16.4.4 List Operations

Build Lists

The function cons builds lists. If its second argument is a list, it returns a new list with the first argument added to the front.

> (cons ’a ’(b c d))
(A B C D)

We can build up lists by consing new elements onto an empty list. The list function is just a more convenient way of consing several things onto nil.

> (cons ’a (cons ’b nil))
(A B)
> (list ’a ’b)
(A B)

The primitive functions for extracting the elemnets of lists are car and cdr. The car of a list is the first element, and the cdr is everything after the first element.

> (car ’(a b c))
A
> (cdr ’(a b c))
(B C)

You can use combinations of car and cdr to reach any element of a list. If you want to get the third element, you could say:

> (car (cdr (cdr ’(a b c d))))
C

You can do the same thing by calling third:

> (third ’(a b c d))
C