Question

I have to reverse the elements of a simple (single-dimension) list. I know there’s a built-in reverse function but I can’t use it for this.

Here’s my attempt:

(defun LISTREVERSE (LISTR)
    (cond
        ((< (length LISTR) 2) LISTR) ; listr is 1 atom or smaller
        (t (cons (LISTREVERSE (cdr LISTR)) (car LISTR))) ; move first to the end
    )
)

Output pretty close, but is wrong.

[88]> (LISTREVERSE '(0 1 2 3)) 
((((3) . 2) . 1) . 0)

So I tried to use append instead of cons:

(t (append (LISTREVERSE (cdr LISTR)) (car LISTR)))

But got this error:

*** - APPEND: A proper list must not end with 2

Any help?

Answer 1

I can give you a couple of pointers, because this looks like homework:

• The base case of the recursion is when the list is empty (null), and not when there are less than two elements in the list
• Consider defining a helper function with an extra parameter, an “accumulator” initialized in the empty list. For each element in the original list, cons it at the head of the accumulator. When the input list is empty, return the accumulator

As an aside note, the above solution is tail-recursive.