Lisp Exercises Involving List Manipulation

Question

I trying to complete this exercise;

Write a Lisp function that takes as input a list of elements, such as (A B C)

, and returns a list in which the position of each element follows it, such as (A 1 B 2 C 3)

I'm trying to do it with two functions, however its not working correctly, I'm just getting the same list. Here is my code:

(defun insert (index var userList)
    (if (or (eql userList nil) (eql index 1))
            (cons var userList)
            (cons (car userList) (insert (- index 1) var (cdr userList)))))


(defun insertIndex (userList)
    (setq len (length userList))
    (loop for x from 1 to len
        do (insert x x userList)))

The insert functions seems to work fine on its own, but it seems like it doesn't do anything with loop. I'm new lisp and any help would be appreciated, thanks in advance.

Answer 1

Positions in Lisp start with 0. In insertIndex the variable len is not defined. The LOOP does not return any useful value.

If you want to solve it with recursion, the solution is much simpler.

You need to test for the end condition. If the list is empty, return the empty list.

Otherwise create a new list with the FIRST element, the current position and the result of calling the function on the rest of the list and the position increased by one.

(LIST* 1 2 '(3 4)) is shorter for (cons 1 (cons 2 '(3 4))).

Here is the example with a local function. To create a top-level function with DEFUN is now your task. You only need to rewrite the code a bit. LABELS introduces a potentially recursive local function.

(labels ((pos-list (list pos)
           (if (null list)
               '()
               (list* (first list)
                      pos
                      (pos-list (rest list) (1+ pos))))))
  (pos-list '(a b c d e f) 0))