Pascal Bourguignon wrote:

> > this simple function i'm trying to write is giving me headaches!
> > basically i want to do something that does:
> > (variations 'x '(y z)) -> ((x y z) (y x z) (y z x))
> >
> > i come from a procedural programming background and find functional
> > programming very confusing (especially recursion). can someone give
> > me some hints? my attempts at this make no sense so pasting them here
> > would only confirm my newbish forray into LSIP. thanks for any help!
>
> (defun variations (item list)
> (if (null list)
> (list (list item))
> (cons (cons item list)
> (mapcar (lambda (rest) (cons (car list) rest))
> (variations item (cdr list))))))

A monkey (Peter Seibel <peter@javamonkey.com>) wrote:

> (defun variations (x list)
> (loop for cons on (cons nil list) collecting
> (nconc (ldiff list (cdr cons)) (cons x (cdr cons)))))
>
> Okay, so that's arguably obfuscated Lisp. But you should never pass up
> a chance to combine LOOP, LDIFF, and abuse of Common Lisp's Lisp-2
> nature.

By using a Lisp instead of CL (COBOL-Like), we can make it shorter.

Gauche Scheme or Racket:

(require srfi/1) ;; iota for Racket

(define (variations x seq)
(map (lambda (i) `(,@(take seq i) ,x ,@(drop seq i)))
(iota (+ 1 (length seq)))))

(variations '- '(a b c))
===>
((- a b c) (a - b c) (a b - c) (a b c -))