Why does this LISP code emit periods or complain that lists must not end in 4?

Question

I just started learning LISP today just for the heck of it, so I am completely new to it. I do have experience in other languages though. I tried to write a function that returns a list exactly as is, except without the last element.

While I intend to rewrite this function anyway since I'm sure there's a simpler way of doing it, my version produced some very unusual output. For the record, I'm using the CLISP environment.

(defun my-butlast (L)
    (if (null (rest L))
      nil
      (if (eq nil (my-butlast (rest L)))
         (first L)
         (cons (first L) (my-butlast (rest L)))
      )
    )
)

(my-butlast '(1 2 3 4 5))

This produced the output (1 2 3 . 4)

And so my question is, where did the point come from?

Also, if I try to run (length (my-butlast '(1 2 3 4))) I get a mystifying error: A proper list must not end with 4. What does that mean?

You don't need the second if. Change the alternative to the second if alternative since (list x) is the same as (cons x '()) — Sylwester, Aug 15 '13 at 07:37
Please spend a few minutes learning [how to format Lisp code](http://dept-info.labri.u-bordeaux.fr/~idurand/enseignement/PFS/Common/Strandh-Tutorial/indentation.html). — danlei, Aug 18 '13 at 10:37

score 6 · Accepted Answer · answered Aug 15 '13 at 05:41

. is used in the representation of a cons whose cdr is not NIL. E.g.

(cons 1 2)

is displayed as

(1 . 2)

Consider what happens in your function if you do

(my-butlast '(1 2))

The test (eq nil (my-butlast (rest L)) will be true, so it returns (first L). Notice that this is just the number 1, not a list containing 1. You need to change to:

(if (eq nil (my-butlast (rest L)))
    (list (first L))
    (cons (first L) (my-butlast (rest L)))
)

Incidentally, it's more idiomatic to write (null (my-butlast (rest L))).

There's a bit more detail about printed representation of lists and proper versus improper lists in [my answer to a distinct, but related question](http://stackoverflow.com/a/16379759/1281433). — Joshua Taylor, Aug 15 '13 at 11:40

score 2 · Answer 2 · answered Aug 15 '13 at 05:39

Try doing I believe for your base case (it's been a while since I wrote lisp):

(list (first L))

(first L) will not return a list and cons of one element to another will create the structure you are looking at. Essentially your linked list is ending in [3|4] instead of [3|->] [4|0] with my lame ascii box diagrams.

Why does this LISP code emit periods or complain that lists must not end in 4?

2 Answers2