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Manipulating Lists

We've already seen the procedure list that constructs a list out of several items. Here are some more procedures for working with lists.

Returning the first element of a list: first

The procedure first returns the first element of a list:

CL-USER > (first '(23 34 45))
23

There are similar procedures second, third, and so on up to tenth for getting later items in a list.

Returning all but the first element of a list: rest

The procedure rest takes a list and returns the list minus the first element:

CL-USER > (rest '(23 34 45))
(34 45)

Note that first and rest were traditionally called car and cdr; you may see references to these alternative names in Lisp.

Returning the nth element of a list: nth

The procedure nth takes a number and a list and returns the nth element of the list, counting the first element as zero:

CL-USER > (nth 1 '(23 34 45))
34

Note that usually Lisp counts things starting from zero; the procedures first and second are exceptions to this, so:

(nth 0 lst) is equivalent to (first lst)

and (nth 1 lst) is equivalent to (second lst).

Finding the length of a list: length

The procedure length returns the length of a list; for example:

CL-USER > (length '(1 2 3))
3

Constructing lists: cons

The procedure cons takes two parameters - an object and a list - and returns a new list with the object added to the front of the list. For example:

CL-USER > (cons 1 '(2 3 4 5 6))
(1 2 3 4 5 6)

Note that the first object can itself be a list:

CL-USER > (cons '(0 1) '(2 3 4 5 6))
((0 1) 2 3 4 5 6)

Joining lists: append

The procedure append takes any number of lists, and joins them all together into a single list. For example:

CL-USER > (append '(1 3 5 7) '(2 4 6 8))
(1 3 5 7 2 4 6 8)

Reverse a list: reverse

The procedure reverse takes a list and reverses it:

CL-USER > (reverse '(1 2 3 4))
(4 3 2 1)

Combining the list procedures

Using these procedures for manipulating lists, we can define a new procedure to perform just about any operation on lists that we need.

For example, let's define a procedure insert that inserts an item between the first and second items in a list. So:

CL-USER 1 > (insert 2 '(1 3 4 5 6))

should give:

(1 2 3 4 5 6)

We can do it as follows:

(defun insert (item lst)
  (cons (first lst)
        (cons item
              (rest lst))))

Exercises

1. Swap the first two items in a list

Write a procedure swap to exchange the first two items of a list. Check that:

(swap '(9 8 7 6))

gives:

(8 9 7 6)

2. Duplicate the first item in a list

Write a procedure dup to duplicate the first item in a list, Check that:

(dup '(0 1 2 3))

gives:

(0 0 1 2 3)

3. Return a random item from a list

Write a procedure random-elt that returns a random element of a list. For example:

(random-elt '(11 22 33 44))

would randomly return one of the four numbers. Hint: use nth and random.

4. Return the last item in a list

Write a procedure last-elt which returns the last item in a list. For example:

(last-elt '(1 2 3 4))

should return 4.


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