Ss Lab Viva Questions

[Type the document title] Viva Questions 1. What is lex? What is the input to it? What is its output? 2. What is Yacc? What is the input to it? What is its output? 3. What are regular expressions? 4. What is a grammar? 5. What does y.tab.h do? 6. What does lex.yy.c. do? 7. Explain this regular expression [^ tn]+ (Anything can be asked) 8. Give the regular expression to recognize a word. (Anything can be asked) 9. Explain the code. (Any statement can be asked to be explained…ex: yywrap, yylex, yyin…etc) 10. What is the functionality of input() and output() functions? 11. How do you parse a command line using lex? 12. What are token definitions? Certain Basic Concepts and Definitions:

1. What is a Parser? A Parser for a Grammar is a program which takes in the Language string as it's input and produces either a corresponding Parse tree or an Error. 2. What is the Syntax of a Language? Langua ge? The Rules which tells whether a string is a valid v alid Program or not are called the Syntax. 3. What is the Semantics of a Language? The Rules which gives meaning to programs are called the Semantics of a Language. 4. What are tokens? When a string representing a program is broken into sequence of substrings, such that each substring represents a constant, identifier, operator , keyword etc of the language, these substrings are called the tokens of the Language. 5. What is the Lexical Analysis? The Function of a lexical lex ical Analyzer is to read the input stream representing the S ource  program, one character ata a time and to translate it into valid tokens 6. How can we represent a token in a language? The Tokens in a Language are represented by a set of Regular Expressions. A regular  expression specifies a set of strings to be matched. It contains text characters and operator  characters.The Advantage of using regular expression is that a recognizer can be automatically generated. Dept. Of CSE, AciT-Bangalore

Page 1

[Type the document title]

7. How are the tokens recognized? The tokens which are represented by an Regular Expressions are recognized in an input string by means of a state transition Diagram and Finite Automata. 8. Are Lexical Analysis and Parsing two different Passes ? These two can form two different passes of a Parser. The Lexical analysis can store all the recognized tokens in an intermediate file and give it to the Parser as an input. However it is more convenient to have the lexical Analyzer as a coroutine or a subroutine which the Parser  calls whenever it requires a token. 9. What is the significance of the yywrap() function ? The yywrap() function is executed when the lexical analyzer reaches the end of the input file. It is generally used to print a summary of lexical analysis or to open another file for  reading. The yywrap() function should return 0 if it has arranged for additional input, 1 if the end of the input has been reached. 10. What are regular expression? The following are the most general notations used for expressing a R.E. Symbol Description | OR (alternation) () Group of Subexpression * 0 or more Occurrences ? 0 or 1 Occurrence + 1 or more Occurrences {n,m} n-m Occurrences Suppose we want to express the 'C' identifiers as a regular Expression:identifier=letter(letter|digit)* The Rules for writting R.E are:* An operator character may be turned into a text character by enclosing it in quotes, or by preceding it with a (backslash). *a/b matches "a" but only if followed by b (the b is not matched) *a$ matches "a" only if "a" occurs at the end of a line *^a matches "a" only if "a" occurs at the beginning of a line *[abc] matches any charcter that is an "a", "b" or "c" *[^abc] matches any charcter but "a", "b" and "c". *ab?c matches abc and ac * Within the square brackets most operators lose their special meanings except "" and "-". the "^" which takes there special meaning.

Dept. Of CSE, AciT-Bangalore

Page 2

[Type the document title] * "n" always matches newline, with or without the quotes. If you want to match the character "" followed by "n", use \n. Things to remember: 1. If there is no R.E for the input string , it will be copied to the standard output. 2. The Lex resolves the ambiguity in case of matching by choosing the longest match first or by choosing the rule given first. 3. All the matched expressions are contained in yytext whose length is yyleng. YACC-Yet Another Compiler-Compiler

Yacc is the Utility which generates the function 'yyparse' which is indeed the Parser. Yacc describes a context free , LALR(1) grammar and supports both bottom-up and top-down parsing. The general format for the YACC file is very similar to that of the Lex file. 1.Declarations 2.GrammarRules 3.Subroutines In Declarations apart from the legal 'C' declarations there are few Yacc specific declarations which  begin with a %sign. 1.%union It defines the Stack type for the Parser. It is a union of various datas/structures/ objects. 2.%token These are the terminals returned by the yylex function to the yacc. A token can also have type associated with it for good type checking and syntax directed translation. A type of a token can be specified as %token tokenName. 3.%type The type of a non-terminal symbol in the Grammar rule can be specified with this. The format is %type non-terminal. 4.%noassoc Specifies that there is no associativity of a terminal symbol. 5.%left Specifies the left associativity of aTerminal Symbol 6. %right Specifies the right associativity of a Terminal Symbol. 7. %start Specifies the L.H.S non-terminal symbol of a production rule which should be taken as the starting point of the grammar rules. 8. %prec Changes the precedence level associated with a particular rule to that of the following token name or literal.

Dept. Of CSE, AciT-Bangalore

Page 3