Re: .Re: simple lisp function

HenHanna <HenHanna@dev.null> writes:

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 '- '())          ===>   ((-))

>
>
I remember writing this in Lisp and Python.  a few years ago.
>
>
Is Scheme (or Gauche) used outside of the Academia?     What kind of people
(other than Grad students, Researchers in
Prog.Lang design)
              would know about  Split-at  and  List-ec  and  Receive ?

lisp programmers?

CL-USER> (mapcon #'list '(X 1 2 3 4))
((X 1 2 3 4) (1 2 3 4) (2 3 4) (3 4) (4))