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From: "B. Pym" <No_spamming@noWhere_7073.org>
Newsgroups: comp.lang.lisp
Subject: Re: .Re: simple lisp function
Date: Thu, 6 Jun 2024 23:31:05 -0000 (UTC)
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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))))