Re: All Possible Combinations

[Nordlöw <per.nordlow@gmail.com>]

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