Subject: fundamental principles of counting - 2nd try
From: Richard Fateman
Date: Fri, 06 Aug 2010 06:39:49 -0700
Look around for lisp programs, perhaps.
For example..
(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)))