Creating AutoCAD Command Functions with autolisp

Not only can you create new AutoLISP functions, but in a similar manner, you can also create new AutoCAD commands! That is, the function name can be typed directly at the Command: prompt without using AutoLISP syntax. Creating a command-line function is simply a modified form of (defun).

Explanation: the (defun C: ) function

(defun C:name ()
<expression1>
…
<expressionn>
);end defun

Notes

• The function name argument must have a name that starts with “C:” .
• The function must be defined with a nil argument list (empty list).
• The function is invoked by typing the name at the “Command:” prompt.

Example

By modifying BOX1.LSP we can make it a command-line function:

(defun C:BOX1 ()
(setq PT1 (getpoint “Lower left corner: ”))
(setq PT2 (getpoint “Lower right corner: ”))
(setq PT3 (getpoint “Upper right corner: ”))
(setq PT4 (getpoint “Upper left corner: ”))
(command “line” PT1 PT2 PT3 PT4 “close”)
)

Command: (load “BOX1”)
C: BOX1
Command: BOX1

AutoLISP vs. AutoCAD Functions

As you create new functions in AutoLISP, you will need to decide whether you want to create a command-line function or not.

Command-line function

• Typed in directly at Command: prompt.
• User is prompted for required information.
• Result is usually to affect drawing directly.
• Cannot normally be used as a subroutine of another function.

Standard AutoLISP function

• Typed as an AutoLISP expression.
• Arguments are specified directly with the function.
• Result is usually some unique value.
• Can be used as a subroutine of other functions.
• Can be used transparently within AutoCAD commands.

Example

DegreesToRadians as AutoCAD command
Command: DegreesToRadians
Enter degrees to be converted: 180
3.14159

DegreesToRadians as AutoLISP function

Command: (DegreesToRadians 180)
3.14159

PRACTICE

Change BOX1.LSP to make it a command-line function and test it. Estimated time for completion: 5 minutes.

• A complete solution to this exercise is on your class disk as BOX1-B.LSP.