Lesson 5.05 – Using Built-in Functions

🧠 Lesson 5-5 – Using Built-in Functions in AutoLISP

Learn essential AutoLISP functions to store values, define commands, and manipulate data.

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📚 What You’ll Learn

By the end of this lesson, you’ll be able to:

• Use key AutoLISP functions like setq, defun, princ, and prompt
• Perform basic math operations using built-in functions
• Combine and manipulate strings
• Understand how these functions fit into real-world AutoCAD routines

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🧠 Why It Matters

AutoLISP offers dozens of built-in functions, but a few form the core of almost every routine. Mastering these will help you write smarter scripts, capture user input, and output clean results to the user.

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🛠️ Tools You’ll Use

Function	Purpose
setq	Assigns values to variables
defun	Defines a new function or command
princ	Silently returns control to AutoCAD
prompt	Displays a message in the command line
+, -, *, /	Perform arithmetic operations
strcat	Combine strings
strlen	Return string length

🧭 Lesson Structure

1️⃣ setq – Assign Values

setq (short for set quoted) is used to store a value in a variable.

(setq a 10)
(setq name "AutoLISP")

Example	Description

(setq x 5)

Assign 5 to x

(setq pt '(0 0))

Assign a list (0, 0) to pt

(setq full (strcat "Hello, " name))

Combine and assign text

2️⃣ defun – Define a Function

Use defun to define a new AutoLISP function.

(defun c:demo ( )
  (prompt "\nRunning demo command...")
  (princ)
)

Syntax	Description

(defun c:demo () ...)

Defines a command that can be typed in AutoCAD

c: prefix

Ensures it works from the command line

3️⃣ prompt vs. princ

Function	Purpose	Example
prompt	Writes a message to command line	(prompt "\\nStart drawing...")
princ	Silently ends a routine	(princ)

🔹 Always end your custom routines with (princ) to avoid cluttering the command line.

4️⃣ Math Functions

AutoLISP supports simple arithmetic with prefix notation.

Operation	Syntax	Result
Add	(+ 2 3)	5
Subtract	(- 10 4)	6
Multiply	(* 3 5)	15
Divide	(/ 10 2)	5.0

You can also assign results to variables:

(setq result (* 4 5))

5️⃣ String Functions

Function	Purpose	Example	Result
strcat	Concatenates (joins) strings	(strcat "Hello, " "world!")	"Hello, world!"
strlen	Returns the length of a string	(strlen "AutoLISP")	8

✅ Lesson Checklist

Task	Completed
Used setq to assign numbers and text	☐
Defined a command using defun and the c: prefix	☐
Displayed a message using prompt	☐
Used princ to end a routine silently	☐
Performed math and string operations	☐

📌 Quick Tips

Tip	Why It Helps
Always use setq for variable assignment	It’s clear and reliable
End every command with (princ)	Keeps the command line clean
Use prompt only for text messages	For visual or interactive output
Use strcat only with strings, not numbers	Avoids type errors

🧩 Real-World Applications

Use Case	Example Usage
Dynamic block naming	Combine prefix and part number using strcat
Area calculator	Use math functions to compute square footage
Layer report tool	Display string-based summaries in command line

📁 Files and Resources

File / Resource	Description
builtins_demo.lsp	Sample routine using all functions from this lesson
math_string_worksheet.txt	Practice examples for math and string expressions
command_template.lsp	Reusable function template with defun, prompt, etc.

📖 Review Table

Function	Description	Example
setq	Store value in variable	(setq a 10)
defun	Define a named function	(defun c:test () ...)
prompt	Show message in AutoCAD command line	(prompt "\\nStart...")
princ	Cleanly exit a routine	(princ)
+, *, -	Perform arithmetic operations	(+ 2 3), (* a b)
strcat	Combine strings	(strcat "Layer: " name)