Introduction One of AutoCAD's greatest assets is its adaptability. You can control just about every aspect of AutoCAD's operations from the appearance of its drawing editor to its variety of menus. A key element of this adaptability is its built-in programming language, AutoLISP. With AutoLISP, you can virtually write your own commands or redefine others. You can think of AutoLISP as a very sophisticated macro-building facility. (Simple macros are like scripts that automate repetitive keystrokes.) You don't need to be a programmer to use AutoLISP. In fact, AutoLISP is designed so that everyday users of AutoCAD can start to use it after a minimum of training. This book makes AutoLISP accessible to AutoCAD users who are looking for a way to enhance and extend their use of AutoCAD.
The ABC’s of AutoLISP by George Omura W
ho should read this book This book introduces nonprogrammers to the use of AutoLISP. If you are an intermediate level AutoCAD user, interested in learning about this powerful tool, then this is the book for you. If you are just beginning to learn AutoCAD, then you should probably become a bit more familiar with AutoCAD before attempting to learn AutoLISP. This book assumes that you have at least an intermediate level of expertise with AutoCAD and are acquainted with simple Windows operations.
H ow to Use This Book Each chapter offers exercises and sample programs that demonstrate some general concept regarding AutoLISP. Through these exercises, the book shows you how programs develop from ideas into finished, running programs. Also, the information you learn in one chapter will build on what you learned in the previous chapter. This way, your knowledge of AutoLISP will be integrated and cohesive, rather than fragmented. For this reason, the best way to use this book is to read each chapter in order and do all of the exercises. Since the topics are oriented toward accomplishing tasks rather than simply focusing on individual functions, you will have a good grasp of how to use AutoLISP in real world situations by the end of this book.
W here to Find the LISP Programs As you read the chapters and do the exercise, you will be asked to enter program code into a file. If you are in a hurry, you can cut and paste the code directly from the chapter you are reading. This will save a good deal of time, but make sure you study the code that you cut and paste. This book was originally published in 1990 by Sybex Inc. It has been reproduced here in an electronic format by the Author for the benefit of Mastering AutoCAD readers everywhere. Enjoy....
The ABC’s of AutoLISP by George Omura Chapter 1: Introducing AutoLISP Featuring
Understanding the AutoLISP Interpreter and Evaluation Expressions and Arguments Variables and Data Types Manipulating Lists with Functions Get Functions If you have never programmed a computer before, you may think that learning AutoLISP will be difficult. Actually, when you use a program such as AutoCAD, you are, in a sense, programming your computer to create and manipulate a database. As you become more familiar with AutoCAD, you may begin to explore the creation of linetypes and hatch patterns, for example. Or you may customize your menu to include your own specialized functions and macros. (Macros are like scripts that the computer follows to perform a predetermined sequence of commands.) At this level, you are delving deeper into the workings of AutoCAD and at the same time programming your computer in a more traditional sense. Using AutoLISP is really just extending your knowledge and use of AutoCAD. In fact, once you learn the basic syntax of AutoLISP, you need only to familiarize yourself with AutoLISP's built-in functions to start writing useful programs. (AutoLISP's syntax is the standard order of elements in its expressions.) You might look at AutoLISP functions as an extension to AutoCAD's library of commands. The more functions you are familiar with, the better equipped you are for using the program effectively. AutoLISP closely resembles Common LISP, the most recent version of the oldest artificial intelligence programming language still in use today. AutoLISP is essentially a pared down version of Common LISP with some additional features unique to AutoCAD. Many consider LISP to be one of the easiest programming languages to learn, partly because of its simple syntax. Since AutoLISP is a subset of common LISP, it is that much easier to learn. In this chapter, you will become familiar with some of the basic elements of AutoLISP by using AutoLISP directly from the AutoCAD command prompt to perform a few simple operations. While doing this, you will be introduced to some of the concepts you will need to know to develop your own AutoLISP applications.
Understanding the Interpreter and Evaluation AutoLISP is accessed through the AutoLISP interpreter. When you enter data at the AutoCAD command prompt, the interpreter first reads it to determine if the data is an AutoLISP formula. If the data turns out to be intended for AutoLISP, then AutoLISP evaluates it, and returns an answer to the screen. This process of reading the command prompt, evaluating the data, then printing to the screen, occurs whenever anything is entered at the command prompt
and is an important part of how AutoLISP functions. In some ways, the interpreter is like a hand-held calculator. Just as with a calculator, the information you wish to have AutoLISP evaluate must follow a certain order. For example, the formula 0.618 plus 1 must be entered as follows: (+ 0.618 1)
Try entering the above formula at the command prompt. AutoLISP evaluates the formula (+ 0.618 1) and returns the answer, 1.618, displaying it on the prompt line. This structure-+ 0.618 1-enclosed by parentheses, is called an expression and it is the basic structure for all AutoLISP programs. Everything intended for the AutoLISP interpreter, from the simplest expression to the most complex program, must be written with this structure. The result returned from evaluating an expression is called the value of the expression.
The Components of an Expression An AutoLISP expression must include an operator of some sort followed by the items to be operated on. An operator is an instruction to take some specific action such as adding two numbers together or dividing one number by another. Examples of mathematical operators include the plus sign (+)for addition and forward slash (/) for division. We will often refer to the operator as a function and the items to be operated on as the arguments to the function or simply, the arguments. So, in the expression (+ 0.618 1), the + is the function and the 0.618 and 1 are the arguments. All AutoLISP expressions, no matter what size, follow this structure and are enclosed by parentheses. Parentheses are important elements of an expression. All parentheses must also be balanced, that is, for each left parenthesis, there must be a right parenthesis. If you enter an unbalanced expression into the AutoLISP interpreter, you get the following prompt: ((_>
where the number of parentheses to the left is the number of parentheses required to complete the expression. If you see this prompt, you must enter the number of closing parentheses indicated in order to return to the command prompt. In this example, you would need to enter two right parentheses to complete the expression. Double quotation marks enclosing text must also be carefully balanced. If an AutoLISP expression is unbalanced, it can be quite difficult to complete it and exit AutoLISP. Figure 1.1 shows the components of the expression you just entered.
Using Arguments and Expressions AutoLISP evaluates everything, not just expressions, but the arguments in expressions as well. This means that in the above example, AutoLISP evaluates the numbers 0.618 and 1 before it applies these numbers to the plus operator. In AutoLISP, numbers evaluate to themselves. This means that when AutoLISP evaluates the number 0.618, 0.618 is returned unchanged. Since AutoLISP evaluates all arguments, expressions can also be used as arguments to a function. For example, enter the following at the command prompt: (/ 1 (+ 0.618 1))
In this example, the divide function (/) is given two arguments-number 1 and an expression (+ 0.618 1). This type of expression is called a complex or nested expression because one expression is contained within another. So in our example, AutoLISP first evaluates the arguments of the expression, which are the expression (+ 0.618 1) and the number 1. It then applies the resulting value of the expression and the number 1 to the divide function and returns the answer of 0.618047 (see figure 1.2 ).
Using Variables Another calculator-like capability of the interpreter is its ability to remember values. You probably have a calculator that has some memory. This capability allows you to store the value of an equation for future use. In a similar way, you can store values using variables. A variable is like a container that holds a value. That value can change in the course of a program's operation. A simple analogy to this is the title of a government position. The position of president could be thought of as a variable. This variable can be assigned a value, such as Ronald Reagan or Bill Clinton.
Understanding Data Types Variables can take on several types of values or data types. Here is what some of these data types look like in AutoLISP.
Function
l II: 06181)) L ‘fl Arguments are evaluated U 1 1.618 ) I * I Then applied tuthe function
The ABC’s of AutoLISP by George Omura
DATA TYPE EXAMPLE
Integer 24 Real Number 0.618 String ``20 Feet 6 Inches'' List (4.5021 6.3011 0.0) File Descriptor Object Name
