Lesson 5.09 – Modifying Objects with AutoLISP

✏️ Lesson 5.9 – Modifying Objects with AutoLISP

Learn how to use entget, entmod, and entupd to change layers, colors, and positions of existing geometry.

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📚 What You’ll Learn

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

• Retrieve entity data using entget
• Modify properties such as layer, color, or coordinates using entmod
• Use entupd to visually refresh updated entities
• Write scripts that adjust object attributes programmatically

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🧠 Why It Matters

AutoCAD drawings often need cleanup, reorganization, or formatting. AutoLISP lets you automate those edits at scale—changing thousands of entities with a single routine. Whether adjusting layers, colors, or geometry, entmod puts you in control.

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🛠️ Tools You’ll Use

Function	Purpose
entget	Retrieves an entity’s DXF data
entmod	Submits modified data back into the drawing
entupd	Forces a visual update of modified entities
assoc	Finds a DXF group in a list
subst	Substitutes a new item into a list

🧭 Lesson Structure

1️⃣ Retrieve Entity Data with entget

Use entget to get an entity’s DXF structure:

(setq ent (car (entsel "\nSelect an object: ")))
(setq data (entget ent))

This returns a list of key-value pairs, like:

((0 . "LINE") (8 . "0") (62 . 256) (10 0.0 0.0 0.0) ...)

2️⃣ Modify a Property with subst

You can change a value in the DXF list using subst.

Change Color to Red (1):

(setq data (subst (cons 62 1) (assoc 62 data) data))

Code	Description	Value Type

8

Layer

String

62

Color Index (ACI)

Integer

10

Start point

List of floats

Change Layer:

(setq data (subst (cons 8 "MyLayer") (assoc 8 data) data))

3️⃣ Apply Changes with entmod and entupd

Once modified, commit the changes:

(entmod data)
(entupd ent)

• entmod applies the data to the drawing
• entupd forces the object to visually update

✅ Lesson Checklist

Task	Completed
Selected and retrieved entity data using entget	☐
Used assoc to inspect properties	☐
Modified properties using subst and entmod	☐
Used entupd to force screen refresh	☐

📌 Quick Tips

Tip	Why It Helps
Always test if entsel returned a value	Prevents errors if the user cancels selection
Use (assoc code list) to retrieve values	Clean and readable way to find DXF entries
Use (subst ...) carefully	Ensure you’re replacing a matching pair

🧩 Real-World Applications

Scenario	What to Modify	Why
Assign new color scheme	Change 62 values	Visual clarity
Reorganize by layer	Change 8 values	Better structure
Realign geometry	Modify 10, 11 coordinates	Fix design or layout issues

📁 Files and Resources

File / Resource	Description
modify_color_layer.lsp	Routine to change layer and color of selected objects
entget_cheatsheet.docx	Common DXF codes for entity inspection/modification
property_update_template.lsp	Code scaffold for modifying any entity property

📖 Review Table

Function	Description	Example
entget	Get entity DXF data	(entget ent)
entmod	Apply modified DXF list	(entmod data)
entupd	Refresh entity on screen	(entupd ent)
assoc	Find a code/value in a list	(assoc 8 data)
subst	Replace a code/value in a list	(subst (cons 8 "MyLayer") (assoc 8 data) data)