As soon as you're in the Visual LISP environment, you can create a new AutoLISP file or open an existing file. To edit or view a file in the Visual LISP text editor, follow these steps:

When you open a file, the title bar of its window displays a blank paper icon to show that the file has not been changed. If you make any changes to the file, Visual LISP adds a pencil to the image to show that you've edited the file.

To use the AutoLISP program in AutoCAD, you must load it. You can load it from within Visual LISP or from within AutoCAD.

If you're in AutoCAD, you can load AutoLISP files into AutoCAD in two main ways. One method is at the command line. To load circle3.lsp from the command line, type (load "circle3")

When you enter the command to load circle3.lsp, AutoCAD searches all the support paths for a file called circle3.lsp. At installation time, AutoCAD automatically configures the support-file search path to include the path of AutoCAD 2010Support. For a full list of folders in thesupport-file search path, choose Application Button

Figure 34.1. An AutoLISP file open in its own window in the Visual LISP editor.

If your file is not in a folder in AutoCAD's support-file search path, then you must specify the full path name. To specify the full path to a routine, you would enter:

(load "c:/Program Files/AutoCAD 2010/Support/circle3") 

or

(load "c:\Program Files\AutoCAD 2010\Support\circle3") 

The backslash () has special meaning in AutoLISP, so you need to use two of them or a regular slash (/) when specifying a path. (The backslash character tells AutoLISP to interpret the following character as a special control code. For example, to use a double quote in an AutoLISP expression, precede it with a backslash.)

Figure 34.2. The Load/Unload Applications dialog box.

If you check the Add to History check box, AutoCAD lists previously loaded applications in the dialog box — just click the History List tab. Locate and choose the file you want to load, and click Load.

After you load an AutoLISP routine, you can use it. How you use the routine depends on the routine. If the program creates a function, then you can type the function's name on the command line like any other AutoCAD command. Most routines contain prompts to guide you in their use.

STEPS: Loading and Using an AutoLISP Routine

To examine the contents of the circle3.lsp file, open it in the Visual LISP editor. Figure 34.3 shows the circle3 routine. At this point, ignore the color formatting, which is discussed later in this chapter.

Figure 34.3. The circle3 AutoLISP routine.

This figure illustrates several general characteristics of AutoLISP routines:

The following explains the routine in Figure 34.3 line by line:

The Visual LISP editor contains a number of tools that make your programming life simpler. These tools represent the difference from the old way of writing AutoLISP code, which involved creating text files in a text editor. The visual way involves working with tools that help you format and complete your code. Some of the more useful tools include:

Another feature of Visual LISP is the Console window. The Console window usually resides at the bottom of the Visual LISP window. The Console window is similar to the AutoCAD command line. Just as you can enter AutoLISP expressions on the AutoCAD command line, as explained later in this chapter, you can also enter AutoLISP expressions in the Console window and see the results. Each line uses a _$ prompt.

A nice feature of the Console window is that Visual LISP remembers everything that you enter. You can retrieve it by pressing Tab for a forward history or Shift+Tab for a backward history.

To close a file within Visual LISP, click its Close button. (You can also press Ctrl+F4.) To exit Visual LISP, click its Close button or press Alt+F4. As with AutoCAD, Visual LISP warns you about saving your changes.

STEPS: Using the Visual LISP Editor

In AutoLISP's syntax, an operator always comes first, followed by a number of operands. An operator can be thought of as a function that does something, and the operands are the elements on which the function operates. Place a space between each component of the statement.

Another way to describe the syntax of AutoLISP is that, within each set of parentheses, the first item is a function, and what follows are parameters or arguments to that function.

Table 34 lists the basic arithmetic functions. You can use these functions on anything that has a value. For example, if you create a variable with a value of 2, you can use that variable with the arithmetic functions.

Table 34.1 lists only a few fundamental arithmetic operators (or functions). You can find more operators in the AutoLISP Function Synopsis under the Appendixes topic of the AutoLISP Developer's Guide, under Basic Functions

Some fundamental arithmetic operations can provide a feel for the fundamentals. If you type (+ 2 3)

To nest expressions, simply add another set of parentheses to enclose the nested expression. For example, when you enter (* 5 (+ 2 3)), AutoCAD responds with 25. The nested expression is evaluated from the innermost parenthesis pair out; first 2 and 3 are added, and then that sum is multiplied by 5.

Working with floating-point numbers (numbers with decimal points) is as easy as working with integers. If you type (/ 7.3 5.84)

Working with text is as easy as working with numbers. In AutoLISP, a string is simply a series of text characters. Table 34.2 lists some common string functions.

Here are two examples:

(strcat "Today is " "a good day" ".")
"Today is a good day."
(substr "Today is a good day." 12 4)
"good"

STRCAT stands for string concatenate and appends one string with another. Any number of strings can come after the STRCAT function. (There are three strings enclosed in quotes in the above STRCAT example.)

SUBSTR stands for substring, and in the above example it returns the four characters starting at position 12.

Table 34.3 offers you a few more functions that work with numbers and strings.