[slime-devel] transpose-lists

[Helmut at common-lisp.net]

* Edi Weitz [2005-08-09 16:00+0200] writes:

> (defun transpose-lists (lists)
>   "Turn a list-of-lists on its side.
> If the rows are of unequal length, truncate uniformly to the shortest.
>
> For example:
> \(transpose-lists '((ONE TWO THREE) (1 2)))
>   => ((ONE 1) (TWO 2))"
>   (catch 'done
>     (loop with result
>           with collectors = (loop for list in lists
>                                   collect (let ((list list))
>                                             (lambda ()
>                                               (cond ((null list)
>                                                      (throw 'done result))
>                                                     (t (pop list))))))
>           collect (loop for collector in collectors
>                         collect (funcall collector)) into temp-result
>           do (setq result temp-result))))

Uugh, how ugly :-)

How about?

(defun transpose-lists (lists)
  (cond ((some #'null lists) '())
	(t (cons (mapcar #'car lists)
		 (transpose-lists (mapcar #'cdr lists))))))