[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: All Possible Combinations
From: |
Nordlöw |
Subject: |
Re: All Possible Combinations |
Date: |
Wed, 3 Jun 2009 23:08:29 -0700 (PDT) |
User-agent: |
G2/1.0 |
On Jun 3, 8:50 pm, Marc Tfardy <tfar...@very-tfardol.com> wrote:
> Nordlöw schrieb:
>
>
>
> > Hey!
>
> > I want a function that generates all possible combinations (ordering)
> > of the elements in a list (or sequence if possible). Here is my
> > mockup:
>
> > (defun all-combinations (n)
> > "Generate a listing of all the possible combinations of the
> > elements in the sequence N. Time-Complexity is N!"
> > (let (all)
> > all))
>
> > For example (all-combinations '(a b c)) should return '((a b c) (a c
> > b) (b a c) (b c a) (c a b) (c b a))
>
> > Has somebody written such a function, preferrably in an iterative
> > rather than recursive way.
>
> Here you find working common lisp
> version:http://www.perlmonks.org/?node_id=485066
>
> (defun permutations (bag)
> "Return a list of all the permutations of the input."
> ;; If the input is nil, there is only one permutation:
> ;; nil itself
> (if (null bag)
> '(())
> ;; Otherwise, take an element, e, out of the bag.
> ;; Generate all permutations of the remaining elements,
> ;; And add e to the front of each of these.
> ;; Do this for all possible e to generate all permutations.
> (mapcan #'(lambda (e)
> (mapcar #'(lambda (p) (cons e p))
> (permutations
> (remove e bag :count 1))))
> bag)))
>
> Now replace 'remove' with 'remove*' and this works in emacs lisp, too.
>
> regards
> Marc
This gives the error: "e is undefined" in Emacs:
(defun all-combinations-gen (bag)
"Return a list of all the permutations of the input."
(if (null bag) '(())
;; Otherwise, take an element, e, out of the bag.
;; Generate all permutations of the remaining elements,
;; And add e to the front of each of these.
;; Do this for all possible e to generate all permutations.
(mapcan #'(lambda (e)
(mapcar #'(lambda (p) (cons e p))
(permutations
(remove* e bag :count 1))))
bag)))
(all-combinations-gen '(a b c))
/Per