Question

Say, that I have a pre-defined function ‘sum’ elsewhere.

I want to sum two lists after I do (setq a '(4 3 4)) and (setq b '(6 10 9))

And I do (recurse a b).

However, I keep getting ‘nil’ as the return value. What am I doing wrong with this recursion?

Trace isn’t being helpful at the moment.

(defun recurse (x y)
    (cond
         ( (null x) nil) )
         (t   (sum (car x) (car y) ) (recurse (cdr x) (cdr y)) )

    )
)

Answer 1

You need to cons up the results; otherwise they are thrown out.

(defun recurse (x y)
  (cond
    ((null x) nil)
    (t (cons (sum (car x) (car y))
             (recurse (cdr x) (cdr y))))))