s . An e x a m p l e is the f o l l o w i n g p r o g r a m . G i v e n as data ere the s y m b o l i c e x p r e s s i o n s : (a+b) ~ (c-d) a +b *c -d

( a + b)* a+b* (c-d) b+c*(d+c*a*a

which period

c-d

)*b+a

ere formed according t e r m i n a t e s the i n p u t .

to

the

expression

term J

factor I if actor

factor

identifier

d v I letter I Figure

11.a

Expressions

syntax

of

figure

11.a.

A

74

The task is to c o n s t r u c t a p r o g r a m to c o n v e r t t h e e x p r e s s i o n s into postfix form (Polish notation). This is done by constructing an individual conversion ~rocedure for each syntactic construct (expression, term, factor). As these syntactic constructs are defined recursively, their corresponding procedures may activate themselves recursively,

75

{

program 11.4 conversion to

mrooram ~ar

ch

postfix

form

}

postfix(input,output); : char ;

orocedure find; beoin ~eoent read(oh) until ( c h < > ° °) a n d end ; orooedure vat op

expression : char:

or_~ure

t erm :

~ot

eoln(input)

:

oroce,~ure factor, beoiQ i f o h = °(* t h e n beoin find; expression; end #lse write(oh) : find end: {factor}

~nd;

factor while benin end {term}

: oh= °*° find:

do factor:

beoin term ; ~hile (ch='+')oz(ch =°-') beqin op : = c h ; f i n d : end ~.d331: { e x p r e s s i o n }

{oh

=

) }

write('*')

do term:

write(op)

beoin find ; ~eoeat write(" "); expression ; writeln until oh='."

end. a b +cd - * abc~+dab+c*dabcd -*+ a a ~a {a ~

bcdca*a

~+*b*+a +

The binary r eeursive consists consists

JEr e e i s a d a t a s t r u c t u r e that is naturally defined in terms and processed by recursive algorithms. It of a finite set of nodes t h a t is e i t h e r e m p t y or of a n o d e (the r o o t ) w i t h t w o d i s j o i n t binary trees.

76

c a l l e d t h e left and r i g h t s u b t r e e s (6). R e c u r s i v e p r o c e d u r e s for generating and traversing binary trees naturally reflect this m o d e of d e f i n i t i o n . Program 11.5 builds a binary t r e e and t r a v e r s e s it in p r e - , posta n d e n d o r d e r . The t r e e is s p e c i f i e d in p r e o r d e r , i.e. by listing t h e n o d e s ( s i n g l e l e t t e r s in this c a s e ) s t a r t i n g at t h e root a n d f o l l o w i n g first t h e left and t h e n t h e r i g h t s u b t r e e s so t h a t t h e i n p u t c o r r e s p o n d i n g to f i g u r e 11.b is:

abc..deo,fg...hi..jkt..m..n.. where

a point

signifies

Figure

an e m p t y

11.b

subtree.

Binary

tree s t r u c t u r e

77 { program 11.5 binary tree traversal

}

~ro0ra~ traversal(input.output): tvae ptr = tnode; node = ~ecord info : char: llink.rlink : ptr ~nd; ~ar root : ptr; ch : char; ~roced~ preorder(p : ptr); ~eoin if p<>~3il then ~e~iE write (p~ .info ) ; preorder(pf .llink): preorder(pt .rlink) e_~:

{preorder}

~ro_~~ postorder(p : ptr); beoin if p<>~il then ~eoin postorder(p~ ,llink): write(pt .info); postorder(p~ ~ l i n k ) and

~d3~; {postorder} orocedw~ endorder(p : ptr): ~eoin i f p <>13~Ll J~hen be~in endorder (p~ ,llink); endorder(p~ .rlink); write(p~ .info) and

~nd:

{ endorder }

~ r o e e d u r ~ enter(yam p :ptr); beoin read(oh); write(oh) : if

_~P.O. new(p);

ch<>'.'

be~i~

p? .info := ch; e n t e r ( p t .llink); e n t e r (pt .rlink) : end ~dL~a p

:= n i l

end ;{ enter} beain write(" write(" write(" write("

"); "); ") : ");

enter(root); writeln : preorder(root); writeln; postorder(root); writeln; endorder (root); writeln

en__d.

abc.ode..fg...hi.,jkl..m..n.. abodefghijklmn cbedgfaihlkmjn cegfdbilmknjha

78

The reader is cautioned against applying reeursive techniques indiscriminately. Although appearing "clever". they do not always produce the most efficient solutions.

If a procedure P calls a procedure Q a n d Q a l s o c a l l s P. t h e n either P or Q m u s t be " p r e - e n n o u n c e d " by a ~ o r w a r d ~eclaratioo (section 11.C).

The &&andard ~rgcedures in A p p e n d i x A are predeclared in e v e r y implementation of Pascal. Any implementation may feature additional predeclared procedures. Since they are, as all standard objects, a s s u m e d t o be d e c l a r e d in a scope surrounding the user program, no conflict arises from a declaration redefining the same identifier within the program. The standard procedures get, put, read, write, reset, and rewrite were introduced in c h a p t e r 9. R e e d a n d w r i t e a r e f u r t h e r discussed in chapter 12.

B.

Functions

Functions are program parts (in t h e s a m e s e n s e as p r o c e d u r e s ) which compute a single scalar or p o i n t e r v a l u e f o r u s e in t h e evaluation o£ an e x p r e s s i o n . A ~wnction ~ . s ~ specifies the activation of a function and consists of the identifier designating the function a n d a l i s t of a c t u a l p a r a m e t e r s . The parameters are variables, exoressions, procedures, or f u n c t i o n s end are substituted for t h e c o r r e s p o n d i n g formal parameters. The the

function exception

L~Oct~gJ3 -orL W ~

declaration has the same form of t h e _f_wj3c_t~DJ& ~ e a d i n o which

:
type>

as t h e p r o g r a m , has the form:

with

:

( })

section> :
type>

;

As in t h e c a s e of p r o c e d u r e s , t h e l a b e l s in t h e l a b e l d e f i n i t i o n part and all identifiers introduced in t h e f o r m a l p a r a m e t e r part. the constant definition part. the type definition part, the variable, procedure, or f u n c t i o n declaration parts are i~cal to the function declaration, w h i c h is c a l l e d t h e ~ of t h e s e objects. They a r e not k n o w n o u t s i d e their scope. T h e v a l u e s of local variables are undefined at t h e b e g i n n l n B of the statement pert. The identifier specified in the function heading names the function. T h e r e s u l t t y p e m u s t be a s c a l a r , subrange, or p o i n t e r type. Within the function declaration t h e r e m u s t be an e x e c u t e d assignment (of t h e r e s u l t t y p e ) to t h e f u n c t i o n identifier. This assignment "returns" the result of t h e f u n c t i o n .

79

The examples to date have only dealt with variable and value parameters. Also possible are procedure and function parameters. Both must be introduced by a special symbol: the symbol ~rocedun~ signals a farmal procedure parameter: the symbol function, a formal function parameter. The following program finds a zero of a function by bisection: the function is specified at t h e t i m e of t h e c e l l .

{ program 11.6 find zero of a

Pro~ra~

bisect

function

(input,

by

bisection

}

output):

const eps =1a-14; v__ar_ x ,y :reaI; function zero(J~unction f': real; v a t x ,z : r e e l ; e :boalean; b#oin s := f(a)<0; ~eoeat

z if

x :=

:=

a,b:

real):

real:

(a+b)/2.0;

f(x):

(z
then

a

:: x w l s e

b

:= x

WJ3~Jil a b s ( a - b ) < e p s ; zero :--- X e a ~ : {zero} beqin

{main}

read(x,y); r e a d ( x ,y ) ; ~n~.

writeln(x.y,zero(sin,x ,y)); writeln(x ,y ,z e r o ( c o s , x , y ) )

-l.000O00000000e+00 1.00O000000000e+00

1.00O000000000e+00 2.000000000000e+00

-7.105427357601e-15 1.570796326795e+00

An assignment (occurring in a function declaration) to a non-local variable or taa variable parameter is called a ~i9_~ #f£ec~. Such occurrences often disguise the intent of the program and greatly complicate the task of verification. (Some implementations may e v e n a t t e m p t to forbid side effects.) Hence, the use of functions producing side effects is strongly discouraged. As an

example,

consider

program

11.7.

80

{ program 11.7 test side effect mro~ram

sideffect

} (output);

v~ a,z : integer : ~wnction sneaky(x : integer): integer; ~e~in z := z - x ; { s i d e e f f e c t on z} sneaky := s q r ( x ) end; j~e~in z := 10; a := s n e a k y ( z ) ; writeln(a,z); z := I0; a := s n e a k y ( t 0 ) * sneaky(z); z := 10; a := s n e a k y ( z ) * sneaky(t0); end.

100 0 10000

writeln(aoZ), writeln(a,z)

0 0 -10

The next example formulates the exponentiation program 4 . 6 as a f u n c t i o n declaration.

a3gorithm

of

81

{ p r o g r a m 11.8 extent p r o g r a m proora~

4.8

}

expon2(output);

~AE pi,spi:

real;

~unetioIl p o w e r ( x :real; z: r e a l ; benin z := I; while y>O do ,be.q i_a

y :integer):

w_1~i~ n o t b~_~n y

y

end ; := y-l:

:= y

z

div

2:

x

real;

{y>=O}

o dd ( y ) d o

:= s q r ( x )

:= x*z

end ;

power := ,@Pd: { p o w e r }

Z

_be~In pi := 3.14159; writeln(2.0o7,power(2.0,7)); spi := P o w e r ( p i , 2 ) : w r i t e l n (pi ,2.spi ); writeln(spi~,~2,power(spl,2)); w r i t e l n ( p i , 4 , p o w e r (pi ,4) )

2.000000000000e+O0 3.141590000000e+00 9.86958772U100e+O0 3.141590000000e+00

7 2 2 4

1.2UOOOOOOOOOOe+02 9.869587728100e+00 9.740876192266e+01 9.740876192266e+01

The appearance of the function identifier within the function itself implies ~eeursive function.

in an e x p r e s s i o n e x e c u t i o n of the

82

{ p r o g r a m 11.9 recursive formulation

of g c d }

_Pro@ram r e c u r s i v e g c d ( o u t p u t ) ; Mar x,y,n

: integer;

~unction gcd(m,n: integer):integer; h eoin i f n=0 ~b~J3 god := m ~ I s e god := g c d ( n , m m o d n) end; {gcd}

&~#.~J~LwZ.~ t r y ( a , b :integer): b~J3 w r i t e l n (a ,b ,god (e ,b ) ) end ; beoin

try(18,27); try(312,2142); t r y (61 ,53); try (98,868)

end.

18 312

27

9

2142

61 98

53 868

6 1 14

F u n c t i o n c a l l s may o c c u r b e f o r e is a ~ D J 2 j ~ ~eference (section

The & J ~ predeclared implementation

~unction8 in every may feature

the function 11.C).

definition

if t h e r e

of

Appendix A ere assumed to be implementation of Pascal. Any additional predeclared functions.

C. Remarks I. P r o c e d u r e (function) calls may o c c u r b e f o r e the p r o c e d u r e (function) definition if t h e r e is a ~9J&_~J&~ ~ e f e r e n c e . The form is as follows: (Notice that t h e p a r a m e t e r list and eventual result type are written ~l~llv in the forward reference.)

83 p r o g d#_~L~ Q (x : T ) ; ~ro#edure P (y : T ) ,

forward;

Q(a)

~nd : &~ocedure Q; ~e~in P(b)

{parameters

are

not

repeated}

end : b~zj~ P(a): Q(b) end.

2.

Procedures and functions which are used other procedures and functions must have only. (Consequently. it is not necessary to whether a parameter is called by value or by

as parameters to value 9arameters test at run time address.)

3.

A component of a packed structure must not appear as an actual variable parameter. (Consequently. there is no need to pass addresses of partwords, and to test at run time £or the internal representation of the actual variable.)

12 ~N PU T _AND # U T P U T

The problem of communication between man and c o m p u t e r was a l r e a d y m e n t i o n e d in c h a p t e r 9. B o t h l e a r n to w ~ e r s t a n d through what is t e r m e d p a t t e r n [ e c 0 ~ n i t ~ . Unfortunately, the patterns recognized most e a s i l y by men ( d o m i n a n t l y t h o s e of p i c t u r e and sound) are very d i f f e r e n t from t h o s e a c c e p t a b l e t o a c o m p u t e r (electrical impulses). In fact, the expense of physically t r a n s m i t t i n g d a t a - - i m p l y i n g a t r a n s l a t i o n of p a t t e r n s l e g i b l e to man into o n e s l e g i b l e to a c o m p u t e r , a n d v i c e v e r s a - - c a n be as Costly as the processing of the transmitted information. (Consequently, much r e s e a r c h is d e v o t e d t o m i n i m i z i n g the cost by "automatizing" or "'automating" m o r e of t h e t r a n s l a t i o n process.) T h i s t a s k of c o m m u n i c a t i o n is c a l l e d i n p u t a n d o u t p u t h a n d l ~ n g (I /O). The h u m a n can s u b m i t his i n f o r m a t i o n v i a ~ O o u ~ ~ e v i c e & (e.g. key punches, card readers, paper tapes, magnetic tapes, terminals) and r e c e i v e his r e s u l t s v i a ~ A W ~ d e v i c e s (e,g. l l n e p r i n t e r s , card and paper tape punches, terminals, visual display units), Common to b o t h - - a n d d e f i n e d by each i n d i v i d u a l i n s t a l l a t i o n - - I s a set of legible characters (chapter 2). It is over t h i s character set that Pascal defines the two standard textfile variables (program parameters) ~OD~t and ~ (also see c h a p t e r 9). Textfiles may be a c c e s s e d t h r o u g h the s t a n d a r d f i l e p r o c e d u r e s get and put. This can, of c o u r s e be q u i t e c u m b e r s o m e as t h e s e procedures are defined for single character"manipulation. To illustrate, consider one has a natural number s t o r e d in a variable x a n d w i s h e s to p r i n t it on t h e file o u t p u t . Note that t h e p a t t e r n of c h a r a c t e r s d e n o t i n g t h e d e c i m a l r e p r e s e n t a t i o n of the value w i l l be q u i t e d i f f e r e n t from t h a t d e n o t i n g the v a l u e written as a Roman numeral (see p r o o r a m 4 , 7 ) ° g u t as one is usually interested in d e c i m a l n o t a t i o n , it a p p e a r s s e n s i b l e to offer built-in standard transformation procedures that translate a b s t r a c t n u m b e r s (from w h a t e v e r c o m p u t e r - i n t e r n a l representation is u s e d ) i n t o s e q u e n c e s of d e c i m a l d i g i t s a n d v i c e v e r s a . The two standard procedures to facilitate the analysis syntax for calling these can be used with a variable not fixed.

A.

The procedure

Let real,

I,

~ead and ~rite are thereby extended and the formation of textfiles. The procedures is non-standard, for they number of parameters whose types are

read

v l.v2 . . . . . vn denote variables and let f denote a textfile.

read(v I ..... vn) stands r e a d (input,v I . . . . . vn)

for

of

type

char,

integer,

or

85

2.

read(f ,v 1 . . . . . vn) be~in r e a d ( f ,v l ) ;

3.

readln(vl ..... readln(inPut.v

4.

readln(f,v be~in

.,.

vn) 1 .....

stands for ; read(f,vn) stands vn)

for

I ..... vn) stands for r e a d ( f ,v l ) ; ... ; r e a d ( f . v n ) :

The effect is that after vn the remainder of the current values of v l...vn may s t r e t c h 5.

end

If ch is read(f,ch)

a variable stands

begin

oh

of type for

:= ff ; g e t (f)

readln(f)

eild

is read (from the textfile f), line is skipped, (However ° the over several lines .)

char.

then

~nd

If a parameter v is o f t y p e i n t e g e r (or m s u b r e n g e thereof) or real° a sequence of characters, which represents an integer or a r e e l n u m b e r a c c o r d i n g to the Pascal syntax, is read. (Consecutive numbers m u s t be s e p a r a t e d by b l a n k s or e n d s of l i n e s . )

examples:

Read and process a sequence of numbers where the last value is immediately followed by an asterisk. Assume f to be a textfile, x and ch to be variables of types integer (or real) and char respectively. reset (f) ; reoe~& read(f,x,ch): P(x) ~t~_~l ch :°*"

Perhaps knowing special schemata

a more common situation is w h e n o n e has no w a y o f how m a n y d a t a i t e m s a r e to be r e a d ° a n d t h e r e is no symbol that terminates the list. Two convenient f o l l o w . In t h e f i r s t , s i n g l e i t e m s a r e p r o c e s s e d .

reset(f); read(f,x): ~ n o t e o f ( f ) do .b~:~d.a P ( x ) : r e a d ( f ,x ) ~nd

Notice become The

that true,

second

the does schema

last call not return processes

of read, a defined n-tuples

which value of

causes for x.

numbers :

eof(f)

to

86

reset(f);

~la

read(f

not

beojJ3

eof(f)

,xl);

do

read(f ,x2 ..... P(xl ..... xn); read(f ,xl)

xn);

~nd (For the of s i n g l e

B.

above items

The p r o c e d u r e

schema to function m u s t be a m u l t i p l e

properly, of n.)

the

total

number

write

The procedure write appends character strings (one or m o r e characters) to a textfile. L e t pl p 2 . . . . . p n be p a r a m e t e r s of the form defined below (see 5), e n d let f be a t e x t f i l e . Then, when writing o n t o t h e f i l e f:

1.

w r i t e (p 1 . . . . . pn) stands for write (output,p 1 ..... pn)

2.

wrtte(f,p be~i~

1 ..... pn) write(f,pl):

3.

wrlteln(p I ..... writeln(output,p

4.

writeln b~in

Every

e e e where

stands for ; write(f.pn)

the the

parameter

: el : el

e,

e~

pn) stands for I ..... pn)

(f.p 1 ..... pn) write(f ,pl): ...

This has terminating 5.

...

stands for ; write(f,pn):

effect current

of writing l i n e of t h e

pi

be

must

of

one

of

writeln(f)

pl . . . . . textfile f. the

eo~ pn

end

then

forms:

: e2

el,

and

e2 a r e

expressions,

6.

e is the valw_E t o be written a n d may be o f t y o e integer, real, Boolean, o r i t may be a s t r i n g . In the case, write(f,c) stands for ff := c; p u t ( f )

char, first

7.

el--called the minimum field wldth--is an o p t i o n a l control. It m u s t be a n a t u r a l n u m b e r and indicates the minimum number of characters to be written. In g e n e r a l , t h e v a l u e e is written w i t h el c h a r a c t e r s (with preceding blanks). If el is "too small", m o r e s p a c e is a l l o c a t e d , ( R e a l s m u s t be w r i t t e n w i t h at l e a s t o n e p r e c e d i n g blank; however, this restriction does not apply to integer values.) I f no f i e l d l e n g t h is specified, a default value (implementation dependent) is assumed according to t h e t y p e of t h e e x p r e s s i o n e.

87

8.

e2--called the ~raction i~o_q~--is an o p t i o n a l control and is applicable only when e is of t y p e r e a l . It m u s t be a natural n u m b e r and s p e c i f i e s t h e n u m b e r of d i g i t s to f o l l o w t h e d e c i m a l p o i n t . (The n u m b e r is t h e n s a i d to be w r i t t e n in fi×ed-~oint notation.) If no f r a c t i o n l e n g t h is s p e c i f i e d , t h e v a l u e is p r i n t e d in d e c i m a l f l o a t i n g - p o i n t form.

I0. If the value identifier true

e or

is of type Booleen, f a l s e is w r i t t e n .

then

the

standard

13

~ASOAL &000-3.4

The p u r p o s e of t h i s c h a p t e r is to i n t r o d u c e t h o s e f e a t u r e s t h a t are peculiar to the implementation on t h e C o n t r o l D a t a 6 0 0 0 computers. The reader is w a r n e d t h a t r e l i a n c e u p o n a n y of t h e characteristics peculiar to PASCAL 6000-3.4 may r e n d e r his programs unacceptable to other implementations of P a s c a l . O n e is, therefore, advised to use only features described as ~tandard £ a s c a l in t h e p r e v i o u s chapters whenever possible, and certainly when writing "portable" programs. The topics A) B) C) D)

A.

of t h i s

chapter

fall

into

four

categories:

Extensions to the l a n g u a g e Specifications left u n d e f i n e d in t h e p r e c e d i n g chapters Restrictions Additional predefined procedures, functions, and types

Extensions

to

the

language

Pascal

This section defines non-standard language constructs available on the Pascal 6000-3,4 system. Although they may b e o r i e n t e d toward the particular environment provided by the given operating system, they a r e d e s c r i b e d a n d can be u n d e r s t o o d in machine independent terms.

A.1

Segmented

files

A file can be regarded as being subdivided into so-called ~e~ments, i.e. as a sequence of segments, e a c h of w h i c h is itself a s e q u e n c e . P A S C A L 6 0 0 0 - 3 . 4 o f f e r s e f a c i l i t y to d e c l a r e a file as b e i n g ~ e ~ m e n t L d , a n d to r e c o g n i z e s e g m e n t s a n d t h e i r boundaries. E a c h s e g m e n t of s u c h a file is a " ~ w e i c a l r e c o r d " in CDC S C O P E t e r m i n o l o g y . declaration:

an

example: ~vm~ T = Qeomented

The p r e d i c a t e

eas (x)

The

::= & e ~ m e n t e ~

following

~ile

Lile

~

~



char;

function returns the value positioned st the false.

two

standard

true when the file x is end of a segment, otherwise

procedures

are

introduced:

89

putseg(x)

m u s t be c a l l e d the file x has

getseg(x)

is called in o r d e r to i n i t i a t e t h e r e a d i n g of t h e next segment of the file x . It a s s i g n s to t h e buffer variable ~ the f i r s t c o m p o n e n t of t h a t next segment. If no next s e g m e n t e x i s t s , e o f ( x ) becomes true; if a next s e g m e n t e x i s t s but is empty, then eos(x) becomes true and xt is undefined. S u b s e q u e n t c a l l s of w e t ~ x ) w i l l e i t h e r step on t o t h e next c o m p o n e n t or, if it does not exist, cause eos(x) to b e c o m e t r u e .

Get(x) eof(x)

must always

when been

the generation completed, and

not be c a l l e d if e i t h e r i m p l i e s e o s ( x ).

eos(x)

of

a segment

or e o f ( f )

of

is t r u e :

The advantages of a segmented file lie in the possibility positioning the reading or w r i t i n g head (relatively) quickly any. segment in the file. For the purposes of reading (re)writing a segmented file, the standard procedures getseg rewrite are extended to accept two arguments.

of to and and

g e t s e g (x ,n )

initiates the reading of the nth segment c o u n t i n g f r o m t h e c u r r e n t p o s i t i o n of t h e f i l e , n>O implies counting s e g m e n t s in t h e f o r w a r d direction: n
rewrite(x ,n)

i n i t i a t e s t h e ( r e ) w r i t i n g of x at t h e b e g i n n i n g of the nth s e g m e n t c o u n t i n g f r o m the c u r r e n t position. N o t e : r e w r i t e ( x , 1 ) is n £ & e q u i v a l e n t to r e w r i t e ( x ) . The l a t t e r c a u s e s i n i t i a t i o n of ( r e ) w r i t i n g at t h e v e r y b e g i n n i n g of t h e e n t i r e file .

Since files are organized for sequentfal one should not expect getseg and rewrite n<=0 as they are for n>0.

to

(forward) processing, b e as e f f i c i e n t for

The following two program schemas, with the statements W, R, and S, show the operations of writing and reading of a segmented file.

Writing

a segmented

file x :

r e w r i t e (x); ~eat {generate a segment} ~eo~ {generate a component}

W(xt): un~2 p : putseg w~til q

(x)

put(x)

parametric sequential

90

Note: this segment.

Reading

schem~

will

a segmented

reset

(x):

~hila

not

abila

file

eof(x)

bm.oin { p r o c e s s

never

generate

an

empty

file

nor

an

empty

x :

m~ a segment}

Dot eos (x) d s

be_%i[1 { p r o c e s s

S ( x t );

a component}

set(x)

en~ : S: getseg(x) end The next example shows textfile f and copies the file output.

a procedure first n lines

that reads a segmented of e a c h s e g m e n t onto the

orocedure list ; 3&ar i.s : i n t e g e r ; h~iO s := 0; reset(f):

~ b _ i l a zl#-~ e o f ( f )

do

hf..qin s

i

:= s + 1 :

:=

O;

writeln( ° segment 'os); w__hil~ n o t e o s ( f ) a n d ( i < n ) berlin

i

:='i+I;

{copy

w_hi/J~ D o t e o l n ( f ) b,#oin write(ff ):

a

d~

line}

do get(f)

{next

character}

eOJ~ :

writeln; readln(f) {next eo_~ : getseg(f) {next segment}

line}

end

oo~ The s t a n d a r d procedures segmented textfiles.

A.2

External

~£~q.d and ~ r i t e

can a l s o

be a p p l i e d

to

procedures

PASCAL 6000-3.4 provides a facility to access oxt~oal oroeedures, i.e. procedures (functions) that exist outside the user program and have been separately compi3ed. This enables the Pascal programmer to a c c e s s p r o g r a m libraries. The declaration of such a procedure consists of a p r o c e d u r e heading followed by

the

word

"extern"

or

"Fortran".

91

B.

8.1

Specifications

The

left

program

undefined

heading

and

in

the

external

preceding

chapters

files

A PASCAL file variable is implemented as a f i l e in t h e C D C operating system. Local files are allocated on d i s c s t o r e or in the Extended Core Store (ECS). Storage is a l l o c a t e d when they are generated and automatically released when the block to which t h e y a r e l o c a l is t e r m i n a t e d . Files that exist outside the program ( i . e . b e f o r e or a f t e r program execution) m a y be m a d e a v a i l a b l e to t h e p r o g r a m if t h e y are specified as ~ c t u a I ~ a r a m e t e r s in t h e p r o g r a m ceil statement (EXECUTE) of the control card record. They are called external files and ere subsituted for the ~ l ~#ramE~WJ2~ specified in t h e ~ro~j~am ~ a d i n o . The heeding has the following form: ~£~Oram

where



a program


( { , }

parameter

identifier>

is

) ;

either:

-or-

The parameters are formal declared as file variables same w a y a s a c t u a l local file


identifier>

file identifiers; in t h e m a l n p r o g r a m varfables.

*

they must in e x a c t l y

be the

Files denoted by t h e f o r m a l p a r a m e t e r s inowJ2 a n d ~ u t o u ~ have somewhat special status. The following r u l e s m u s t be n o t e d : I .

2,

The program heading ~L~S~ contain the formal parameter output. Contrary to all other external files, the two formal file identifiers ~nout and ~-9_~ ~st Dot be defined in a declaration, because their declaration is automatically assumed t o be: 3&&~ i n p u t ,

3.

a

output:

text;

The procedures reset and rewrite have to the actual files INPUT and OUTPUT.

examp le : Proora~ ~_~_ x ,y

P (output, text

If an actual card record is

x,

no

effect

if

applied

y);

;

parameter in t h e E X E C U T E statement of the control left empty, the corresponding formal parameter in

92

the p r o g r a m h e a d i n g i s t h e n a s s u m e d as t h e a c t u a l "logical file n a m e " . For e x a m p l e , w h e n c e i l i n g a p r o g r a m w i t h t h e h e a d i n g : p r or ~

p (output,f

,g);

then EXECUTE,(,X,) Ss e q u i v a l e n t t o E X E C U T E , P ( O U T P U T , X . G ) . The full s p e c i f i c a t i o n of t h e f i l e p a r a m e t e r s is r e c o m m e n d e d b e c a u s e reliance on default v a l u e s o f t e n l e a d s t o m i s t a k e s that c o u l d easily have been avoided.

B.2

Representation

of f i l e s

In the case of external files it is i m p o r t a n t t o know t h e representation of files chosen by the P A S C A L c o m p i l e r . E v e r y component of a P A S C A L - 6 0 0 0 file o c c u p i e s an i n t e g r a l n u m b e r of ~O-bit words, with t h e e x c e p t i o n of files w i t h c o m p o n e n t t y p e ~har (~extfiles). In t h i s c a s e P A S C A L f i l e s u s e t h e " s t a n d a r d " representation imDosed by CDC°s text file conventions: 10 characters are packed into each word, implying that the procedures put and get i n c l u d e p a c k i n g and u n p a c k i n g o p e r a t i o n s when applied to t e x t f i l e s . The end of a l i n e is r e p r e s e n t e d by at least 12, right-adjusted zero-bits in a word. Files originating from ~ard ~ f o l l o w the s a m e g e n e r a l t e x t f i l e conventions. Note that the operating system ~moves most (but not necessary all) trailing blanks when reading cards, Hence, such files do Dot necessarily c o n s i s t of 8 0 - c h a r a c t e r "card images" Files that are not s e g m e n t e d a r e w r i t t e n as a s i n g l e " l o o i c a l record" (in SCOPE terminology). W h i l e r e a d i n g an u n s e g m e n t e d external file, end-of-record marks are ignored [for an exception, s e e p o i n t 3 b e l o w ] . In s e g m e n t e d f i l e s , each s e g m e n t corresponds to a "logical record". T h e r e is no p r o v i s i o n to specify a "record level".

Use of e x t e r n a l I.

files

If an external file is t o be r e a d ( w r i t t e n ) , t h e n in t h e Case of non~egmented files, reading (writing) must be i n i t i a t e d by t h e s t a t e m e n t (rewrite(x)

r es et (x)

and

in t h e

case

reset(x) getseg(x.n)

of s e g m e n t e d

) files

by

(rewrite(x) > or (rewrite(x.n) )

(This statement is automatically implied for the f i l e s denoted by the f o r m a l p a r a m e t e r s j~omut a n d ~ u t n u t , and must not be s p e c i f i e d by the p r o g r a m m e r . )

2.

Every

external

file

is a u t o m a t i c a l l y

"opened"

by a call

of

93

the OPE r o u t i n e of t h e o p e r a t i n g s y s t e m . I f t h i s o p e n i n g is to be r e s t r i c t e d to t h e r e a d f u n c t i o n - - e . g , in t h e c a s e of a permanent file without write permission--then t h i s has t o be i n d i c a t e d by an a s t e r i s k f o l l o w i n g t h e f i l e p a r a m e t e r in t h e program heading. The asterisk itself constitutes no protection a g a i n s t w r i t i n g on t h e f i l e . example: ~r_Q_gram ~i

testdata (output.

d a t a : "f_ila ~ f r: r e a l ; r

:='data~ ;

data*) :

real ;

get(date)

o . .

3.

If the a c t u a l file n a m e I N P U T is s u b s t i t u t e d corresponding to a formal program parameter, s a y f, t h e n f is t h e c u r r e n t s i n g l e l o g i c a l r e c o r d of t h e f i l e I N P U T .

B.3

The

standard

types

INTEGER

The

standard ~on~

identifier

maxint

=

~axint

is

281474976710655;

defined {

as

= 2**48

-

1 }

The r e a d e r is c a u t i o n e d , h o w e v e r , that t h e CDC c o m p u t e r p r o v i d e s no indication of o v e r f l o w . It is, t h e r e f o r e , the programmer's responsibility to p r o v i d e a c h e c k w h e n e v e r t h i s m i g h t o c c u r . Actually. the machine is c a p a b l e of s t o r i n g i n t e g e r s up t o an absolute value of 2**59, but then ~ the operations of addition (+), subtraction (-), taking the absolute value, multiplication and division by powers of 2 ( i m p l e m e n t e d as shifts), and comparisons a r e c o r r e c t l y e x e c u t e d in t h i s r a n g e (as l o n g as no o v e r f l o w o c c u r s ) . In p a r t i c u l a r , one cannot even p r i n t an i n t e g e r v a l u e i w h e n a b s ( i ) > m a x i n t . This does, however, allow the following test: if a b s ( i )

> maxint

the•

write("

too

big')

REAL

The type ~al is d e f i n e d a c c o r d i n g to CDC 6 0 0 0 f l o a t i n g f o r m a t . P r o v i d e d is a m a n t i s s a w i t h 48 b i t s , c o r r e s m o n d i n g d e c i m a l d i g i t s . The m a x i m u m a b s o l u t e m a g n i t u d e is 1 0 - ' 3 2 2 .

point to 14

94

CHAR

A v a l u e of t y p e ~ h a r is an e l e m e n t in t h e c h a r a c t e r set p r o v i d e d by t h e p a r t i c u l a r i n s t a l l a t i o n . The f o l l o w i n g 3 v e r s i o n s e x i s t : I) The C D C 2) T h e C D C 3) The CDC

Scientific 64-character Scientific 63-character A S C I I 6 4 - c h a r a c t e r set

set set

Table 13.a lists the a v a i l a b l e c h a r a c t e r s and i n d i c a t e s t h e i r o r d e r i n g : ~ o t e : t h e CDC s p e c i f i c a t i o n i m p l i e s an o r d e r i n g of the ASCII characters w h i c h d i f f e r s from the I n t e r n a t i o n a l S t a n d a r d (ISO)!

95

CDC

Scientific

Character

Set

with

64

elements

j~

1

2

3

4

5

6

7

8

9

:

A

a

C

O

E

F

G

H

I

J

K

L

M

N

0

P

q

R

S

T

U

V

W

X

Y

Z

~

I

2

3

4

5

6

7

8

9

+

-

*

/

(

)

$

=

-~

[

]

~

~

m

v

~

<

>

not

used

set

version

, ^

~

Comments: -

~

- 51

:

in

the

- 48

'

at

ETH

-

53

t

at

ETH

-

57

t

at

ETH

ASCII

Character

1

in

63-character

5et

2

63-character

with

set

CDC's

version

ordering

3

4

5

6

7

8

9

:

A

B

C

D

E

F

G

H

I

J

K

L

M

N

0

P

q

R

5

T

U

V

W

X

Y

Z

~

I

2

3

4

5

6

7

8

9

+

/

(

)

,

=

J

Figure

~

13.a

:

,,

CDC

, &

character

'

sets

?

*

~

[

<

>

96

Based upon the above character sets, the following characters are accepted as s y n o n y m s for t h e s t a n d a r d s y m b o l g i v e n in t h e left c o l u m n :

5tandard

Pascal

CDC

scientific -7

not and o__r <> <= >= Figure

B.4

The

13.b:

standard

_< > Alternative representation standard symbols

of

"'write"

If no m i n i m u m f i e l d l e n g t h p a r a m e t e r default values are assumed.

is s p e c i f i e d ,

the

following

default

integer real

10 22

Boolean

10

char

a string a l f a (see

&

A V

procedure

type

ASCII

( w h e r e t h e e x p o n e n t is a l w a y s e x p r e s s e d in t h e f o r m : E~999

)

1

length D.I)

of t h e

string

10

The end of each line in a textfile f m u s t be e x p l i c i t l y indicated by writeln(f)o where writeln(output) m a y be w r i t t e n simply as w r i t e l n . If a t e x t f i l e is to be s e n t to a p r i n t e r ~ no line may c o n t a i n m o r e t h a n 136 c h a r a c t e r s . The f i r s t c h a r a c t e r of each line is interpreted by the printer as a control character and ~s not printed. The following characters are interpreted to mean °+"

blank "0"

no llne f e e d (overprinting) single spacing double spacin~ triple spacing s k i p to top of next p a o e b e f o r e

printing

The procedure writeln(x) is u s e d to m a r k t h e end of a line on fi&e x, The conventions of t h e C D C o p e r a t i n g s y s t e m r e o a r d i n g textfile representation a r e s u c h that t h i s p r o c e d u r e is f o r c e d to emit some extra blanks under certain circumstances. Hence,

97

upon that

reading, a textfile were never explicitly

(as

may c o n t a i n blanks at the end written. (Sorry about this!)

C.

Restrictions

of

1.

The word

2.

T h e b a s e t y p e of a set m u s t be e i t h e r a) a s c a l a r w i t h at m o s t 58 e l e m e n t s (or a s u b r a n g e

se.qmented

March

of

lines

1974)

is r e s e r v e d .

thereof)

OF

b)

c)

3.

a subrange with a minimum element greater than or equal to zero, and a maximum element less than or equal to 58, or a subrange of the type chat with the maximum element less than or equal to the value chr(58).

Standard (built-in) proeedrues or functions are not accepted as actual parameters. For example, in order to run program 11.6 in PASCAL 6 0 0 0 - 3 . 4 , one would have to write auxiliary functions as follows:

functi~233 s i n e ( x : r e e l ) : r e a l ; beqin s i n e := s i n ( x ) ~U~: fw43ction c o s i n e ( x : r e a l ) : r e a l : b#~in c o s i n e := cos (x) ~3~; fu~£~

zero(function ... e~ :

be~in

f: r e a l ;

e,b:

real):

real;

..•

be~in read(x,y); r e a d ( x oy ) ;

4.

It is records

not and

D. Additional

D,I

Additional

The t y p e ~ i f & ~y~

ella

writeln writeln

( x .y , z e r o ( s i n e , x , y ) ) ; (x ,y , z e r o ( c o s i n e , x , y )

p o s s i b l e to c o n s t r u c t a file of f i l e s : h o w e v e r , a r r a y s w i t h f i l e s as c o m p o n e n t s a r e a l l o w e d .

predefined

types,

predefined

procedures,

by:

= ~acked ~rrav[1..10] type of

functions

types

is p r e d e f i n e d

(Hence, a value of word.) The constants characters.

and

ella this

~

char;

is representable type are &trinqs

in exactly of exactly

one 10

98

Applicable comparison, test order values may

on operands of type ella are assignment where = a n d <> t e s t equality and <, <=, according to the underlying character be p r i n t e d by t h e p r o c e d u r e write.

(:=) a n d >=. and > set. Alfa

{ program 13.1 al£a values } ~roeram

egalfa

(output);

vat nl.n2: alfa; beoin write(" names: '); nl := "raymond ": n2 := "debby if n2 < nl then writeln(n2.nl) else writeln (nl,n2) end.

nameS:

debby

Note: It is the following vi~

raymond

not possible to is s u g g e s t e d :

read

alfa

values

directly;

instead,

b u r : a r r a y [ I..10] of c h a r ; a: a l f a ; i: i n t e g e r ;

E e l i :: I t o 10 d o r e a d ( b u f [ i ] ); pack(buf01.a ) {accomplishes read(a)}

D .2 A d d i t i o n a l

predefined

procedures

and

functions

~roce~ures date (a)

assigns

halt

terminates the execution issues a poat-mortem dump.

linellmit(f,x)

f is a t e × t f i l e a n d x is an i n t e g e r expression, The effect is to cause the program to be terminated, if m o r e t h a n x l i n e s a r e a s k e d t o be written on f i l e f.

message (x)

the string x is written (Hence. x should contain at

time (a)

assigns

putseg

.

getseg,

and

to

to the

the

the

alfa

alfa

extensions

variable

variable to

the of

current the

date.

program

and

into the dayfile. most 40 characters.) a the rewrite

current and

reset

time. are

99

discussed

in s e c t i o n

13.A.I.

~unctlons

card(x)

equals number

clock

a function, without parameters, yielding an i n t e g e r v a l u e e q u a l to the c e n t r a l p r o c e s s o r t i m e , expressed in milliseconds, already u s e d by t h e job.

expo(x)

yields an integer the floating~oint value x.

undefined(x)

a Boolean function. Its value real value x is "out of r a n g e " o t h e r w i s e f a l s e [ 7] .

cos(x)

(discussed

the cerdinal±ty of the s e t x (i.e. t h e of e l e m e n t s c o n t a i n e d in t h e s e t x.)

v a l u e e q u a l to t h e e x p o n e n t of representation of the real

in s e c t i o n

13.A.1)

is t r u e w h e n t h e or " i n d e f i n i t e " ,

14 Use ~he ~ASCAL ~ 0 0 0 - 3 . 6 ~ v s t e m

Uow ~ A. C o n t r o l

statements

(for

SCOPE

3.4)

A Pascal job u s u a l l y c o n s i s t s of f o u r s t e p s . F i r s t , the P a s c a l compiler is loaded. The s e c o n d s t e p is t h e c o m p i l a t i o n step, which yields a l i s t i n g of t h e s o u r c e p r o g r a m a n d t h e c o m p i l e d code. In the third step t h e c o m p i l e d c o d e , d e p o s i t e d by t h e compiler on secondary store, is loaded and linked with precompiled routines for input and output handling, which are provided on a "program l i b r a r y " f i l e . F i n a l l y , t h e c o m p i l e d a n d loaded code is executed. These four s t e p s a r e i n i t i a t e d by appropriate orders to the operating system in t h e f o r m of ~ontro~ Qtetements. The e x a c t f o r m of t h e s e s t a t e m e n t s and t h e i r abbreviated f o r m s ( l o a d i n g a n d e x e c u t i o n c a n o f t e n be o r d e r e d by a single statement) depend entirely upon the available operating system, and must therefore be specified by the particular installation. The a c t u a l f i l e p a r a m e t e r s , w h i c h c o r r e s p o n d t o t h e f o r m a l file identifiers l i s t e d in t h e p r o g r a m h e a d i n g , m u s t be s p e c i f i e d in the statement initiating execution of the compiled program ( u s u a l l y an E X E C U T E c o m m a n d ) . The

compiler ~r_~wJE~

itself

is a l s o

a Pascal

program.

Its

heading

is

Pascal(input.output,lgo);

The first formal source p r o g r a m to end t h e t h i r d , the

parameter d e n o t e s t h e file r e p r e s e n t i n g the be c o m p i l e d ; t h e s e c o n d , t h e p r o g r a m l i s t i n g ; compiled "binary", relocatable code.

The COC operating systems allow the omission of a c t u a l parameters in t h e c o n t r o l s t a t e m e n t s . If an a c t u a l f i l e name is omitted, the Pascal convention on p r o g r a m p a r a m e t e r s specifies t h a t t h e f o r m a l file i d e n t i f i e r be u s e d as t h e a c t u a l file n a m e . Hence, the standard files INPUT, OUTPUT, and LGO are automatically a s s u m e d as t h e d e f c u l t ~ i l e s for t h e s o u r c e file, the listing, and the relooatable binary code respectively. Note, however, that these roles m a y be a s s u m e d by o t h e r f i l e s w h e n their names are entered as actual parameters. Note: actual parameters m u s t c o n s i s t of at m o s t 7 c h a r a c t e r s .

B . Compiler

options

The compiler may be i n s t r u c t e d to g e n e r a t e c o d e a c c o r d i n g to c e r t a i n o p t i o n s ; in p a r t i c u l a r , it may be r e q u e s t e d to insert or omit r u n - t i m e t e s t i n s t r u c t i o n s . Compiler directives are written as comments and a r e d e s i g n a t e d as s u c h by a S - c h a r a c t e r as the first c h a r a c t e r of t h e c o m m e n t :

101

{$
sequence>

{$T+.P+


comment>

}

}

The option sequence is a s e q u e n c e of i n s t r u c t i o n s separated by commas. Each instruction consists of a l e t t e r , designating the option, followed either by a p l u s (+) if t h e o p t i o n is t o be activated or a m i n u s (-) if t h e o p t i o n is t o be p a s s ~ v a t e d 0 or by a d i g i t (see X e n d B b e l o w ) .

The T

following include a) b)

e) d)

e)

tests

presently that

available"

chock

= T+

generate Dump (see default

X

run-time

are

all array indexing operations to insure that the index lles within the specified array bounds. all assignments to variables of s u b r a n g e t y p e s to m a k e certain that the assigned value lies within the specified range . all divisions to i n s u r e a g a i n s t zero divisors all automatic integer to reel conversions to a s s u r e t h a t the converted value satisfies : a b s (i) <= m s x i n t all case statements to insure that the case selector corresponds to one of the specified case labels.

default P

options

the code necessary to section 14.C .2) in t h e

write a complete Post~ortem c a s e of a r u n - t i m e error.

= P+

if a digit n (0 <= n <= 6) f o l l o w s t h e X. p a s s t h e f i r s t n parameter discriptors in the registers XO to X(n-1) (the first in XO, t h e s e c o n d in XI, etc',). O t h e r w i s e p a s s t h e m in the locations with the addresses B6+3 to B6+n+2. n>O reduces t h e s i z e of t h e c o d e p r o d u c e d by t h e c o m p i l e r and probably also s l i g h t ly improves the code. However', the programmer must be a w a r e t h a t w i t h n > O . t h e c o m p i l e r cannot use the registers XO t o X m l n ( n - l , i - 2 ) for the passing of t h e ith parameter. It is therefore possible t h a t for n > O , t h e compiler gives the message " r u n n i n g o u t of r e g i s t e r s " : where f o r n = O o it w o u l d n o t . default

E

= X4

allows the programmer to control the symbols for the entry points to the object code modules (procedures and functions) that he declares in h i s p r o g r a m . . The following conventions hold : -- M o d u l e s declared as point name equal to first seven characters.

"extern" or the procedure

"fortran" get an e n t r y identifier cut to the

102

-- L o c a l value module EE+

modules of the name):

get an E-option

A u n i q u e s y m b o l is The first seven taken.

entry (at

point name the moment

generated characters

by

depending on t h e of analyzing the

the compiler of the module

name

ere

Whenever the cut module n a m e is t a k e n ( E + a n d " e x t e r n " or "fortran"), it is t h e p r o g r a m m e r s responsibility to a v o i d t h e occurrence of duplicate entry point symbols. default L

U

:

controls

the

default

= L+

listing

of

the

program

text.

a l l o w s t h e u s e r to r e s t r i c t t h e n u m b e r of r e l e v a n t characters in every source line to ?2. T h e r e m a i n d e r o f t h e l i n e is treated as a comment. With Uthe number of relevant characters is 120. T h e r e s t of t h e l i n e is t h e n t r e a t e d as a comment. default

B

E-

= U-

used to s p e c i f y a T o w e r l i m i t for t h e s i z e of f i l e b u f f e r s . If after the B a digit d (I<=d<=9) occurs, the bufffer size S, computed by the compiler, is g u a r a n t e e d t o be S > 1 2 8 . d words . default

= B4

As the program, specific

compiler instructions may be w r i t t e n anywhere in t h e it is p o s s i b l e to activate the options selectively over parts of the program.

C.

messages

0.1

Error

Compiler

The compiler indicates a detected e r r o r by an the relevant place in the text, followed corresponds to the messaDes in A p p e n d i x E .

C.2

Run~ime

When the generates the case

(Post-Mortem

arrow, pointino to by a n u m b c r , w h i c h

Dump)

compiler o p t i o n P is t u r n e d on ( i . e . P + ) , t h e c o m p i l e r code that c a n be u s e d t o p r i n t a r e a d a b l e "dump'" in that a run-time error occurs. The dump includes the

103

following

information:

a)

the

cause

b)

a description of e a c h o f t h e p r o c e d u r e s (functions) that is activated at t h e t i m e of t h e t r a p . T h e s e a p p e a r in t h e reverse o r d e r of t h e i r c a l l s a n d c o n s i s t of: I) t h e n a m e o f t h e p r o c e d u r e 2) t h e l o c a t i o n of its call 3) a list of t h e n a m e s a n d v a l u e s o f t h e l o c a l v a r i a b l e s and parameters.

c)

the

values

of

the

of t h e

trap

and

global

where

it

variables

occurred

in

the

main

program.

Only variables and parameters of the types integer, real, Boolean, char, and alfa. Pointers are either " n i l " or h a v e an octal value (address). For other scaler variables, the ordinal number of their current value is p r i n t e d , When, for any one procedure, the option P is turned off (P-). t h e n o n l y t h e procedure name and the location of its c a l l a p p e a r in t h e d u m p . In the recent)

case three

of r e c u r s i v e procedure calls, only the last occurrences of each procedure are listed,

(most

104 References 1. N. Wirth,

~he

~r_@R~@mina L~u~ Informatica, 1, 3 5 - 6 3 . 1 9 7 1 .

~&cal.

Acta

~ ~roorammi~_~ L~noua~ ~aal (~,~zised ~p_~), Berichte der Fachgruppe Computer-Wissenschaften ETH Z u r i c h , 5, 1973. 2.

"Program Development by Stepwise Refinement", ~omm. ~CM ~ , 2 2 1 - 2 2 7 , April 1 9 7 1 .

3.

System~J2~

~roqrammino,

Prentice-Hall,

Inc,

Academic

Inc.

1973. 4.

O.d.

Dahl,

E .W. D i j k s t r e , ~tructured

C.A.R. Hoare. ~£oorammiQ,&.

Press

1972. 5. C.A .R . H o a r e and N. W i r t h , Programming ~nfo~ma~ica,

6, D . E . 7.

Knuth.

SCOPE R e f e r e n c e

"An A x i o m a t i c Definition Language Pascal", 2, 335-355, 1973.

of

the ~£ta

~ &r_~ ~ ~wmouter ~roa~&~_mino, vol 1 Edwj3~~ &3~gorithms, Addison-Wesley, 196U. Manual,

CDC 6000 V e r s i o n

Corporation,

1973.

4.3.1,

Control

Data

Standard

File

handling

Appendix A ~&edures ~nd

~unctions

procedures

put (f)

appends the value of t h e b u f f e r v a r i a b l e file f, and is applicable only if execution, e a r ( f ) is t r u e . e o f ( f ) r e m a i n s f~ b e c o m e s u n d e f i n e d ,

get(f)

advances the current file position to the next component, and assigns the value of this component to the buffer variable f~ . I f no n e x t c o m p o n e n t exists, then e o f ( f ) b e c o m e s t r u e , a n d t h e v a l u e of ft is u n d e f i n e d . Applicable o n l y if e a r ( f ) is f a l s e p r i o r t o its e x e c u t i o n .

reset

resets the current file p o s i t i o n to its b e g i n n i n g for the purpose of reading, i.e. a s s i g n s to t h e buffer variable f~ the value of the first element of f. ear(f) becomes false if f is not empty: otherwise, f~ is undefined and ear(f) remains true.

(f)

f~ t o prior true,

rewrite(f)

replaces the current eof(f) becomes true,

page(f)

instructs the printer to s k i p to t h e t o p page before printing the next l i n e of t h e f,

read, readln, discussed in

Dynamic

write, chapter

allocation

writeln 12.

v a l u e of f w i t h t h e e m p t y f i l e . a n d a new f i l e m a y be w r i t t e n .

applicable

on

textfiles

of a n e w textfile

and

are

procedure

new ( p )

allocates a new v a r i a b l e v and assigns the pointer reference of v to the Pointer variable p. If the type of v is a record type with variants, the form

new(p ,t 1 .....

tn)

dispose(p)

dispose(p

are

the to and

,t I .... ,tn)

can be used variant with tag field contiguously declaration.

to a l l o c a t e a v a r i a b l e of t h e t a g f i e l d v a l u e s t 1 . . , t n . The values must be listed and in the order of t h e i r

returns the dynamic referenced by P .

variable

returns the dynamic referenced by p and second form of new.

variable v t h a t is was crested by the (Implementations may

v

that

is

106 choose

Data

transfer

to

ignore

this

statement,)

procedures

paek(a,i,z)

if

a i s an a r r a y variable of type arrav[m..n] ~ T and z is a variable of type aacke~ ~_~Z[u..v] ~ T w h e r e n-m >= v - ~ , then this is equivalent .f_or j

u n p a c k (z ,a , i )

:= u J~D. v d o z [ j ]

:=

and is equivalent

to

for

v do a[j-u+i]

j

::

u to

e[j-u+i]

:=

z[j]

(In both c a s e s , j d e n o t e s an a u x i l i a r y not o c c u r i n g e l s e w h e r e in t h e p r o g r a m . )

Arithmetic

to

variable

functions

abs ( x )

c o m p u t e s t h e a b s o l u t e v a l u e of x. The t y p e o f t h e r e s u l t is t h e s a m e as t h a t of x, w h i c h m u s t be e i t h e r i n t e o e or real.

sqr(x)

c o m p u t e s x * x . T h e t y p e of t h e r e s u l t is t h e of x, w h i c h must be e i t h e r i n t e g e r or r e a l .

sin(x)

for the integer.

same

f o l l o w i n g , t h e t y p e o f x m u s t be e i t h e r The t y p e o f t h e r e s u l t is always real.

as

that

real

or

cos(x) aretan

(x)

exp ( x ) ln(x) sqrt(x)

(natural (square

Predicates

logarithm) root)

(Boolean

functions)

odd ( x )

the t y p e of x m u s t be i n t e g e r ; is o d d , o t h e r w i s e f a l s e .

eoln(f)

returns f, the false.

eof(f)

returns the value true when, the "end-of-file" is reached;

the

result

is t r u e

if x

the v a l u e t r u e w h e n , w h i l e r e a d i n g the t e x t f i l e end of t h e c u r r e n t line is r e a c h e d ; otherwise,

while reading the otherwise, false.

file

f.

107

Transfer

functions

trunc(x)

x must be of t y p e r e a l ; t h e r e s u l t is t h e g r e a t e s t integer less than or equal to x for x > = O . a n d t h e l e a s t i n t e g e r g r e a t e r or e q u a l to x for x < O .

round(x)

x must be of t y p e r e a l : t h e r e s u l t , is t h e v a l u e x r o u n d e d . That is, r o u n d ( x ) = t r u n c ( x + 0 . 5 ) , for trunc(x-O.5), for

of t y p e

integer,

x > 0 x < 0

ord (x)

the ordinal number v a l u e s d e f i n e d by t h e

chr(x)

x must be of type integer, a n d t h e r e s u l t is t h e c h a r a c t e r w h o s e o r d i n a l n u m b e r is x (if it e x i s t s ) .

Further

standard

of the a r g u m e n t t y p e of x.

x

in t h e

set

of

functions

succ(x)

x is is t h e

of any s c a l a r t y p e (except r e a l ) , a n d s u c c e s s o r v a l u e of x (if it e x i s t s ) .

the

result

pred(x)

x is is t h e

of any s c a l a r t y p e ( e x c e p t r e a l ) , a n d t h e predecessor v a l u e of x (if it e x i s t s ) .

result

108

~ / i v

go e r a ~

o_

arithmetic: + (unary) - (unary) +

/ mad

Appendix B of Operations

£oeration

~Yoe

of # o e r a n d ( s )

assignment

any t y p e e x c e p t file t y p e s

identity sign inversion

integer

addition subtraction multiplication

integer

or r e a l

integer division real division modulus

integer integer integer

or

equality inequality

scalar, string. s e t . or p o i n t e r

less t h a n greater than

scalar

or r e a l

KesN![ type ---

same

as

operand

real

integer or r e a l

integer real integer

relational: = <>

or s t r i n g Boolean

less or e q u a l -orset i n c l u s i o n greater or equal

<=

>=

scalar

or s t r i n g

set scalar

or s t r i n g

-or-

ia

logical: not or

aad

set

inclusion

set

set

membership

f i r s t o p e r a n d is any scalar, the s e c o n d is its s e t type

negation disjunction conjunction

Boolean

union set difference intersection

any

Boolean

set: +

set

type

T

109 Appendix Yables

A.

Table

of

standard

Constants : false , true.

Types: integer,

C

identifiers

maxint

Boolean,

real,

char,

text

Program parameters: input, output

Functions: abs, arctan, ord, pred,

chr, round,

Procedures : get . n e w , pack. rewrite, unpack,

B.

Table

of

cos, sin,

eof, sqr,

page, write,

exp, succ,

put , read, writeln

word-delimiters

m~ arraK ~a_q~n

eoln. sqrt,

(reserved

readln,

nil not of

y.~e

~unctioq

~E

tvo~

~OJ3S t div do dow nt Q eIs e

_onto if in ~abe ~ /nod

o~Gk#d oroceduns o r o ~r a m rw_~or d r_e o w2.c~

until var ~bila ~ith

predefined

reset ,

words)

aad file for

C. Non-standard,

In, odd, trunc

sa~ thezl ~o

identifiers

in

PASCAL 6 0 0 0 - 3 . 4

Types: alfa

Functions: card,

clock,

Procedures: data, getseg,

eos, halt,

expo,

undefined

iinelimit,

message,

putseg,

time

110 Appendix ~vnt ax

~_acku~_-N~_IZ~

F~Lm

(RNF)

Note: the following BNF formalism, a n d not

I

::=

{

: ::

belonginu Pascal.

to

the

denote possible repetition times. In g e n e r a l .

of

the

enclosed

{B}

is a s h o r t A

are mete-symbols of t h e l a n o u a g e

!

The curly brackets symbols z e r o or m o r e A

symbols symbols

form

for

: := < e m p t y >

the

purely

recursive

rule:

[ AB

BNF



: := < p r o g r a m



heading>

identifier>



::=

::= ~ o ~ r _ ~

(

identifier> } );

: := < i d e n t i f i e r > :'=

{


or

digit>}


or



:::


digit>

heading>

~

declaration part> ::= ~abel ~, } ;



::=






I

integer>

definition part> ::= I c~nst { :

::=



definition>}

=

::: < u n s i g n e d number> I I I I number>

::=


integer>

I
real>

;

111


integer>


factor>



::=


+

::=


{}

integer>

I

::=

::=

'

definition> ::=

{}

: := < i d e n t i f i e r >


type>

°

type> ::= < s c a l a r


type>


::=

type>

::=

type>

::=

I

type>
type>

type>


::= D e c o r d

list> ::= < f i x e d


part>

::=

section>

part>

type>

I

type>

type>

{,

list>

~J3~[

I



type>}

type>

l

] of

type>




I

part>

{ ;
identifier>

{.

;
part>


field>

{ ; }


I

section>}
identifier>}



::= c a s e



~
section>

::= < f i e l d


)

: := < t y p e >



I

type>

: :=


:'=

structured

structured

type> ::= ~rev [

type>

type>

type>

.. < c o n s t a n t >



I
{ ° }



structured


;

: := < i d e n t i f i e r >

~Lecked


type>

~
(

identifier>


type>

definition>}

=

I


integer>



definition part> ::-- < e m p t y > I Jiyoe < t y p e d e f i n i t i o n > { ;



~ -

identifier>






reel> ::= < u n s i g n e d integer> . {} I . {} E E



::=

identifier>

of

:

112


field>



::= ::=


label


label>




list>

type>

identifier>

label

list>

: :-- < c a s e

: ~

: (
label>

{,

list>


label>}

: := < c o n s t a n t > : := s e t

of


type>


type>

::=


type>


type>

::=

file



type>


declaration part> ::= < e m p t y > I ~&WJ& < v a r i a b l e declaration> { ;


identifier>

: := < i d e n t i f i e r >

and function {
or f u n c t i o n declaration> declaration>


::=

::=


declaration>}

}


::=

heading>

heading>

section>

group>

:



heading> ::= f u n c t i o n : f~tio~ ( { ; } ) : ; type>


::=

part>


identifier>


statement>

::= < u n l a b e l l e d statement> I : :'= < s i m p l e

statement>

I


statement>

J

statement>

::=


I



{ ° }


:

declaration>

I I ~unction }

group> ::=

;

;}

heading> ::= & ~ w ~ e d u r e ; I &rocedure ( } ) ;

declaration>


{.

declaration part> ::= function declaration>

parameter section> ::= va~ &~ocedure



~
declaration>



::=

~



) i

; I

113



I
to

statement>

I

::= < v a r i a b l e > := < e x p r e s s i o n > := < e x p r e s s i o n >


::-- < e n t i r e v a r i a b l e >

variable>


::=

identifier>


variable> ::=


variable> ::= < a r r a y { , } ] variable>


designator>


variable>


buffer>


variable>


I

identifier>


variable>

variable>

I
designator>

I

[

: := < v a r i a b l e > ::= ::=

identifier>


variable>





I
I


identifier>



: := < f i l e

variable>



variable>

variable>

.


::=

::=

variable>

: := < p o i n t e r

::=

variable>



::= < s i m p l e e x p r e s s i o n > ~
operator>

::=

=

I <>

I <

I

<=

I >=


expression> ::= < t e r m > I


operator>



::=




::=

+

I -

I

operator>

::=

I

>

in

I

1

I o~
operator> I ~L~

I a~d



::= < v a r i a b l e > I I I not


constant>

desionator> ::= < f u n c t i o n ~dentifier> I (

::= < u n s i g n e d number> identifier> I nil



I (

I

I

)

114

{ ,
parameter>}


identifier>

: :=



[
list>

::=


list>





identifier>

parameter>
::=

[ . J

}

I

,.





::= < e x p r e s s i o n > I I identifier> I

to

statement>

::= o o t o


statement>

::=



]

statement> ::= < p r o c e d u r e identifier> I ( { . } )





: := < e x m r e s s i o n >



::=

)





: :=

::= < c o m p o u n d statement> I
statement>


::= b ~ g ~ n

statement>

::=




l statement>

I

[ ; }

statement>

I
ond

statement>

statement> ::= if < e x p r e s s i o n > then I if JZ~J~D < s t a t e m e n t > e3se


statement> : := c a s e [ ; } ~nd


list element>


label

list>

::=

::=



::=
statement>

::= w h i l e

label

label>
of

list>

{,

label>

statement>

statement> ::--z:e~_@_~JZ < s t a t e m e n t > until variable>

list

element>

:






}

J
do

I

statement}



{ ;

}


statement> ::= f o r

:=


list> ::= < i n i t i a l v a l u e > t o < f i n a l v a l u e > dgwnto


J

list>

do

I

115


variable>


value>


value>

::=

::= ::=

statement> variable





::= ~ i t h list>

::=


variable variable>

list>

~



{ .
variable>}

consta~l

unsigned con~ta~tt

unslgned number

unsigned integer

identifier

PASCAL

~

constant identifier]

1

~

field list

type

~

~

I ~ G - - -® F~q

-

"@--'Ityp°~

.[type identifierI

I 1.

rm

~©~'ype'°o°','d'@ 1

simple type

O~

term

factor

variable

~-
~lu..~..°° ........ I

i

parameter

list

L i ~

,I

l.[i~pt°oxp

m

O©QQ@(~® ~ I

oxp .... i o ~

----•simpio

expression

simple expression

.......

i

............

",4

~~-~~

statement

rogtawl

lock

. . . . . .

]

=

0

i

119 Appendix E ~:~j~umber ~ummar~

I: 2: 3: 4: 5: 6: 7: 8: 9: 10: 11: 12: 13: 14: 15: 16: I?: 18: 19: 20: 21:

50: 51: 52: 53: 54: 55: 56: 57: 58: 59:

101: 102: 103: 104: 105: 106: I07: 108: I09: 110: 111: 112: 113: 114: 115: 116: 117:

e r r o r in s i m p l e t y p e identifier expected "program' expected ")" expected ":" expected illegal symbol e r r o r in p a r a m e t e r l i s t "of" e x p e c t e d "(° e x p e c t e d e r r o r in t y p e "[" e x p e c t e d '] " e x p e c t e d "end" e x p e c t e d ";" e x p e c t e d integer expected "=" e x p e c t e d "begin" expected e r r o r in d e c l a r a t i o n p a r t e r r o r in f i e l d - l i s t ' " expected "~" expected

error in constant ":=" expected "then" expected "until" expected "do" expected 'to'/'downte ° expected "if" e x p e c t e d "file" e x p e c t e d e r r o r in f a c t o r e r r o r in v a r i a b l e

identifier declared twice low b o u n d e x c e e d s h i g h b o u n d i d e n t i f i e r is not of a p p r o p r i a t e c l a s s i d e n t i f i e r not d e c l a r e d s i g n not a l l o w e d number expected incompatible subran~e types file not a l l o w e d h e r e t y p e m u s t not be r e a l t a g f i e l d t y p e must be s c a l a r or s u b r a n g e incompatible with tagfield type i n d e x t y p e must not be r e a l i n d e x t y p e must be s c a l a r or s u b r a n g e b a s e t y p e must not be r e a l b a s e t y p e must be s c a l e r or s u b r a n o e e r r o r in t y p e of s t a n d a r d p r o c e d u r e p a r a m e t e r unsatisfied forward reference

120

118: 119: 120: 121: 122:

f o r w a r d r e f e r e n c e t y p e i d e n t i f i e r in v a r i a b l e d e c l a r a t i o n f o r w a r d d e c l a r e d : r e p e t i t i o n of p a r a m e t e r list not a l l o w e d f u n c t i o n r e s u l t t y p e must be s c a l a r , s u b r a n g e or p o i n t e r file v a l u e p a r a m e t e r not a l l o w e d f o r w a r d d e c l a r e d f u n c t i o n ; r e p e t i t i o n of r e s u l t t y p e not allowed

123: 124: 125: 126: 127: 128: 129: 130: 131: 132: 133:. 134: 135: 136: 137: 138: 139: 140: 141: 142: 143: 144: 145: 146: 147: 148: 149: 150: 151: 152: 153: 154: 155: 156: 157: 158: 159: 160: 161: 162: 163: 164: 165: 166: 167: 168: 169: 170: 171: 172:

m i s s i n g r e s u l t t y p e in f u n c t i o n d e c l a r a t i o n F - f o r m a t for r e a l o n l y e r r o r in t y p e of s t a n d a r d f u n c t i o n p a r a m e t e r n u m b e r of p a r a m e t e r s does not a g r e e w i t h d e c l a r a t i o n illegal parameter substitution r e s u l t t y p e of p a r a m e t e r f u n c t i o n does not a g r e e w i t h declaration t y p e c o n f l i c t of o p e r a n d s e x p r e s s i o n is not of set t y p e t e s t s on e q u a l i t y a l l o w e d o n l y s t r i c t i n c l u s i o n not a l l o w e d file c o m p a r i s o n not a l l o w e d i l l e g a l t y p e of o p e r a n d ( s ) t y p e of o p e r a n d m u s t be B o o l e a n set e l e m e n t t y p e m u s t be s c a l a r or s u b r e n g e set e l e m e n t t y p e s not c o m p a t i b l e t y p e of v a r i a b l e is not a r r a y i n d e x t y p e is not c o m p a t i b l e w i t h d e c l a r a t i o n t y p e of v a r i a b l e is not r e c o r d t y p e of v a r i a b l e m u s t be file or p o i n t e r illegal parameter substitution i l l e g a l t y p e of loop c o n t r o l v a r i a b l e i l l e g a l t y p e of e x p r e s s i o n type conflict a s s i g n m e n t of f i l e s not a l l o w e d label type incompatible with selecting expression s u b r a n g e bounds m u s t be s c a l a r i n d e x t y p e must not be i n t e g e r a s s i g n m e n t to s t a n d a r d f u n c t i o n is not a l l o w e d a s s i g n m e n t to f o r m a l f u n c t i o n is not a l l o w e d no s u c h f i e l d in t h i s r e c o r d t y p e e r r o r in r e a d a c t u a l p a r a m e t e r must be a v a r i a b l e c o n t r o l v a r i a b l e m u s t not be f o r m a l m u l t i d e f i n e d case l a b e l too m a n y cases in c a s e s t a t e m e n t missing corresponding variant declaration r e a l or s t r i n g t a g f i e l d s not a l l o w e d p r e v i o u s d e c l a r a t i o n w a s not f o r w a r d again forward declared p a r a m e t e r s i z e must be c o n s t a n t m i s s i n g v a r i a n t in d e c l a r a t i o n s u b s t i t u t i o n of s t a n d a r d p r o c / f u n c not a l l o w e d multidefined label mu!tideclared label u n d e c l a r e d label undefined label e r r o r in b a s e set value parameter expected s t a n d a r d file was r e d e c l a r e d undeclared external file

121

173: 174: 175: 176:

F o r t r a n p r o c e d u r e or f u n c t i o n e x p e c t e d P a s c a l p r o c e d u r e or f u n c t i o n e x p e c t e d m i s s i n g f i l e " i n p u t " in p r o g r a m h e a d i n g m i s s i n g f i l e " o u t p u t " in p r o g r a m h e a d i n g

201: 202: 203: 204:

e r r o r in r e a l c o n s t a n t : digit expected s t r i n g c o n s t a n t m u s t not e x c e e d s o u r c e integer constant exceeds range 8 or 9 in o c t a l n u m b e r

250: 251: 252: 253: 254: 255: 256: 257: 258: 259:

too m a n y n e s t e d s c o p e s of i d e n t i f i e r s too many nested procedures and/or functions too many forward references of p r o c e d u r e entries procedure too long t o o m a n y long c o n s t a n t s in t h i s p r o c e d u r e t o o m a n y e r r o r s on t h i s s o u r c e line too many external references too many externals too many local files expression too c o m p l i c a t e d

300: 301: 302: 303: 304:

d i v i s i o n by z e r o no c a s e p r o v i d e d for t h i s v a l u e i n d e x e x p r e s s i o n out of b o u n d s v a l u e to be a s s i g n e d is out of b o u n d s e l e m e n t e x p r e s s i o n o u t of r a n g e

398: 399:

implementation restriction variable dimension arrays

not

line

implemented

122

Appendix F ~roorammin~ ~amo~&

[

PASCAL

6000-3.4

}

~rocedure rdr(~Aar ?: t e x t ; ~A~r x: r e a l ) ; { r e a d r e a l n u m b e r s in " f r e e f o r m a t ' } ~onst t48 = 281494976910656; limit = 56294995342131 ; z = 27; { ord('O °) } l i m l = 322; { maximum exponent } lim2 = -292; { minimum exponent } tvme posint = 0..323; v a ~ c h : c h a r : y: r e a l ; a , i , e : integer: s,ss: boolean: { signs }

function ten(e: posint): real; ~ a r i: i n t e g e r ; t : real; ~agln i := O; t := 1 . 0 ; ~#~ea& ~ odd(e) ~hen ~ a i af O: t := t * 1 . 0 e l ; 1: t := t * 1 . 0 e 2 : 2: t := t * 1 . 0 e 4 ; 3: t := t * 1 . 0 e 8 ; 4: t := t * 1.0e16; 5: t := t * 1 . 0 e 3 2 ; 6: t := t * 1.0e64; 9: t := t * 1.0e128; 8: t := t * 1,0e256 and : e := e ~ i v 2; i := i+I ~ e = O; ten := t en~ ;

{

=

lO**e,

0
~eein

if eof(f) ~hen be~in

message halt

( "**tried

end ; {skip leading blanks} while ( f t = • °) ~O_~ (not nDJl e o f ( f ) t h e n

to

read

eof(f))

past

do

eos/eo?');

get(f);

begin ch if

:= ft ch = benin

; s

~hen := t r u e :

get(f);

and e l ~ b e o i n s := f a l s e ; if eh = "+ ° t h e n ]~egi6 get (f): ch

and end :

:-- f~

ch

:= ft

i

123

i ~ i ~ o t (ch i n [ "0 ° . . ' 9 be~io message(°~*digi end ; a := O; e := O: redeemS_ i f

a

< limit

g e t (f);

ch

°] ) then t expected');

then else := ft

un£il if_ ch

not(ch ia ['0'..'9']), = ' . * J~heo

beoin

{ read

Kbite

fraction

eh i n

a e

:= l O ~ a := e + 1 ;

} get(f);

[ °0°..

°9']

halt;

ch

+ ord(ch)-z

:=

ff ;

do

D~oin

i f a < l i m i t tl3wJ& b e a i n a := 1 0 ~ a + e r d ( e h ) - z end ; get(f); ch := f T

; e

:=

e-1

end ead ; i £ c h = "e" t h e n be~in { read scale i := 0;

if

ch =

"-"

factor

} get(f):

ch

:=

fT ;

thell

be-qill s s := t r u e ; get(f); c h := f~ and ela~ b a a / / & s s := f a l s e ; i f c h = "+" ~ a ~ h a ~ i z l g e t (f): c h := fT and end ; ~hil_~ ch in [ "0'..'9"] do bg__qin i f i < l i m i t then bemin i := I 0 - i + o r d ( c h ) - z e13~;

~ e t ( f ) ; ch

:=

ft

and ; i f s s i h a n e := e - i a ~ e := e + i ; and ; i f e < lira2 ~h@q beqin a := O: e := 0 end ~lse i f e > l i m l the[l be&Lie m e s s a z e ( ' ~ ' ~ n u m b e r t o o l a r g e ' ) ; halt { 0 < a < 2"~'-49 } i f a > = t q U ~ h e n y := ( ( a + 1 ) d i v 2) ~ 2 . 0 e l s e y := a ; i f s t h e n y := - y : e < 0 t h e n x := y / t e n ( - e ) i_f e < > 0 t_h2Jl x := y ~ t e n ( e ) e l s e x := y ; and ; end ;

end;

124

[ PASCAL

6000-3.4

}

~racedure wre(var f:text; x: reel; n: i n t e g e r ) ; { w r i t e r e a l n u m b e r x in n c h a r a c t e r s } ~st t48 = 281474976710656; [ 2**48 } realt48 = 2,81474976710656e14; z = 27: { ord('O') } tYPe posint : 0..323; c,d.e,eO,el,e2,i: integer; ~WJ3~tio~ ten(e: posint): real; ~ i: i n t e g e r ; t : real; ~ i : : O; t : = 1 . 0 ; ~eoBat i ~ o d d ( e ) ~hg~_~ ~ a s e i ~_[ O: t := t * 1 . 0 e l ; I: t := t * 1 . 0 e 2 : 2: t := t * 1 . 0 e 4 ; 3: t := t * 1 . 0 e 8 ; 4: t :: t * 1.0e16; 5: t := t * 1 . 0 e 3 2 ; 6: t := t * 1 . 0 e 6 4 ; 7: t := t * 1 . 0 e 1 2 8 ; 8: t := t * 1 . 0 e 2 5 6 end ; e := e ~J~3L 2; i := i+I ~ntil e : O; t e n := t end { ten } ;

{

10**e.

0
bF..g.iZl { at l e a s t 10 c h a r a c t e r s needed: b+9,9e+999 if u n d e f i n e d ( x ) then b e a i n f o r i : = 1 t ~ n - 1 d ~ b e a i n ft ::" "; p u t ( f ) ft :='u °; p u t ( f ) end al~a i f x =0 t h ~ a h ~ ~Zhil: n > I W o bg~w.in ft := " "; p u t ( f ) ; n : = n-1 end ; ft : = ° 0 ° ; p u t ( f )

} end;

baaia if n <= 10 ~ h e n n : = 3 eZ~_~ n : = n - 7 ; react f~ := ° °; put(f); n := n-1 until n <: 14: { 2 <: n <: 14 : n u m b e r of d i g i t s t o be p r i n t e d beqin { test sign, then obtain exponent } iJ[ x < 0 t h e n b e a i n ft .= - 0 put(f); x := -x OD~ e l s e b~_q~n ft :: " "; ~ut(f) end; e :: expo(x) * 7 7 d~AL 2 5 6 ; { "1og(2) } if e >: 0 then ~eq!n e :: e+1; x := x/ten(e) e n d e l s e x := x * t e n ( - e ) ; w h i l e x < 0,I d o b~_qin x := 1 0 . O - x ; e := e-1 end ; •

•

•

•

}

}

125

{ 0 . 1 <= x < 1.0 } case n o£ { rounding } 2: x := x+O.5e-2; 3: x := x+O.5e-3; 4 : x :-- x + O . 5 e - 4 ; 5: x := x + O . 5 e - 5 ; 6: x := x + O . S e - 6 ; ?: x := x + O . S e - ? ; U: x := x + O . 5 e - 8 ; 9: x := x + O . S e - 9 ; 1 0 : x := x + O . S e - l O ; 11: x := x+O.Se-11; 12: x := x+O.5e-12; 13: x := x+O.5e-t3; 14: x := x+O.5e-14 e~ ; if x >= 1.0 ~hen pekin x := x * 0 . 1 ; e := e + 1 end ; c := trunc(x~eait48); c := 10-c; d := c d~v t48; ff := chr(d+z); put(f); ff := " "; put(f); f o ~ i := 2 J&~ n d o begLn c := ( c " - d . t 4 8 ) * 10; d := c d ~ v t 4 8 ; f~ := c h r ( d + z ) ; put(f) end ; f~ := "e'; put(f); e := e - l ; i£ e < 0 then beoin f~ •. = " - : p u t ( f ) ~ -= -~; end else beoin ft := ~ "+ ; put(f) ~3d; el := e * 205 div 2048; e2 := e - 10{-el; eO : = e l * 2 0 5 d&3L 2 0 4 8 7 et := el - lO~eO; ff := chr(eO+z); put(f), ff := chr(el+z); put(f); ff := chr(e2+z); put(f) end end =rid { wr }

;

~NOEX

When a Appendix

reference A), then

xl xl is which chapter figure;

in this index is not a section name ( e . g . the #e£erence may be o f t h e f o l l o w i n g forms:

xl.x2

xl.x2.x3

always the chapter n u m b e r , x 2 may b e a c a p i t a l letter in case i t may be f o l l o w e d by x3, a number, and refer~s to s section. When x 2 i s a s m a l l letter, the reference is a when x2 is a number, the reference is a program.

alfa (PASCAL 6 0 0 0 - 3 . 4 ) 13.D.1 array types 6 assignment statement 4.A binary tree 11.A block 0 BNF d e f i n i t i o n s Appendix D Boolean 2.A case statement 4.D.2 char 2.D character sets 13.8.3 comment 1 compound statement 4.R compile# error messages 14.C . 1 , Appendix compiler options (PASCAL 6 0 0 0 - 3 . 4 ) 13.B conditional statements 4.D constant declaration part 3.C control statements (PASCAL 6 0 0 0 - 3 , 4 ) 14.A control variable 4.C.3 data types 2 declaration part 3 empty statement 4.B equivalence 2.A expression 4.A

E

i d e n t i f i e r s , table of s t a n d a r d Appendix C if s t a t e m e n t 4,D.I implication 2.A input 9.B integer 2.B I /0 12 field list ? figures after (list insertion) I0.C a l t e r n a t i v e r e p r e s e n t a t i o n of standard symhols ASCII character set (with CDC's ordering) 13.a before (list insertion) 10.b block s t r u c t u r e O.b binary tree structure 11.b CDC s c i e n t i f i c character set (with 64 elements) expressions 11.a identifier 1.a linked list 10.a syntax

diagram

of

program

structure

O.a

13 .b

13 .a

127

two s a m p l e people 7.a u n s i g n e d number 1.b file types 9 13.B .1 e x t e r n a l files (PASCAL 0000-3.4) 13.A .1 s e q m e n t e d files (PASCAL 6 0 0 0 - 3 . 4 ) 13 .D .2 r e p r e s e n t a t i o n in PASCAL 6 0 0 0 - 3 . 4 textfiles 9.A for s t a t e m e n t 4.C.3 forward r e f e r e n c e 11.C functions 11.B d e c l a r a t i o n part 3.F designator 11,D heading 11,B p r e d e f i n e d (PASCAL 6000-3.4) 13.D.2) s t a n d a r d , table of Appendix A global v a r i a b l e s 11.A goto s t a t e m e n t 4.E labels case 7.A declaration part 3.B goto 3 . B . 4~E lists (linked) 10 local v a r i a b l e s 11.A name p r e c e d e n c e 11.A notation I numbers I operator p r e c e d e n c e 18,A o p e r a t o r s , s u m m a r y of Appendix B output 9.B Pecked structures 6 parameters 11.A PASCAL 6 0 0 0 - 3 . 4 13, 14 pointer types 10 procedures 11.A d e c l a r a t i o n part 3.F 13 .A .2 external p r o c e d u r e s (PASCAL 6000-3.4) heading 11.A p r e d e f i n e d (PASCAL 6 0 0 0 - 3 . 4 ) 13.D.2 s t a n d a r d , table of Appendix A p r o c e d u r e statement 11.A programs and p r o g r a m parts beginend 4.1 bisect 11.6 complex 7.1 convert 3.1 cosine 4.5 egalfa 13.1 egfor 4.4 egrepeat 4,3 egwhile 4.2 exponentiation 4.8 expon2 11.8 examples of goto 4.E forward reference 11,C frequency count 9.1 graph1 4.9 graph2 6.2

128

inflation 0.1 insert 9.2 matrixmul 6.3 minmax 6 .I minmax2 11 ,I minmax3 11 .2 parameters 11.3 primes 8.2 pointers, construction via 10 merge two files 9 postfix 11 .4 recursivegcd 1 1 .9 roman 4 .? setop 8.1 sideffect 11 . 7 sum f i l e of real numbers 9 summing 4.6 tree traversal 11.5 program heading 3.A (PASCAL 6 0 0 0 - 3 . 4 13.B.1) record types ? repet ire statements 4.C reserved words--see word-delimiters restrictions (PASCAL 6 0 0 0 - 3 . 4 ) 13.C read. the standard procedure 12.A real 2 .C relational operator 2.A, 4 . A repeat statement 4.0.2 run-time error messages 14.C .2 scalar types 5.A schemata read a text 9.A read a text from "input" 9.A reading arbitrary number of numerical items from a textfile reading a segmented file 1 3 . A .1 reed and write a se£;mented file 13.A.1 write a text 9.A write a text x to y 9.A write a text onto "output" 9.A w r i t e a s e g m e n t e d file 13,A.I scope O separators I set o p e r a t o r s 8 set t y p e s 8 side effect 11,B standard identifiers Appendix C string 1 6 subrange types 5,B syntax diagrams Appendix D tables block structure 0 d e f a u l t v a l u e for f i e l d w i d t h 13.B.4 o p e r a t i o n s on t e x t f i l e s 9.A printer control characters 9.B, 13.B .4 special symbols I truth values 2.A

129 type--also see d a t a t y p e s declaration part 3.D 13 °D .1 predefined (PASCAL 6 0 0 0 - 3 . 4 ) 13 .B .3 s t a n d a r d (PASCAL 6 0 0 0 - 3 . 4 ) variable d e c l a r a t i o n part 3.E vocabulary I while s t a t e m e n t 4.C.I with s t a t e m e n t 7.A w o r d - d e l i m i t e r s , table of Appendix C write, the s t a n d a r d p r o c e d u r e 12.B (PASCAL 6000-3.4 13.8.4)

REPORT

~reface

to

t~e

Revised

Repot&

The language PASCAL has now been in use for several years, during which considerable experience has been gained through its use, its teaching, and its implementation. Although many r e a s o n s suggest that a language should be k e p t u n c h a n g e d as s o o n as i t has gained a user community, it w o u l d be u n w i s e to ignore this experience and to refrain from making good use of it. This Report therefore describes a revised language which included some changes suggested by t h e w o r k o f t h e l a s t t w o y e a r s . It is still of the form of the original definition, a n d in f a c t t h e changes are very few and relatively minor. They concern the following subjects : -

Constant sense of

parameters Algol 60).

are

replaced

by

value

parameters

(in

the

-

The class structure is eliminated: pointer variables bound to a data type instead of a c l a s s v a r i a b l e .

-

The handling of files variable f~ always has condition e o f ( f ) is t r u e .

-

Packed records end packed arrays are introduced. As a consequence, the type ella becomes a special c~se of a packed character array. The generalization has some consequences on the denotation o£ s t r i n g s (formely called alfa constants).

-

Programs require parameters.

a

-

All

a declaration.

Moreover. renaming

labels

of

require

is a

program

there are a few minor the powerset structure

changed defined

heading

such value

with

are

that the buffer except when t h e

external

syntactic changes, to set structure.

files

such

as

as

the

Implementation efforts on various computers have brought the problem of portability and machine independence of s o f t w a r e systems to our closer attention. Nlany of t h e a b o v e m e n t i o n e d changes, and also some additional restrictions, were adopted and imposed in the interest of program portability and machine independent definability. T h e y m a d e ~t p o s s i b l e to define almost the entire language by e set of abstract a x i o m s a n d r u l e s of inference. Such a rigorous definition is n e c e s s a r y to be a b l e to prove properties of proorams. This rigour and machine independence has n o t a b l y b e e n a c h i e v e d without sacrifice in t h e efficiency of program execution. In o r d e r to p r o v i d e a c o m m o n b a s e f o r i m p l e m e n t a t i o n s on v a r i o u s computers, the language defined in t h i s R e v i s e d R e p o r t is c a l l e d ~tand&r~ ~E~j&A~, This standard is d e f i n e d in t e r m s of t h e I S O character set. The

chapter

on

PASCAL f o r

the

CDC 6 0 0 0

computer

has

been

removed

134

from

the

Report

and r e p l a c e d

by a general

chapter

on s u g g e s t e d

standards for implementation and program interchange. This standard specifies ways to represent programs in t e r m s of a v a i l a b l e c h a r a c t e r s e t s , a n d l i s t s a n u m b e r of r e s t r i c t i o n s on the language with the i n t e n t of s i m p l i f y i n g implementations. Programs to be used on several computers where P A S C A L is a v a i l a b l e s h o u l d a d h e r e to t h i s s t a n d a r d . The two procedures Kead and ~rite together with the new procedures readln, w r i t e l n , a n d eoln h a v e b e e n i n c l u d e d in t h e set of standard procedures a n d a r e d e s c r i b e d in a new C h a p t e r 12. T h e y n o w c o n s t i t u t e a b i n d i n g s t a n d a r d for l e g i b l e input a n d output. T h e l a t t e r t h r e e a r e u s e d to c o n t r o l t h e l i n e s t r u c t u r e of text f i l e s , a n d h a v e b e c o m e n e c e s s a r y , b e c a u s e the S t a n d a r d language cannot depend on the existence of a ~ine control charccter (eol) in t h e c h a r a c t e r s e t .

~efereoces

"The Programming Language INFORMATICA, 1. 3 5 - 6 3 , (1971)o

N. W i r t h ,

"Systematisches Verlag, Stuttgart. "Systematic

Englewood C .A .R .

Hoare

and

N .

Wlrth,

Programming INFORMATICA, N.

Wirth.

"An

2,

Teubner

Prentice-Hall,

1973. Axiomatic

Language 335-355.

"The Design SOFTWARE-Practice (1971).

Programmieren",

ACTA

1972.

Prooramming",

Cliffs,

PASCAL".

Definition

PASCAL", (1973).

o£ a PASCAL and Experience,

of t h e

ACTA

Compiler", !, 309-333

135

Contents

I. I n t r o d u c t i o n 2. Summary 3.

Notation,

of the language terminology,

4. I d e n t i f i e r s ,

5.

Constant

Numbers,

and vocabulary and

Strings

definitions

6. Data type definitions 6.1. Simple types 6.2. Structured types 6.3. Pointer types

7.

D e c l a r a t i o n s and d e n o t a t i o n s 7.1. E n t i r e variables 7.2. Component v a r i a b l e s 7.3. Referenced v a r i a b I e s

of v a r i a b l e s

8. E x p r e s s i o n s 8.1. Operators U.2. F u n c t i o n d e s i g n a t o r s 9. S t a t e m e n t s 9.1. Simple statements 9.2. Structured s t a t e m e n t s 10.

Procedure declarations 10.1. Standard procedures

11,

Function declarations 11.1 Standard functions

12. Input 13.

and Output

Programs

14. A s t a n d a r d for i m p l e m e n t a t i o n p r o g r a m interehanoe 15. I n d e x

and

136

I. I n t r o d u c t i o n

The development of the l a n g u a g e ~ a s c a l is b a s e d on t w o p r i n c i p a l aims. The first is t w o m a k e a v a i l a b l e a l a n g u a g e s u i t a b l e to teach programming as a systematic discipline b a s e d on c e r t a i n fundamental concepts clearly and naturally reflected by t h e language. The second is to develop implementations of t h i s language which are both reliable a n d e f f i c i e n t on p r e s e n t l y available computers. The desire for a new language for t h e p u r p o s e of t e a c h i n g programming is due to my d i s s a t i s f a c t i o n with the present]y used major languages w h o s e f e a t u r e s and c o n s t r u c t s too often cannot be e x p l a i n e d logically and convincingly a n d w h i c h t o o o f t e n defy systematic reasoning. Along with this dissatisfaction goes my conviction that the l a n g u a g e in w h i c h t h e s t u d e n t is t a u g h t to express his ideas profoundly influences his h a b i t s of t h o u g h t and invention, and that the disorder governing these languaRes directly imposes itself onto the programming style of t h e students. There is of course plenty of r e a s o n to be c a u t i o u s w i t h t h e introduction of yet another programming language, and the objection against teaching programming in a l a n g u a g e w h i c h is not widely used and accepted has undoubtedly some justification, at least based on short term commercial reasoning. H o w e v e r , t h e c h o i c e of a l a n g u a g e for t e a c h i n g b a s e d on its w i d e s p r e a d acceptance and availability, together with the fact t h a t t h e l a n g u a g e m o s t w i d e Z y t a u g h t is t h e r e a f t e r g o i n g to be the one most widely used, forms the s a f e s t r e c i p e for stagnation in a s u b j e c t of s u c h p r o f o u n d p e d a g o g i c a l influence. I consider it therefore well worth-while to m a k e an e f f o r t to break this vicious circle, Of course a new l a n g u a g e s h o u l d not be d e v e l o p e d just for t h e sake of novelty; e x i s t i n g l a n g u a g e s s h o u l d be u s e d as a b a s i s for d e v e l o p m e n t w h e r e v e r t h e y m e e t t h e c r i t e r i a m e n t i o n e d a n d do not impede a systematic structure. In t h a t s e n s e A l g o l 60 w a s used as a basis for P a s c a l , s i n c e it m e e t s t h e d e m a n d s w i t h respect to teaching to a much h i g h e r d e g r e e t h a n any o t h e r standard language. Thus the principles of s t r u c t u r i n g , a n d in fact t h e f o r m of e x p r e s s i o n s , a r e c o p i e d f r o m A l g o l 60, It w a s , however not d e e m e d a p p r o p r i a t e to a d o p t A l g o l 60 as a s u b s e t of Pascal: certain construction princ~p!e~.particulerly t h o s e of declarations, w o u l d have b e e n i n c o m p a t i b l e with those allowin~ a natural and convenient representation of t h e a d d i t i o n a l features of P a s c a l , The main extensions relative to A l g o l 60 lie in t h e d o m a i n of data structuring facilities, s i n c e t h e i r l a c k in A l g o l 60 was considered as t h e p r i m e c a u s e for its r e l a t i v e l y n a r r o w r a n g e of applicabil~ty. The introduction of r e c o r d and f i l e s t r u c t u r e s should make it p o s s i b l e to s o l v e c o m m e r c i a l t y p e p r o b l e m s w i t h Pascal, or at least to e m p l o y it s u c c e s s f u l l y to d e m o n s t r a t e such problems in a p r o g r a m m i n g course.

137

2.

Summary

of

the

language

An algorithm or computer program consists of t w o e s s e n t i a l parts, a description o£ ~tiw_o_& which are to be performed, and e description of the #ata, which are manipulated by these actions. Actions are described by so-called &~q~_Ements, and data are described by so-called ~clara~on~ and ~#finitions.

The data are represented by values of variables. Every variable occurring in a statement must be introduced by a ~ a r i a b l ~ ~ ~ o n which associates an i d e n t i f i e r and a data type with that variable. The ~at& ~voe essentially defines t h e s e t of v a l u e s w h i c h m a y be a s s u m e d by t h a t v a r i a b l e . A d a t a t y p e m a y in P a s c a l be e i t h e r d i r e c t l y described in t h e v a r i a b l e declaration, or it m a y be r e f e r e n c e d by a t y p e i d e n t i f i e r , in w h i c h c a s e t h i s identifier m u s t be d e s c r i b e d by an e x p l i c i t ~ v o e d e f i n i t i o n . The basic data types are the &calar types. Their definition indicates an o r d e r e d s e t of v a l u e s , i.e. introduces identifiers standing for each value in t h e s e t . A p a r t f r o m t h e d e f i n a b l e scalar types, there exist four ~tandar~ #asi& ~voes : ~oolean, integer, ghar, and ~ealo Except for t h e t y p e B o o l e a n , their values are not d e n o t e d by i d e n t i f i e r s , but instead by n u m b e r s and quotations respectively. These are syntactically distinct from identifiers. The set of values of type char is t h e character set available on a p a r t i c u l a r installation.

A type indicating

may also be d e f i n e d as a subranoe of a scalar type the smallest and the largest value of the subrange.

by

~truc[ured ty_&es are defined by d e s c r i b i n g t h e t y p e s of t h e i r components and by i n d i c a t i n g a ~tructuriQ&__metho~. The various structuring methods d i f f e r in t h e s e l e c t i o n mechanism s e r v i n g to select the components of a v a r i a b l e of t h e s t r u c t u r e d t y p e . In Pascal, there are four basic structuring methods available: array structure, record structure, set structure, and file structure. In an ~_~av ~tructure, all components are of the same type. A component is s e l e c t e d by an a r r a y s e l e c t o r , or ~ U ~ m u t a b l e i~!, whose type is i n d i c a t e d in t h e a r r a y t y p e d e f i n i t i o n and which must be s c a l a r . It is u s u a l l y a programmer-defined scalar type, or a subrange of the type integer. Given a value of the index type, an array selector yields a v a l u e of t h e c o m p o n e n t type. Every array variable can therefore be r e g a r d e d as a m a p p i n g o f the index type onto the component t y p e . T h e t i m e n e e d e d for a selection d o e s not d e p e n d on t h e v a l u e of t h e s e l e c t o r (index). The array structure is therefore celled a ~ando~-~£~_~ess ~Jj2u c t ur_~. In a ~#_¢_~d s ~ _ ~ u c t u r e , the components (called ~ield&) are not necessarily of the same type. In order t h a t t h e t y p e of a selected component be evident from the program text (without executing the program), a record selector is n o t a c o m p u t a b l e value, but instead is an identifier uniquely denoting the component to be selected. These component identifiers are

138

declared in the record type definition. Again, the time needed to access a selected component does not d e p e n d on t h e s e l e c t o r , a n d t h e r e c o r d is t h e r e f o r e a l s o a r a n d o m - a c c e s s structure. A record type may be specified as consisting of several ~ariaots. This i m p l i e s t h a t d i f f e r e n t v a r i a b l e s , althought said to be of t h e s a m e t y p e . may a s s u m e s t r u c t u r e s w h i c h d i f f e r in a c e r t a i n m a n n e r . T h e d i f f e r e n c e m a y c o n s i s t of a d i f f e r e n t n u m b e r and d i f f e r e n t t y p e s of c o m p o n e n t s . The v a r i a n t w h i c h is a s s u m e d by t h e c u r r e n t v a l u e of a r e c o r d v a r i a b l e m a y be i n d i c a t e d by a component field which is c o m m o n to a l l v a r i a n t s a n d is c a l l e d the ~aofie~. Usually. the p a r t c o m m o n t o all v a r i a n t s w i l l c o n s i s t of s e v a r a l c o m p o n e n t s , including the tag field. A ~ u c t u r e defines of its b a s e t y p e . i.e. base type. The base type be a programmer-defined integer.

t h e s e t of v a l u e s w h i c h is t h e p o w e r s e t t h e s e t of a l l s u b s e t s of v a l u e s of t h e m u s t be a s c a l a r t y p e . a n d w i l l u s u a l l y s c a l a r t y p e or a s u b r a n g e of the t y p e

A ~ile_~ruc~_W~ is a ~W_g3&~O~e of c o m p o n e n t s of t h e s a m e t y p e . A natural ordering of the components is defined t h r o u g h the sequence. At any instance, only one component is d i r e c t l y accessible. The other components are made accessible by progressing sequentially t h r o u g h t h e f i l e . A f i l e is g e n e r a t e d by sequentially a p p e n d i n g c o m p o n e n t s at its e n d . C o n s e q u e n t l y . the file type definition does not determine t h e n u m b e r of components, Variables declared in e x p l i c i t d e c l a r a t i o n s are called ~atic, The d e c l a r a t i o n a s s o c i a t e s an i d e n t i f i e r w i t h t h e v a r i a b l e w h i c h is u s e d to r e f e r to t h e v a r i a b l e . In e o n s t r a t , v a r i a b l e s may be generated by an e x e c u t a b l e s t a t e m e n t . Such a ~ m i ~ generation yields a so-called ooi~te~ (a subtstitute for an e x p l i c i t identifier) which subsequently s e r v e s to r e f e r to t h e v a r i a b l e . This pointer may be assigned to other variables, namely variables of type oointer. Every pointer variable may assume values p o i n t i n g to v a r i a b l e s of t h e s a m e t y p e T o n l y , a n d it is said to be h~d to t h i s t y p e T. It m a y , h o w e v e r , a l s o a s s u m e the value ~ . which points to no v a r i a b l e . B e c a u s e p o i n t e r variables may a l s o o c c u r as c o m p o n e n t s of s t r u c t u r e d variables, which are themselves dynamically # e n e r a t e d , t h e u s e of p o i n t e r s permits the representation of f i n i t e B r a p h s in full g e n e r a l i t y . The most fundamental statement is the ~ i ~ m e n t _ ~ a t e m e n t . It specifies t h a t a n e w l y c o m p u t e d v a l u e be a s s i g n e d to a v a r i a b l e (or components of a variable). The value is obtained by evaluating an ~xoressio~. Expressions consist of v a r i a b l e s , constants, sets, operators and functions operating on the denoted quantities and producing new values. Variables, constants, and f u n c t i o n s a r e e i t h e r d e c l a r e d in t h e p r o g r a m or are standard e n t i t i e s . P a s c a l d e f i n e s a f i x e d s e t of o p e r a t o r s , each of w h i c h can be r e g a r d e d as d e s c r i b i n g a m a p p i n g from t h e operand types into the result t y p e . The set of o p e r a t o r s is subdivided i n t o g r o u p s of I.

~r_~t hM @~ ie

operators

of

addition,

subtraction,

sign

139 multiplication,

inversion, remainder. 2.

~olean

operators

of

division,

negation,

union

and

(or),

computinc

and

the

conjunction

(and). 3.

~et

~oerators

4.

~e!atiQ~L~ membership operations

of u n i o n ,

intersection,

and

set

Doerato~s of e q u a l i t y , i n e q u a l i t y , and set inclusion. The results are of t y p e ~ o o l e a ~ .

difference. ordering, set of r e l a t i o n a l

The & r o ~ ~ ~ causes the execution of the designated procedure (see below). Assignment and procedure statements are the components or building blocks of ~ u c t u r e d ~tatemeots, which specify sequential, selective, or r e p e a t e d e x e c u t i o n of their components. Sequential execution of statements is specified by the ~omeo~ ~tatement, conditional or s e l e c t i v e execution by the if &tatemen[ and the ~&~_~ ~ t a t e m e o t , and repeated e x e c u t i o n by t h e ~ # # e ~ t ~ t a t e m e o t , t h e ~ h i ! e &tatemeoj~, and the ~ &~atement. The if s t a t e m e n t s e r v e s to m a k e t h e execution of a statement d e p e n d e n t on t h e v a l u e of a O o o l e a n expression, and the case statement allows for the selection among many statements according to t h e v a l u e of a s e l e c t o r . The for statement is used when t h e n u m b e r of i t e r a t i o n s is k n o w beforehand, and the repeat and while statements are used otherwise. A statement can be g i v e n a n a m e ( i d e n t i f i e r ) , a n d be r e f e r e n c e d through that identifier. The statement is then called a orocedur_~, and its d e c l a r a t i o n a ~rocedur~ ~laration. Such a declaration may additionally contain a set of variable declarations, type definitions and further procedure declarations. The v a r i a b l e s , types end procedures thus declared can be referenced only within the procedure ~tself, and are therefore c a l l e d ~ o c a l to t h e p r o c e d u r e . Their identifiers have significance only within the program text which constitutes the procedure declaration and which is c a l l e d t h e & ~ o o e of t h e s e identifiers. Since procedures may be d e c l a r e d l o c a l to o t h e r procedures, s c o p e s may be n e s t e d . E n t i t i e s w h i c h a r e d e c l a r e d in the main p r o g r a m , i.e. not l o c a l to s o m e p r o c e d u r e , are called ~lobal. A procedure has a f i x e d n u m b e r of p a r a m e t e r s , each of which is denoted within the procedure by an i d e n t i f i e r called the ~ a l ~arameter. Upon an activation of the procedure statement, an actual quantity has to be i n d i c a t e d for e a c h parameter which can be referenced from within the procedure through the formal parameter. T h i s q u a n t i t y is c a l l e d t h e #ctu@.~ ~aram~j~W_~, There are three kinds of parameters: value parameters, variable parameters, and procedure or function parameters. In the first case, the actual parameter is an expression which is evaluated once. The formal parameter represents a local variable to which the result of this evaluation is a s s i g n e d b e f o r e t h e e x e c u t i o n of t h e p r o c e d u r e (or function). In the case of a variable parameter, the actual parameter is a v a r i a b l e a n d t h e f o r m a l p a r a m e t e r s t a n d s for t h i s variable. P o s s i b l e i n d i c e s a r e e v a l u a t e d b e f o r e e x e c u t i o n of t h e procedure (or function). In t h e c a s e of p r o c e d u r e or f u n c t i o n

140 parameters, identifier.

the

actual

parameter

is

a

procedure

or

function

~unctions are declared analogously to procedures. The o n l y difference lies in the fact that a function yields a result which is confined to a scalar or p o i n t e r t y p e a n d must be specified in t h e f u n c t i o n d e c l a r a t i o n . F u n c t i o n s may t h e r e f o r e be used as c o n s t i t u e n t s of e x p r e s s i o n s . In o r d e r to e l i m i n a t e side-effects, assignments to non-local variables should be avoided within function declarations.

3. N o t a t i o n ,

terminology,

and

vocabulary

According to traditional Backus-Naur form. syntactic constructs are denoted by English words enclosed between the angular brackets < and > . These w o r d s also d e s c r i b e t h e n a t u r e or meaning of the construct, and are used in the accompanying description of s e m a n t i c s . P o s s i b l e r e p e t i t i o n of a c o n s t r u c t is indicated by e n c l o s i n g t h e c o n s t r u c t w i t h i n m e t a b r a c k e t s { and } . The s y m b o l < e m p t y > d e n o t e s t h e null s e q u e n c e of s y m b o l s . The basic classified



::=

vocabulary of Pascal consists into letters, digits, and special

of basic symbols.

symbols

AI~IclDIEIF1GIHIIIJIKILIMINIOIPIQIRISlTlUlVl wtxlYlZlalblcldlelflgthliljlklllmlnlolPlqlrl sltlulvlwlxlylz

: : = OI 11213141 5161 7 1 8 1 9 : := + II * I t I = 1 <> I < t > t <= t >= I ( I ) I [ 1 ] 1 { I } I := I • 1, 1 : i : I " t f I di_v. I mad I nil I i n I o n I a n d I not I i f I ~ a ~ n f a S s e i ca.~e 1 o£ I r_eoeat I until I while I do I fan I to 1 dg_~DJ~o I b e a i n I e n d I wiJih I ~ o t o I ~ o n s t I v~j~ I t y p e I a r r a y ~ ~#-~J&r-~ I s£.i I f i l e I f u n c t i o n I P£_Qced~u~e I l a b e l I ~acd2#_~ ~ ~ m o ~ r a m The c o n s t r u c t { < a n y s e q u e n c e of s y m b o l s not c o n t a i n i n g "}"> } may be i n s e r t e d b e t w e e n a n y t w o i d e n t i f i e r s , n u m b e r s (cf. 4), or special s y m b o l s . It is c a l l e d a ~ o m m e n t and may be r e m o v e d from the p r o g r a m t e x t w i t h o u t a l t e r i n g its m e a n i n g . The s y m b o l s { and } do not o c c u r o t h e r w i s e in t h e l a n g u a g e , a n d w h e n a p p e a r i n g in syntactic descriptions they are m e t e - s y m b o l s l i k e I and ::= The s y m b o l p a i r s (~ and *) a r e u s e d as s y n o n y m s for { a n d } .

4. I d e n t i f i e r s ,

Numbers,

and

Strings

Identifiers serve to denote constants, types, variables, procedures and functions. Their association must be u n i q u e within their scope of v a l i d i t y , i.e. w i t h i n t h e P r o c e d u r e or function in which they are declared (of. 10 a n d 1 1 ) .

141

::= < l e t t e r > { < l e t t e r Or d i g i t > } : := < l e t t e r > I The usual decimal notation is u s e d constants of the data types inteoer letter E preceding the scale factor to t h e p o w e r o f " .

for numbers, which are the and ~&l (see 6.1.2.). The is p r o n o u n c e d as " t i m e s 10

::= < d i g i t > { < d i g i t > } : := < d i g i t s e q u e n c e > ::= < u n s i g n e d integer>. .E E ::= < u n s i g n e d integer> I ::= < u n s i g n e d integer> I ::= + I Examples 1

:

100

Sequences &~r_i~g.&. constants Consisting the types

O. I

8 7 . 3 5 E +8

of characters enclosed by quote mmrks a r e c a l l e d Strings consisting of a single character are the of the standard type char (see 6.1.2). Strings of n (>1) e n c l o s e d c h a r a c t e r s are the constants of (see 6.2.1)

&ac~.~t array Note:

5E-3

[1..hi

aZ

char

I f t h e s t r i n g is t o c o n t a i n a quote m a r k is to be w r i t t e n twice.

Examples

: :=

"{

mark,

}

then

this

quote

'

: "A' °; . . . . . 'PASCAL" °TI{IS

5. C o n s t a n t

A constant constant.

IS

A STRING

"

definitions

definition

introduces

an

identifier

as

a synonym

::= < i d e n t i f i e r > ::= < u n s i g n e d number> ~ ] I ::= < i d e n t i f i e r > =

to

a

I

142

6. D a t a

type

definitions

A data t y p e d e t e r m i n e s t h e set t y p e m a y a s s u m e and a s s o c i a t e s

of v a l u e s w h i c h v a r i a b l e s of that an i d e n t i f i e r w i t h t h e t y p e .

< t y p e > ::= < s i m p l e t y p e > I < s t r u c t u r e d t y p e > < t y p e d e f i n i t i o n > ::= < i d e n t i f i e r > = < t y p e >

6.1.

type>

S i m o Z ~ ~y_a_~


type>

::= < s c a l a r t y p e > I < s u b r a n g e i d e n t i f i e r > ::= < i d e n t i f i e r >

Sr~ar

type defines identifiers which


type>

::=

an o r d e r e d s e t of v a l u e s denote these values. (

I

to all

scalar

enumeration

types

of

)

Friday.

(except

real)

are

:

t h e s u c c e e d i n g v a l u e (in t h e e n u m e r a t i o n ) t h e p r e c e d i n g v a l u e (in the e n u m e r a t i o n )

SUCC

pred 6.1.2.

applying

by

{.}

Examples: (red, o r a n g e , y e l l o w , g r e e n , blue) (club. d i a m o n d , h e a r t , s p a d e ) (Monday j Tuesday, Wednesday, Thursday, Saturday, Sunday) Functions

type>

types

A scalar

the

I
~&&odard

The f o l l o w i n g

~voes

types

are s t a n d a r d

in P a s c a l :

integer

The values are a subset of t h e w h o l e n u m b e r s d e f i n e d by i n d i v i d u a l i m p l e m e n t a t i o n s . Its v a l u e s are the i n t e g e r s (see 4).

real

Its values are a subset of the r e a l n u m b e r s depending on the p a r t i c u Z a r i m p l e m e n t a t i o n . The v a l u e s a r e d e n o t e d by r e a l n u m b e r s (see 4).

Boolean

Its values identifiers

char

Its v a l u e s are a set of c h a r a c t e r s d e t e r m i n e d by particular implementations. T h e y are d e n o t e d by the c h a r a c t e r s t h e m s e l v e s e n c l o s e d w i t h i n q u o t e s .

are true

the truth and f a l s e .

values

denoted

by the

t43

6.1.3.

~ubr~noe

types

A type may be defined as a subrange of indication of the least and the largest The first constant specifies the lower greater than the upper bound.
type>

1..100 -10 .. Nonday

Examples:

6.2.

::=

~#_gJ~red



..

another scalar type by value in the subrange. bound, and must not be



+10 .. Friday

types

A structure type is characterised by the type(s) of its components and by its structuring method. Moreover, a structured type definition may c o n t a i n an indication of the preferred data representation. If a definition is prefixed with the symbol packed, this has no effect on the meaning of a program, but is a !lint to the compiler that storage should be economised even at the price of some loss in efficiency of access, and even if this may expand the code necessary for expressin~ access to components of the structure.
type> ::= lacked ::= I I I

6.2.1.

~rrav

I

types

An array type is a structure consisting of a f i x e d n u m b e r of components w h i c h a r e a l l of t h e s a m e t y p e , c a l l e d t h e ~£_W_EJ&oeot ~yoe. The elements of the array are designated by i n d i c e s , values belonging to the so-called index ~p_~. The array type definition specifies the component t y p e as w e l l as t h e i n d e x type.


type>

::=

::= If n index ~-_~_j~O~&na~, Examples:

arra~ [ ::=

{,
types ere specified, the array and a component is designated by #rray #r~av #tray

[ 1..100] of real [1..10,1..20] of 0..99 [Boolean] ~[ color

type>}]

type is n indices.

~

called

144

6.2.2.

_Rec~L~__J~vo e&

A record type is a structure consisting of a fixed number of components, possibly of different types. The record type definition specifies for each component, called a _f~, its type and an identifier which denotes i t . T h e s c o p e of t h e s e so-called f ~ identifiers is t h e r e c o r d definition itself, and they are also accessible within a field designator (ef. 7.2) referring to a r e c o r d v a r i a b l e of t h i s t y p e . A record type may have several variants, in w h i c h c a s e a c e r t a i n field m a y be d e s i g n a t e d as t h e ]2~_q £ ~ e l d , w h o s e v a l u e i n d i c a t e s which variant is a s s u m e d by t h e r e c o r d v a r i a b l e at a g i v e n t i m e . Each variant structure is i d e n t i f i e d by a c a s e l a b e l w h i c h is a constant o f t h e t y p e of t h e t a g f i e l d .

::= E e e o r d end ::= < f i x e d p a r t > I ; ::= < r e c o r d s e c t i o n > [ ;} : := { ,} : I ::= ~ a s e < t a g f i e l d > < t y p e i d e n t i f i e r > of { ;} ::= < c a s e l a b e l l i s t > : () I ::= { ,} : := ::= < i d e n t i f i e r > : I

Examples:

rWcor~[

I

day: 1..31; month: 1 ,,12; y e a r : Inte~qer

en~ record

name, firstname: age : 0..99: married: Boolean

alfa;

and record

x,y : real; area : real; ~O~ s : shape o~ trlangle: (side : real; inclination, angle1, angle2: rectangle: (sidel, skde2: real; skew, angle3: angle) ; circle: (diameter: real) and

6.2.3.

angle);

_Se~,__t~y_p_es

A set type defines the range of values which is the oowerset of its so-called ~ ~Y.~Za. Base types m u s t not be s t r u c t u r e d types. Operators applicable to a l l s e t t y p e s a r e :

145

+

union set difference intersection membership

-

* iS

The s e t d i f f e r e n c e x-y x w h i c h a r e not m e m b e r s

is d e f i n e d of y,

as

the

set

of

all

elements

of

::= s e t o f < b a s e t y p e > < b a s e t y p e > ::= < s i m p l e t y p e >

6.2.4.

Lila__~yoes

A file type definition specifies sequence of components which are number of components, called the fixed by t h e file t y p e d e f i n i t i o n , called ~m&&Z.
type>

::= f i l ~

~Z

a structure consisting of a all of t h e s a m e t y p e , T h e J~@/3~th o f t h e f i l e , is not A file with 0 components is



Files with component type char are called ~extfilw_&, end are a special case insofar as t h e c o m p o n e n t r a n g e o f v a l u e s m u s t be considered as extended by a m a r k e r d e n o t i n g the end of a line. This marker allows textfiles t o be s u b s t r u c t u r e d into lines. The t y p e ~82£~ i s a s t a n d a r d type predeclared as

~X~

6.3.

text

= ~ile

~f

char

~ o i n t ~ r tvoes

Variables which are declared in a program (see 7.) are accessible by their identifier, They exist during the entire execution process of the procedure (scope) to which the variable is local, and these variables are therefore celled stati~ (or statically allocated), In contrast, variables may also be generated dynamically, i,e, without any correlation to t h e structure of t h e p r o g r a m , T h e s e ~ z n a m i c v a r i a b l e s are generated by the s t a n d a r d p r o c e d u r e ~#j~ (see 1 0 . I . 2 , ) : s i n c e t h e y do not occur in an explicit variable declaration, they c a n n o t be referred to by a name. Instead. access is achieved via a so-called &D.i~ v a l u e w h i c h is p r o v i d e d u p o n g e n e r a t i o n of t h e dynamic variable, A p o i n t e r t y p e t h u s c o n s i s t s o £ an u n b o u n d e d set of values pointing to elements of the same t y p e , No operations a r e d e f i n e d on p o i n t e r s e x c e p t t h e t e s t for e q u a l i t y ,

The pointer to no e l e m e n t
value ~ll at all, type>

belongs

::= ~ < t y p e

to

every

identifier>

pointer

type;

it p o i n t s

146

Examples

of

type

color sex text shape card alfa complex p arson

= = -= = = = =

definitions: (red, yellow, green, blue) (male, female) f~le ~_~ c h a r (triangle, rectangle, circle) ~rray [1..80] o~ char ~ a e k e d ~&rJ~eJL [ 1 , . 1 0 ] of char record re.ira: real ~nd 2ecord name, firstname: ella; age : integer; married :Boolean; father, child, sibling: fperson: case s : sex of male: (enlisted, bold: Boolean); female: (pregnant : Boolean; size: arrav[1..3] of i n t e g e r ) ea~

7. D e c l a r a t i o n s

denotations

of variables

Variable declarations consist the new variables, followed by

of e l i s t of their type.


and

declaration>

::=

identifiers

{.}

denoting

:

Every declaration of a f i l e v a r i a b l e f with components of t y p e T implies the additional declaration of a so-called ~uffer ~ariable of type T. This buffer variable is d e n o t e d by ft a n d serves to append components to t h e f i l e d u r i n g g e n e r a t i o n , and to access the file during inspection (see ? . 2 . 3 . a n d I 0 . I . 1 . ) . Examples: X ,y ,Z : r e a l u,v: complex i,j: integer k: 0 . . 9 p ,q : B o o l e a n operator : (plus, minus, a : ~rJ&~L[O,.63] ~ real b: ~ r a y [ c o l o r , B o o l e a n ] c: c o l o r f: f&l~ of char huel,hue2: & a t O~ c o l o r pl p2: ~person

times) £~

complex

Denotations of v a r i a b l e s either designate an e n t i r e v a r i a b l e , a component of a variable, or a v a r i a b l e referenced by a p o i n t e r (see 6.3). Variables occurring in examples in subsequent chapters a r e a s s u m e d t o be d e c l a r e d as i n d i c a t e d above,

::=

I

variable>

I

147

7.I. An

Entlre~riables

entire

variable

is

denoted

::=

7.2.

by

its

identifier.

::= < i d e n t i f i e r >

C prop o n ~ n t _ _ ~ a r i a b l e ~

A component a selector depends on

of a v a r i a b l e is d e n o t e d by t h e v a r i a b l e specifying the component. The form of the structuring type of the variable.

::=

?.2.1.

i~g~gd


variable> buffer>

n-dimensional array variable by n i n d e x e x p r e s s i o n s .

::= [ ::= < v a r i a b l e >

Examples

I

v~riables

A component of an variable followed

The types index types

followed by the selector

of the declared

index in t h e

expressions definition

is

denoted

by

the

must correspond with of t h e a r r a y t y p e .

the

{,}]

:

a [ 12] a[i+j] b [ r e d ,true]

?.2.2.

_Field

desionator~

A component of variable followed

a record variable is by t h e f i e l d i d e n t i f i e r

Examples

:

U .re

hired,true] p 2~ .s i z e

.im

denoted by t h e r e c o r d of t h e c o m p o n e n t .

::= < r e c o r d variable>. ::= < i d e n t i f i e r >

identifier>

148

?.2.3,

Eile

~uffers

At any time, o n l y the one c o m p o n e n t d e t e r m i n e d by the c u r r e n t file position (read/write head) is d i r e c t l y a c c e s s i b l e . This component is called the current file component and is r e p r e s e n t e d by t h e f i l e ' s ~ f _ ~ ~ariable.
b u f f e r > ::= ~ v a r i a b l e > : := < v a r i a b l e >

7.3. ~ ~ e d

variables

< r e f e r e n c e d v a r i a b l e > ::= < p o i n t e r < p o i n t e r v a r i a b l e > ::= < v a r i a b l e >

variable>t

If p is a pointer variable which is b o u n d to a t y p e T , p denotes t h a t v a r i a b l e a n d its p o i n t e r v a l u e , w h e r e a s p~ d e n o t e s the v a r i a b l e of t y p e T r e f e r e n c e d by m. Examples: p~ .father p I~ .sibling~ .child

8. E x p r e s s i o n s

Expressions are constructs denoting r u l e s of c o m p u t a t i o n for obtaining values of v a r i a b l e s and g e n e r a t i n g new v a l u e s by t h e application of o p e r a t o r s . E x p r e s s i o n s c o n s i s t o f o p e r a n d s , i.e. v a r i a b l e s and c o n s t a n t s , o p e r a t o r s , and f u n c t i o n s . The r u l e s of c o m p o s i t i o n s p e c i f y o p e r a t o r ~ 2 ~ £ ~ # ~ & according to four c l a s s e s of o p e r a t o r s . The o p e r a t o r ~ J 2 has the h i g h e s t precedence, followed by the so-called multiplying operators, then the so-called adding operators, and finally, with the lowest precedence, the relational operators. Sequences of operators of the same precedence are e x e c u t e d from left to right. The rules of p r e c e d e n c e a r e r e f l e c t e d by the f o l l o w i n g syntax:

149


constant>

::= < u n s i g n e d number> I I I ~il ::= < v a r i a b l e > I 1 I I () I ~ot < s e t > ::= [ < e l e m e n t l i s t > ] < e l e m e n t l i s t > ::= < e l e m e n t > {,} I ::= < e x p r e s s i o n > I .. < t e r m > ::= < f a c t o r > I < t e r m > < m u l t i p l y i n g operator> ::= < t e r m > I I ::= < s i m p l e e x p r e s s i o n > I Expressions which a r e m e m b e r s o f a s e t m u s t a l l be of t h e s a m e type, which is t h e b a s e t y p e of t h e s e t . [] d e n o t e s t h e e m p t y set, and [x.,y] d e n o t e s t h e s e t of a l l v a l u e s in t h e i n t e r v a l X,o.y

,

Examples: Factors:

X

15 ( x +y +z ) sin (x+y) [ r e d .c , g r e e n ] [ 1,5, 10.,19,23] n~J& p x *y

Terms :

i/(1-i) p nz q (x<=y) ~[

Simple

expressions

< z )

x +y -x hue 1 + hue2 i*j + I

:

x = 1.5 p <=q (i
Expressions:

8.1.

(y

09..~r3.tors

If both operands of the arithmetic operators of addition, subtraction and multiplication are of type integer (or a subrange thereof), t h e n t h e r e s u l t is of t y p e i n t e g e r . I f o n e o f the oPerands is of t y p e r e a l , t h e n t h e r e s u l t is a l s o of t y p e real. 8.1.1.

The omer_wtor

not

150

The

operator

8.1.2.

~

denotes

~ul&iolyinQ


Ioperatorl

negation

of

its

Boolean

operand.

opera,Or&

operator>

::=

operation

~ I / I ~

type

multiplication set intersection

] m~L~t I ~ n d

of o p e r a n d s

type

of resultI

real. integer any set type T

real,

real

integer

T

/

division

real,

dlv

division with truncation

integer

integer

mod

modulus

integer

integer

and

logical

Boolean

Boolean

8,1.3.

~ddio,~__~eretors


"and"

operator>

Ioperatorl

integer

::= + I - I ~ Z

operation

I type

of o p e r a n d s

) type

of r e s u l t l

..............................................................

I 1

÷

I I I I I

I

I

I

I addition set union

I integer, real I any set type T

1 integer, ~ T

I

I

I

I subtraction I set difference

] integer, real I a n y set t y p e T

I integer, I T

I

I

I

] Ooolean

I Boolean

I

r

I

I

] logical

I

"or"

I

When used inversion,

as and

operators with one operand only, + denotes the identity operation.

~3.I.4. _ R e l a t i o n a l
I reel

I real

::=

=

I <>

I < I <=

T >=

I I

I

- denotes

ooeratQ~&

operator>

1 I

I > I in

sign

151

I

operator

I

I =

<>

T

l

<

>

l

I I

<-->=

type

operands

I

scalar

or

subrange

type

!

Boolean

I

T

I

! T

l 1

I any I and

I

t

that

result

I any

I in I

Notice

of

scalar

its set

I i

or s u b r a n g e t y p e I type respectivelyl

Boolean

I

t

all

scalar

types

I

define ordered

sets

of v a l u e s .

The operators <>. < = . >= s t a n d for unequal, less or equal,and oreater or equal respectively. The operators <= a n d >= may a l s o be used for comparing values of set type. and then denote set inclusion. tf p and q are Boolean expressions, p = q denotes their

equivalence, and that f a l s e < t r u e ) The relational compare (packed)

and then sequence

denote of t h e

8.2. E # n ~ t i o n

p

<=

q denotes

im#llcation

operators = <> < <= arrays with components

> >= may of type

alphabetical ordering according u n d e r l y i n g set of c h a r a c t e r s .

of q by p.

also char

to

(Note

be used to (strings),

the

collating

dEsianator&

A f u n c t i o n d e s i g n a t o r s p e c i f i e s the a c t i v a t i o n of a f u n c t i o n . It consists of t h e i d e n t i f i e r d e s i g n a t i n g t h e f u n c t i o n and a l i s t of actual procedures,

parameters. and

corresponding

formal

The parameters functions, and

parameters

(cf.

are variables, are substituted

9.1.2.o

I0, a n d

< f u n c t i o n d e s i g n a t o r > ::= < f u n c t i o n i d e n t i f i e r > I ({ , ::= < i d e n t i f i e r >

expressions. for the

11).

parameter>})

Sum(a .100) SCD ( 1 4 7 , k )

Examples :

s i n (x+y) eof (f) ord(ff

)

9. S t a t e m e n t s

Statements #xec~&bi&. referenced

denote algorithmic actions, They may be prefixed by by g o t o s t a t e m e n t s .

a

and are label

said which

to can

be be

152

::= I : ::= < s i m p l e statement> ::= < u n s i g n e d integer>

9,1.

~&mo!e

I statement>

statements

A simple statement is a s t a t e m e n t of w h i c h no p a r t c o n s t i t u t e s another statement. The empty statement consists of no s y m b o l s and denotes no a c t i o n . ::= < a s s i g n m e n t I ::= < e m p t y > 9.1.1.

Assignment

The variable (or the identical type, with the

x

~ro~_~dure

:=

value

:= < e x p r e s s i o n > := < e x p r e s s i o n >

function) following

of

I

and the expression m u s t be exceptions being permitted:

of the variable is real. and is i n t e g e r or a s u b r a n g e thereof. of t h e e x p r e s s i o n is a s u b r a n g e of or v i c e - v e r s a .

p i hue

9.1.2,

I

serves to r e p l a c e the current specified as an e x p r e s s i o n .

::=

Examples:

I

statemeoJza

The assignment statement a variable by a new v a l u e

I. t h e type expression 2. t h e type variable,

statement> statement>

of

the

type

of

the

the

type

of

the

y+z

:= ( I < = i ) a n d ( i < I 0 0 ) := s q r ( k ) - (i'j) := [ b l u e . s u c c (c )]

statements

A procedure statement serves to execute the procedure denoted by the procedure identifier, The procedure statement may c o n t a i n a list of a~ual ~aramet~a which are substituted in place of their corresponding ~orma~ ~ar~et@~S defined in the procedure declaration (cf. 10). The correspondence is established by the positions of the parameters in the lists of actual and formal Parameters respectively, There exist four kinds of parameters: so-called value parameters, variable parameters, procedure parameters (the a c t u a l p a r a m e t e r is a P r o c e d u r e identifier), and function parameters (the actual parameter is a function identifier). In an

the case expression

of

a ~alue ~jc_~m__~, the (of which a variable

actual parameter m u s t be is a simple case). The

153

corresponding formal parameter represents a l o c a l v a r i a b l e of t h e c a l l e d p r o c e d u r e , a n d t h e c u r r e n t v a l u e of t h e e x p r e s s i o n is initially assigned to t h i s v a r i a b l e . In t h e c a s e of a ~ l e ~arameter, the actual parameter must be a v a r i a b l e , and the corresponding formal parameter represents this actual variable during the e n t i r e e x e c u t i o n of t h e p r o c e d u r e . If t h i s v a r i a b l e is a component of an array, its index is evaluated when the procedure is called, A variable parameter must be used whenever the parameter represents a result of the procedure. Components of a packed variable parameters.

structure

must

not

appear

as

actual

::= < p r o c e d u r e i d e n t i f i e r > I ( {,}) ::: < i d e n t i f i e r > ::: < e x p r e s s i o n > I I I Examples :

9.1.3.

next Transpose(a on,m) Bisect (fct.-1.0,+1.0.x)

_Onto s t a t e m e n t s

A gore statement serves to indicate should continue at a n o t h e r p a r t of the t h e p l a c e of t h e l a b e l .
statement>

The following labels:

:::

£ot£

restrictions

hold

I. T h e scope of a label is defined, it is therefore procedure. 2. E v e r y label heading of statement.

9.2.

must the

that further processinc p r o g r a m t e x t , n a m e l y at

concerninD

the not

the

procedure possible

applicability

w i t h i n w h i c h it to jump into

of

is a

be s p e c i f i e d in a l a b e l d e c l a r a t i o n in t h e procedure in which the label marks a

~r_w_~t~@3!~tatemen~:

Structured statements are constructs composed of other statements which have to be executed either in sequence (compound statement), conditionally (conditional statements), or repeatedly (repetitive statements). ::: < c o m p o u n d s t a t e m e n t > I I

I

154

9,2.1,

~I~Ol&~Dd

statements

The compound s t a t e m e n t s p e c i f i e s t h a t its c o m p o n e n t s t a t e m e n t s are to be e x e c u t e d in t h e s a m e s e q u e n c e as t h e y a r e w r i t t e n . The s y m b o l s ~ . ~ & ~ a n d ~031 act as s t a t e m e n t b r a c k e t s ,
statement>

Example: 9.2.2,

~

z

::= ~ _ E i n := x

; x



:= y;

y

{ ;}

~

:= z ~

~&~_~tional_~J~.O~

A conditional statement its c o m p o n e n t s t a t e m e n t s .

selects

for

execution

a single

one

of

::= I

9 , 2 , 2 . I , If__~&~d~ements The if s t a t e m e n t s p e c i f i e s t h a t a s t a t e m e n t be e x e c u t e d only if a. c e r t a i n condition (Boolean expression) is t r u e . If it is false, then either no statement is to be e x e c u t e d , or t h e statement f o l l o w i n g t h e s y m b o l ~ I s ~ is to be e x e c u t e d .
statement> ::= & ~ ~ ~

The e x p r e s s i o n Boolean.

between

~ote: The syntactic i~

is

< e x p r e s s i o n > &b~l! < s t a t e m e n t > I ~l&~

the

ambiguity

symbols

arising

~b_~i i L ~

resolved

by

interpreting

~ ~Uia ~e~iA i~ end

~

from

and ~ h e &

the

iheo

construct

be of t y p e

construct



the

must

~

as





equivalent

~.~

to



Examples : i~ if

9,2,2,2.

~ e

x < 1 . 5 t h e n z := x + y e l s e z : = p 1 <> n j ! &he~l p I : = p I f . f a t h e r

1 .5

stat@ments

The case statement c o n s i s t s of an e x p r e s s i o n (the s e l e c t o r ) a n d a list of s t a t e m e n t s , e a c h b e i n g l a b e l l e d by a c o n s t a n t of t h e type of the selector, It s p e c i f i e s t h a t t h e o n e s t a t e m e n t be executed whose label is equal to the c u r r e n t v a l u e of the

155 selector

.

::= ~ , ~ D~ { ;~ ~A~ ::= < c a s e l a b e l l i s t > : < c a s e l a b e l l i s t > ::= < c ~ s e l a b e l > { , < c a s e label> ] E x a m p 1as : c & s e o p e r a t o r of" plus : x := x+y ; m i n u s : x := x - y ; t i m e s : x := x * y end

case 1: 2: 3: 4:

i of x := x := x := x :=

1

sin(x); cos(x); exp(x); ln(x)

LoAd

9.2.3.

~#3z~ti~ve

s ~ t ~ & ~

Repetitive statements specify that certain statements a r e t o be executed repeatedly. If the number of repetitions is k n o w n beforehand, i.e. before the repetitions are started, the for statement is the appropriate construct to express this situation; otherwise the while or r e p e a t s t a t e m e n t s h o u l d be used . ::= < w h ~ l e s t a t e m e n t > [

9.2.3.1o

[

~ile__~tatemen~&


statement>

::= ~ , ~ i £



d£



The expression controlling repetition m u s t be of t y p e g o o l e a n . The statement is repeatedly executed until the expression becomes false. I£ its value is f a l s e at t h e b e g i n n i n g , the statement is not e x e c u t e d at a 1 1 . T h e w h i l e s t a t e m e n t ~hile is

B do

equivalent

i£

S

to

B £hea be£in 8 ; wbile end

B do

S

156 Examples: whil~

a[i]

while beoin

i > O ~j& if odd(i) then i := i ~ L ~ 2:

x

<> x d o

i

:= i + I

z

:= z ' x ;

:= s q r ( x )

and ~]3ile ~ e e l ( f ) da P ( f t ); g e t ( f )

~W~O

9.2.3.2.

~931eat

statemeoj&&

: := reoeat

{ :}

until



The expression controlling repetition m u s t be o f t y p e B o o l e a n . The sequence of statements between t h e s y m b o l s [9~2eat a n d w n t i ~ is repeatedly executed (and at l e a s t o n c e ) u n t i l t h e e x p r e s s i o n becomes true. The repeat statement reoeat is

S ~il

equivalent

B

to

beoin S : i f no*. B t h e n r~oeat S un~il end

B

E x a m p les : zeoeal

~.~.~l

:= i m e ~ := j ; := k j = O

z~ea~ ~Dtil

eof(f)

9.2.3.3.

k i j

P(ft);

J:

get(f)

Egj~_~_~t e m e n t s

The for statement indicates executed while a progression w h i c h is c a l l e d t h e c o o ~ _ ~

that a statement is t o be r e p e a t e d l y of v a l u e s is a s s i g n e d to a v a r i a b l e ~r_iW_b~ of t h e f o r s t a t e m e n t .

157


statement> ::= := < f o r l i s t > ~ list> ::= < i n i t i a l v a l u e > ~ O < f i n a l v a l u e > I ~owot~ ::= < i d e n t i f i e r > ::= < e x p r e s s i o n > ::= < e x p r e s s i o n > The control variable, the initial value, and the final value must be o f t h e s a m e s e a l e r t y p e (or s u b r a n g e thereof), and must n o t be a l t e r e d by t h e r e p e a t e d statement. T h e y c a n n o t be of t y p e real. A for

statement

for

is

and

v

equivalent v

:= el;

a

for

fD~l v is

:=

:= el

:= el;

Examples

for

the

to

e2 do

to

the

S; v

form

of

:= s u c c ( v ) ;

S;

of

the

do~jl~J& e 2 d o to S;

the v

9.2.4.

statements .,.

; v

:= e2;

S

...

; v

:= e2;

S

form

S

statement

:= p r e d ( S ) ;

S;

:

i

:= 2 t o

100

do ii

f e z i := I t o n #j& fD~l J := I t o n d o b e n i n x := 0 ; for_ k := I t o n d o c[i.j] :-- x an~ for

S

sequence

statement

equivalent v

el

of

c

:= r e d

to

blue

x

do

a[i]

> max

:= x + a [ i , k ]

tbru& m a x

:= a [ i ]

*b[k,j] ;

Q (c)

~h_~tatements


::= ~ i t h < r e c o r d v a r i a b l e list> dO l i s t > ::= < r e c o r d variable>{ ,}

Within the component statement of the with statement, the components ( f i e l d s ) of t h e r e c o r d variable specified by t h e w i t h Clause can be denoted by their field identifier only, i.e. without preceding them with the denotation of t h e e n t i r e r e c o r d variable. The with clause effectively opens the scope containing the field identifiers of the specified record variable, so that the field identifiers m a y o c c u r as v a r i a b l e identifiers.

158

Example: ~£~b date d~ ~ m o n t h = 12 ~ e n ~ m o n t h := ~se is

month

equivalent i~

I; y e a r

:= y e a r

+

I

:= m o n t h + 1 to

date.month ~ 12 ~ ~d~ date.month :=

~]_.~ d a t e . m o n t h

1;

date.year

:= d a t e . y e a r + 1

:= d a t e . m o n t h + l

No assignments may be m a d e in t h e q u a l i f i e d statement to elements of the record variable list. However, assignments possible to t h e c o m p o n e n t s of t h e s e v a r i a b l e s .

10.

Procedure

Procedure associate procedure

any are

declarations

declarations s e r v e to d e f i n e identifiers w i t h t h e m so t h a t statements.

p a r t s of they can

programs and be a c t i v a t e d

: := < p r o c e d u r e heading> ::= < l a b e l d e c l a r a t i o n oart>

to by

part>

The ~ ~ ~ d ~ specifies the identifier naming the procedure and the formal parameter identifiers (if ~ n y ) . The parameters are either value-, variable-, procedureor function parameters (cf. also 9,1,2,), Procedures and functions which are u s e d as p a r a m e t e r s to o t h e r p r o c e d u r e s end functions must have value parameters only. ::= ~ 2 o c w ~ d u r e < i d e n t i f i e r > ; 1 ~2~r~W~ ( I ;}) : ::= 1 ~&~ ~nc~on ~£ce~ur~ { ,t ::= {,} A parameter constituents The

i ~

group without preceding are value parameters. de~l~ration

~it

:

specifier

specifies

all

implies

labels

that

which

its

mark

a

159

statement
in

the

statement

declaration part> ::= < e m p t y > iah#3. < l a b e l > { . < l a b e l > } ;

The £~nsJi&Oj~ definition definitions l o c a l to t h e
to


part.

~&~& contains procedure.

definition part> : :=

The ~&r_i&hl~ l o c a l to t h e

type

constant

~&~ contains declaration.

vat

all

::= { ;
&~_~ ~ o m ~&f_~Wd3 function declarations

synonym

definition>}

definitions

part> : := < e m p t y > I definition> { :

~cle£ation procedure

The &2~£~r_@ procedure and declaration.

all

I { ;
~efioij~ion &arl contains all the procedure declaration. definition ty_~
I

which

declarations

I declaration>t &&2& to

are

}:

variable

local

:

;

contains all the procedure

::= { :} ::= ~ The &~atg@~ELt executed upon statement.
~r_J2 specifies an activation

part>

::=

the algorithmic of the procedure


actions to be by a p r o c e d u r e

statement>

All identifiers introduced in the formal parameter part, the constant definition part, the type definition part, the variable-, procedure or f u n c t i o n declaration parts are l~l to the procedure declaration which is c a l l e d t h e &_q~Z~ of t h e s e identifiers. T h e y a r e not k n o w n o u t s i d e t h e i r s c o p e . In t h e c a s e of local variables, their values ere undefined at t h e b e c i n n i n ~ of t h e s t a t e m e n t part. The use of within its procedure.

the procedure identifier in a p r o c e d u r e declaration implies recursive execution

statement of the

160

Examples

of

procedure

declarations:

~ro_~dure readinteger (3Aor f : t e x t ; M a r va_~r i . j : i n t e g e r ; ~_qin w _ h i l e f¢ = " " d o g e t ( f ) : i := O: .~bile f t ~O. [ ' 0 " . . "9"] do begio j := o r d ( f ~ )- o r d ( ' O ' ) ; i :: I 0 - i + j ; g e t (f) end ; x := i and

x:

integer)

~rocedwj~e Bisect(functio~ f: real; a.b: real; va~ m: real; beqin {assume f(a) < 0 and f(b) > 0 } whi]j& abs (a-b) > IE-10*abs(a) do b~g[in m :: ( a + b ) / 2 . 0 : if f ( m ) < O Jzh~J3 a : = m e l s e b ::m end ; Z :: m ~d

Mar

;

z:

real);

or~.~e~_~re GCD (m 0 n : i n t e g e r ; v o ~ x 0y , z : i n t e g e r ) : ~ al,a2, bl,b2,c,d ,q , r : i n t e g e r ; {m>=O, n>O} be~in {Greatest Common D i v i s o r x of m and n, Extended Euclid's Algorithm} al :: O; a 2 : = I ; b l ;:I; b 2 : = O; e : : m: d : : n : ~hi~ d <> 0 d o beo3J3 {a1*m + b1*n = d, a2*m + b2*n = e. god(cod) : gcd(mon)} q := c ~ d ; r :-- c rnDj~ d ; a 2 := a 2 - q * a I; b 2 := b 2 - q * b l ; o :: d; d := r : r :: a l ; e l :: a 2 : a 2 :: r ; r := b 1 : b l := b 2 ; b 2 := r end ; x :: c ; y := a 2 ; z := b 2 { x = gcd(m,n) = y*m + z*n } end

10.1.

~Zld~rd

erocedwres

Standard procedures are supposed to be predeclared in every implementation of Pascal. Any implementation may feature additional predeclared procedures. Since they are, as all standard quantities, assumed as declared in a scope surrounding the program, no conflict arises from a declaration redefining the same identifier within the program, The standard procedures are l~sted and explained below.

161

10.1.1.

Eile

handling appends

put(f)

orocedur~ the

value

of

the

buffer

variable

f~

to

the

file f. The effect is defined o n l y if p r i o r to execution the predicate eof(f) is true. eof(f) r e m a i n s t r u e . a n d t h e v a l u e of f~ b e c o m e s u n d e f i n e d . get(f)

a d v a n c e s the c u r r e n t f i l e p o s i t i o n (read/write head) to t h e next c o m p o n e n t , and a s s i g n s t h e v a l u e of t h i s component to the buffer variable ft . if no n e x t component exists, then eof(f) becomes true. and the value of ft is not d e f i n e d . T h e e f f e c t of g e t ( f ) is defined only if ear(f) = false prior to its execution. (see 1 1 . 1 . 2 )

reset (f)

resets the current file p o s i t i o n to its b e g i n n i n g end assigns to t h e b u f f e r v a r i a b l e f~ t h e v a l u e of the first e l e m e n t of f. e o f ( f ) b e c o m e s f a l s e , if f is not empty; otherwise f~ is not d e f i n e d , a n d eof(f) remains true.

rewrite(f)

d i s c a r d s t h e c u r r e n t v a l u e of f s u c h t h a t m a y be g e n e r a t e d , eof(f) becomes true.

Concerning and

ne.

page,

the see

(p)

textfile

procedures

Chapter

read,

write,

allocates

,t 1 .....

tn)

a

can

variant dispose(p)

Let

the

and

new

be

with

a and

a:

&~&~

[m..n]

z:

oack~

azrav

n-m

>=

~or

j

the

writeln,

variable

v

and

used

tag

to

field

allocate

values

assigns

the

If t h e form

type

a

variable

pointer

of v

of

is

the

t 1 ..... t n .

a n d d i s p o s e ( p , t 1 , . . . . t n ) can be u s e d to i n d i c a t e t h a t storaoe occupied by t h e v a r i a b l e r e f e r e n c e d by t h e pointer p is no l o n g e r n e e d e d . (Implementations may use this information to r e t r i e v e s t o r a g e , or t h e y may i g n o r e it,)

variables

where

readln,

file

12.

to v to the p o i n t e r v a r i a b l e p. a record type with variants, the new(p

a new

v-u :=

statement

of

be

v

the do

declared

by

T

[u..v]

o Then u to

z

of

T

statement z[j]

unpack(z,a,i)

:=

pack (a ,i ,z )

a[j-~+i] means

means

162

f~l~ J

:= u t o

where j denotes the program.

11. F u n c t i o n

an

v do

a[J-u+i]

auxiliary

:: z[j]

variable

not

occurring

elsewhere

in

declarations

Function declarations serve to define parts of the program which compute a scalar value or a pointer value. Functions are activated by the evaluation of a f u n c t i o n designator (cf. 8 . 2 ) w h i c h is a c o n s t i t u e n t o f an e x p r e s s i o n .
declaration>

The function heading function, the formal the function.

:::


soecifies parameters

the of

heading>

identifier naming the function, and

the the

type

::: ~WJl&~i9_~ < i d e n t i f i e r > : < r e s u l t type>; ~tion ( { :}) : ; ::= < t y p e i d e n t i f i e r >

of

I

The t y p e of t h e f u n c t i o n m u s t be a s c a l a r , s u b r a n g e , or p o i n t e r type. Within the function declaration t h e r e m u s t be at l e a s t o n e assignment statement assigning a value to the function identifier. This assignment determines the result of the function. Occurrence of the function identifier in a f u n c t i o n designator within its d e c l a r a t i o n implies recursive execution of the function. Examples: fu~l~J~ioa S q r t (x : r e a l ) : r e a l : vgr xU.x1: real: b~_~i~ x l := x; {x>l. Newton's method} ~e~zoat xO :: x l; x l := (xO+ x/xO)*O.5 until abs(xl-xO) < eps*xl ; Sqrt := x 0 ~nd

f u n ~ d ~ i o ~ N~ax(e : v e c t o r ; n: v~ x : r e a l ; i: i n t e g e r : b a ~ i n x := a [ 1 ] : _fD/i i := 2 t ~ n do

b.~lin_ if

{x X

= max(a[ I] ..... < a[i]

end : {X = max(a[ I~!ax : = x end

integer):

I] .....

~h~ii x a[n]

a[i-1])} := a [ i ] )}

real;

163

function be~in if en,~d

G C D (m,n : i n t e g e r ) : i n t e g e r : n = O J~hen G C D :-- m # ! s e G O D

:= G C O (n,m

function Power(x: r e a l ; y: i n t e g e r ) : v&r w.z : reel; i : integer ; ~.~in w : = x ; z := 1: i : = y ; mhJJ.~ i > 0 d o if odd(i) then i := i d i v 2 ; w := sqr(w) end : {Z = x ~ y } Power := z

z

:=

real

: {y

mod

n)

>=

O}

z'w;

~nd

11.1.

~&ndard

functig_Q&

Standard functions are supposed to be predeclered implementation of Pascal. Any implementation may additional predeclared functions (cf. also 10.1).

The

standard

11.1.1,

Ar_~_bmetic

abs(x)

sqr

are

listed

the absolute either ~Ca~ is the type

computes x~2. or ~ n t e ~ e r , and of x.

s in (x) cos (x) exp (x) ln(x ) sqrt (x) erctan (x)

the the

and

explained

below:

fg_ectiojl&

computes must be the result

(x)

11.1.2.

functions

in every feature

type type

or of

value of iO~e~er, x.

x . The t y p e o f x and the type of

T h e t y p e o f x m u s t be t h e t y p e of t h e r e s u l t

of x m u s t be e i t h e r ~ e a l of t h e r e s u l t is ~ e a l .

either is t h e

or 3 n t e g e r ,

~ type

and

~e~_~tes

odd(x)

the true,

type if x

of is

x m u s t be ~ o t e ~ w J : . e n d t h e odd, and false otherwise.

eof(f)

eof(f) indicates, end-of-file status.

eoln(f)

indicates the chapter 12).

end

whether

of

a

the

line

file

in

result

f is

e textfile

in

is

the

(see

164

11.1.3.

~&sfer

trune(x)

the Part

(x)

x

is

truncated

argument

x

is

to

rounded

x m u s t be o f t y p e i n t e g e r , a n d char) is the character whose (if it e x i s t s ) .

Fw.~_he~

standard

succ(x)

x is result

of is

pred(x)

x is result exists).

of is

12.

value

its

integral

to

the

nearest

x m u s t be of a s c a l e r t y p e ( i n c l u d i n g Boolean and char), and the result (of t y p e i n t e g e r ) is t h e ordinal number of t h e v a l u e x in t h e s e t d e f i n e d by t h e t y p e o f x.

chr (×)

11.1.4.

real .

the real integer.

round (x)

ord

funct~j&~&

Input

end

the result (of t y p e ordinal n u m b e r is x

functions any scalar the successor any the

or subrange type, and the v a l u e of x (if it e x i s t s ) .

scalar or predecessor

subrange value

type, of x

and t h e (if it

output

The basis of l e g i b l e i n p u t a n d o u t p u t a r e t e x t f i l e s (cf.6.2.4) that are passed as program parameters (cf. 13) t o a PASCAL program and in its environment represent some i n p u t or output device such as e terminal, a card reader, or a line printer. In order to facilitate the handling of textfiles, the four standard procedures ~£.~3~, writE, ~eadlo, and ~rite!~ are introduced in addition t o ~e~ a n d ~W~. T h e y c a n be a p p l i e d to textfiles only; however, these textfiles must not necessarily represent input/output devices, but cam also be local files. The new procedures are used with a non-standard syntax for their parameter lists, allowing, among other things, for a v a r i a b l e number of parameters. Moreover, the parameters must not necessarily be of t y p e c h a r , b u t m a y a l s o be of c e r t a i n o t h e r types, in which case the date transfer is a c c o m p a n i e d by an implicit data conversion operation. If the first parameter is s file variable, then this is t h e f i l e to be r e a d or w r i t t e n . Otherwise, the standard files iO&ut and ~g~&~& are automatically assumed as default values in t h e c a s e s of r e a d i n g and writing respectively. These two files are predeclared as vat

input 0 output : text

Textfiles represent a special c a s e a m o n g f i l e t y p e s i n s o f a r as texts are substructured into l i n e s by s o - c a l l e d line markers (cf. b.2.4.). If, u p o n r e a d i n g a textfile f, t h e f i l e p o s i t i o n

165

is a d v a n c e d to a l i n e m a r k e r , t h a t is p a s t t h e l a s t c h a r a c t e r of a line, then the value of the buffer variable ft becomes a blank, and the standard function ~#_~O(~) ( ~ n d £ £ l i n e ) yields the value true. Advancing the file position once more assigns to f~ the first character of the next line, and eoln(f) yields false (unless the next line consists of 0 c h a r a c t e r s ) . Line markers, not being elements o f t y p e c h a r , c a n o n l y be g e n e r a t e d by t h e p r o c e d u r e ~r~teln,

12.1.

!~__~r_Q~edure

r~d~

The following rules hold for the textfile and vl...vn denote variables (or s u b r a n g e of i n t e g e r ) , or r e a l .

1. r e a d ( v 1

....

,vn)

is

equivalent

if to

4.

if v is a v a r i a b l e of type integer (or s u b r a n g e of integer) or real, then read(f.v) implies the reading f r o m f of a sequence of characters which form a number according to t h e syntax of PASCAL (cf. 4.) a n d t h e a s s i g n m e n t of t h a t n u m b e r to v. P r e c e d i n g blanks and line markers are skipped,

!~£,__2~&~ure

1. r e a d l n ( v l 2.

.....

readln(f,vl

of

vn)

.....

is

~b&2,

then

read(f,v

read(f,v)

1);

is

...

;

equivalent

r ~

is

vn)

read(f,v 3, r e a d l n ( f )

type

to

vn)

3.

is a v a r i a b l e := f? ; g e t ( f )

equivalent

1 .....

read(f,v 1 ..... read(f,vn)

12.2.

is

read(input,v

f denotes a char, integer

2.

v v

vn)

to

procedure ~ ; of t h e t y p e s

equivalent is

equivalent

1 .....

equivalent

to

vn):

readln(input,vl

.....

vn)

to readln

(f)

to

~ h i ] d ~ D_~& e o l n ( f )

~

get(f);

get(f) Readln of the

12.3.

is used to next line,

read

and

subsequently

skip

to

the

beginning

!b~_~roe~dure~r_i~

The following rules hold for the procedure ~i~-~; f denotes a textfile, p 1 ..... pn d e n o t e s o - c a l l e d write-parameters, e denotes an e x p r e s s i o n , m and n denote expressions of t y p e i n t e g e r .

1. w r i t e ( p

1.....

pn)

is

equivalent

to

write(output,p1

.....

pn)

166

2. w r i t e ( f , p

1 ..... pn) write(f

3.

The

is

equivalent

.pl);

..,

write-parameters e :m

p

;

write(f,pn)

have

e :m : n

to

the

followin~

forms:

e

e represents t h e v a l u e t o be " w r i t t e n " on t h e f i l e f, a n d m and n are so-celled field width parameters. If t h e v a l u e e, which is either a number, a character, a Boolean value, or e string requires less than m characters for its representation, then an a d e q u a t e n u m b e r of b l a n k s is i s s u e d such that exactly m characters are written. If m is o m i t t e d , an implementation-defined d e f a u l t v a l u e w i l l be m s s u m e !. T h e form with the width parameter n is applicable on~y if e is of type reel (see r u l e 6 ) . 4.

if

e is of t y p e c h a r , t h e n write(fo e : m ) is e q u i v a l e n t to f~ := °: put(f); (repeated m-1 times) ft : = e ; p u t (f) N o t ~ : t h e d e f a u l t v a l u e f o r m is in t h i s c a s e I,

5.

If e is of t y p e i~_~_W_g~ (or a s u b r a n o e of i n t e g e r ) , then the decimal representation of t h e n u m b e r e w i l l he w r i t t e n on t h e file f, preceded by an appropriate number of b l a n k s as specified by m.

6.

If e is of t y p e f e e l , a decimal representation of t h e n u m b e r e is w r i t t e n on t h e f i l e f, p r e c e d e d by a n a p p r o p r i a t e number of blanks as specified by m. If t h e p a r a m e t e r n is m i s s i n g ( s e e r u l e 3), e f l o a t i n o ~ o i n t representation eonsistino of a coefficient and a s c a l e f a c t o r w i l l be c h o s e n . Otherwise a fixed~oint representation with n dioits after the decimal p o i n t is o b t a i n e d .

2.

if e is of type ~ i ~ , then written on t h e f i l e f, p r e c e d e d b l a n k s as s p e c i f i e d by m,

C.

if e is written

12.4.

an on

(packed) the file

array f.

of

t h e w o r d s T R U E or by an a p p r o p r i a t e

characters,

then

the

FALSE are n u m b e r of

string

e is

T h e _ ~ r o c e d u r e _ ~ r it e l a

I. w r i t e l n

(p I ..... pn)

is

2. w r i t e l n ( f , p w r i t e l n (f)

1,...,pn)

3. w r i t e l n ( f )

appends

a

equivalent ~s

line

to

equivalent

marker

writeln to

( o u t p u t ,p I ..... p n )

write(f,p

(cf.6.2,4)

to

the

I ..... p n ) ;

file

f.

167

12.5.

~ditional_grocedu~es

page(f)

13.

top

causes skipping to the t e x t f i l e f is p r i n t e d .

of

a

new

page.

when

the

Programs

A Pascal program for its heading.



::=

has

the


form

of

a procedure

heading>



heading> ::= ~raoram parameters>

::=

declaration

.

(


{ ,

except

parameters>)



;

}

The identifier f o l l o w i n g t h e s y m b o l ~roorej~ is t h e p r o g r a m n a m e ; it has no f u r t h e r s i g n i f i c a n c e inside the program. The p r o o r a m parameters denote entities that exist outside the program, and through which the program communicates w i t h its e n v i r o n m e n t . These entities (usually files) are called ~9/:~ , a n d must be declared in the block which constitutes the program like ordinary local variables. The two standard files i~&W_~ a n d ~ u t o u ~ m u s t not be d e c l a r e d (cf. 12)0 but have to be l i s t e d as p a r a m e t e r s in t h e p r o g r a m heading, if they are used. The initialising statements reset(input) and r e w r i t e ( o u t p u t ) are automatically generated and must not be s p e c i f i e d by t h e p r o g r a m m e r . Examples: or~g~, c o p y (f ,~ ) ; v_all f.g: _file o f r e a l ; ~ e o i ~ r e s e t (f); r e w r i t e ( g ) ; w h i l e n o t e o f ( f ) do ~eoio gT : = f ~ ; p u t ( g ) ; oad •

orw_qram e o p y t e x t (input , o u t p u t ) , )jar oh : c h a r ; begin w~LiiO D o t e o f ( i n p u t ) lip ba_q~a while not eoln(input) do bwlljJ3 r e a d ( c h ) ; w r i t e ( c h ) ea~ ; readln; writeln eo/

get(f)

168

14. A s t a n d a r d

for

implementation

and

program

interchange

A primary motivation for the development of P A S C A L w a s t h e n e e d for a powerful end flexible l a n g u a g e t h a t c o u l d be r e a s o n a b l y efficiently implemented on m o s t c o m p u t e r s . Its features w e r e to be defined without reference to any particular machine in o r d e r to facilitate the interchange of programs. The following s e t of Proposed restrictions is designed as a guideline for implementors and for programmers who anticipate that their programs be used on d i f f e r e n t computers. The purpose of t h e s e standards is to increase the likelihood that different implementations will be compatible, and that programs are transferable from one installation to a n o t h e r .

1. I d e n t i f i e r s denoting first U characters. 2. L a b e l s

consist

of

at

distinct

most

objects

must

differ

over

their

4 digits.

3.

The implementor may s e t a l i m i t to t h e s i z e of a b a s e t y p e o v e r w h i c h a s e t can be d e f i n e d . (Consequently, a bit pattern representation may reasonably be u s e d f o r s e t s . )

4.

The first character on each line of printfiles m a y be interpreted as a p r i n t e r control character with the following meanings: blank : single spacing °0' : double spacing "1" : print on top of next page

Representations should obey the

of P A S C A L in t e r m s following rules:

of

available

5.

Word symbols sequence of They may not

such as ~eqiA, ~nd, etc. letters (without surrounding be used as identifiers.

6.

Blanks, ends of lines, and comments are considered as separators. An arbitrary number of separators may occur between any two consecutive PASCAL symbols with the following restriction: no separators must occur within ~dentifiers, numbers, and word symbols.

7.

At least consecutive

- are escape

character

sets

written as characters).

one separator must occur between any identifiers, numbers, or w o r l d s y m b o l s .

pair

a

of

169

15. I n d e x

actual parameter adding operator array type array variable assignment statement base t y p e block case l a b e l c a s e l a b e l list case l i s t e l e m e n t case statement component type component variable compound statement conditional statement constant constant definition c o n s t a n t d e f i n i t i o n part constant identifier control v a r i a b l e digit digit s e q u e n c e element e l e m e n t list empty statement entire variable expression factor field designator field identifier f i e l d list file b u f f e r file t y p e file v a r i a b l e final v a l u e fixed p a r t for list formal Parameter section for s t a t e m e n t function declaration function designator furct ton h e a d i n g function identifier goto s t a t e m e n t identifier if s t a t e m e n t index t y p e indexed variable initial value label label d e c l a r a t i o n part letter l e t t e r or digit multiplying operator

9.1.2. 8.1.3 6.2.1 7.2.1 9.1.1 6.2.3 10. 6.2.2 9.2.2.2 9.2.2.2 9.2.2.2 6.2.1 7.2 9,2.1 9,2.2 5. 5. 10. 5. 9.2.3.3 3. 4. 8. 8, 9.1 7.I 8. 8. 7.2.2 72.2 62.2 72.3 62.4 72.3 92.3.] 62.2 92.3.3 10. 9.2.3.3 11. 8.2 11. B.2 9.1.3 4. 9.2.2.1 0.2.1 7.2,1 9.2.3.3 9. 10. 3. 4. 8.1.2

and

6.2.2

170 p a r a m e t e r group pointer variable pointer t y p e p r o c e d u r e and function d e c l a r a t i o n part Procedure declaration procedure heading p r o c e d u r e identifier p r o c e d u r e or function d e c l a r a t i o n p r o c e d u r e statement program program h e a d i n g program P a r a m e t e r s record s e c t i o n record type record variable record variable list referenced variable r e l a t i o n a l operator repeat s t a t e m e n t repetitive statement result type scale factor scalar t y p e set set t y p e sign simple e x p r e s s i o n simple s t a t e m e n t simple type special symbol statement statement part string structured statement structured type subrange type tag field term type type d e f i n i t i o n type d e f i n i t i o n pert type i d e n t i f i e r variable variable d e c l a r a t i o n variable d e c l a r a t i o n part variable i d e n t i f i e r variant variant part u n l a b e l l e d statement unpacked s t r u c t u r e d type unsigned constant unsigned integer unsigned number unsigned real with s t a t e m e n t while s t a t e m e n t

10. ?.~ 6.3 10. 10. 10. 9.1.2 10. 9.1.2 13. 13. 13. 6.2.2 6.2.2 7.2.2 9.2.4 7.3 8.1.4 9.2.3.2 9.2.3 11. 4. 6.1 .t 6.2.3 4. U. 9.1 6.1 3. 9. 10. 4. 9.2 6.2 6.1 .3

6.2.2 8. 6. 6. 10. 6.1 7. 7. 10. 7.1 6.2.2 6.2.2 9. 6.2 8. 4. 4. 4. 9.2.4 9.2.3.1