Re: .Re: simple lisp function

On 6/6/2024, B. Pym wrote:

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))))))

 
Gauche Scheme:
 
(use srfi-1)  ;; split-at
(use srfi-42) ;; list-ec
 
(define (variations x lst)
  (list-ec (: i (+ 1 (length lst)))
    (receive (a b) (split-at lst i)
      `(,@a ,x ,@b))))
 
(variations '- '(a b c))
  ===>
((- a b c) (a - b c) (a b - c) (a b c -))
 
(variations '- '(a))
  ===>
((- a) (a -))
 
(variations '- '())
  ===>
((-))
 

Gauche or Racket Scheme:

(use srfi-42) ;; list-ec for Gauche
  or
(require srfi/42) ;; list-ec for Racket

(define (variations item seq)
  (list-ec (: i (+ 1 (length seq)))
    `(,@(take seq i) ,item ,@(drop seq i))))