Page 66 - DCAP310_INTRODUCTION_TO_ARTIFICIAL_INTELLIGENCE_AND_EXPERT_SYSTEMS

Introduction to Artificial Intelligence & Expert Systems

Notes 4.3 Basic List Manipulation Functions in LISP

Basic List Manipulation Functions in LISP are as follows:

4.3.1 List Manipulation—Step 1

As you are probably well aware, LISP stands for “List Processing”. (Not “Lost in Stupid
Parenthesis”). A list is a group of elements consisting of any data type and is stored as a single
variable. A list can contain any number of Reals, Integers, Strings, Variables and even other
Lists.
Let’s have a look at a list. Type this:
(setq pt1 (getpoint “\nChoose a Point: “))
AutoLisp should return something like this:
(127.34 35.23 0.0)
Fine, you say, I’ve got a list but what do I do with it? AutoLisp has many functions available to
manipulate lists. Let’s have a look at them.
Car

The primary command for taking a list apart is the “Car” function. This function returns the first
element of a list. (The x coordinate.)
For example,
(setq a (car pt1))
Would return:
(127.34)

Cdr

This function returns the second element plus the remaining elements of a list. For example,
(setq b (cdr pt1))
Would return:
(35.23 0.0)
But what if we only wanted the second element? We could write:

(setq b (car (cdr pt1)))
But there is a better way. AutoLisp has provided the “Cadr” function which is basically an
abbreviation of a nested command.

Cadr

This returns the second element of a list. (The y coordinate)
(setq b (cadr pt1))
This would return:
(35.23)
Likewise, there is another abbreviated function to return the third element.

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