List Manipulation - Page I.

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.

Caddr

This returns the third element of a list. (The z coordinate.)

	(setq c (caddr pt1))

Would return :

	(0.0)

AutoLisp has other functions that will retrieve values from lists of more than three elements. (Caar, cadar, etc). You can, though, us another function to access any element of a list. This is the "nth" function.

nth

The syntax for the nth function is as follows :

	(nth num list)

"num" is the number of the element to return. Just remember that zero is the first element. For example given the list :

	(setq d '("M10" "M20" "M30" 10.25))
	(setq e (nth 0 d))

Would return :

	("M10")

And likewise :

	(setq f (nth 3 d))

Would return :

	(10.25)

On the next page we will look at a practical example of using these functions.