Question

I wrote my silly function which returns a list without the last element in common lisp. Is there any more elegant solution to this problem?

Here’s my code:

(defun list-without-last (l)
  (if (> (length (rest l)) 0)
      (append (list (first l)) (list-without-last (rest l)))
      nil))

Answer 1

Your function has two problems:

• you are using LENGTH. LENGTH has to scan the whole list.

• you are using APPEND. Try to use CONS. CONS is simpler.

Common Lisp also already provides this function. It is called BUTLAST.

In real code we also would not use recursion. The stack size would limit the length of lists we can process.

An iterative version using the LOOP macro:

CL-USER> (defun my-butlast (list)
           (loop for l on list
                 while (rest l)
                 collect (first l)))
MY-BUTLAST                                                                                                                                      
CL-USER> (compile 'my-butlast)
MY-BUTLAST                                                                                                                                      
NIL                                                                                                                                             
NIL                                                                                                                                             
CL-USER> (my-butlast '(1 2 3 4 5))
(1 2 3 4)                                                                                                                                       
CL-USER> (my-butlast '(1))
NIL                                                                                                                                             
CL-USER> (my-butlast '(1 2))
(1)