Question

In common lisp I can do this:

(mapcar #'cons '(1 2 3) '(a b c))

=> ((1 . A) (2 . B) (3 . C))

How do I do the same thing in elisp? When I try, I get an error:

(wrong-number-of-arguments mapcar 3)

If elisp’s mapcar can only work on one list at a time, what is the idomatic way to combine two lists into an alist?

Answer 1

You want mapcar*, which accepts one or more sequences (not just lists as in Common Lisp), and for one sequence argument works just like the regular mapcar.

(mapcar* #'cons '(1 2 3) '(a b c))
((1 . A) (2 . B) (3 . C))

And even if it weren’t defined, you could easily roll your own:

(defun mapcar* (f &rest xs)
  "MAPCAR for multiple sequences"
  (if (not (memq nil xs))
    (cons (apply f (mapcar 'car xs))
      (apply 'mapcar* f (mapcar 'cdr xs)))))