John Gilson wrote:
(defun power-set (set)
(let ((power-set (list ()))) ; power set always contains empty set
(labels
((insert (subset)
(loop for elt in set
until (eql elt (first subset))
collect (cons elt subset)))
(power-set-length-n (length-m-sets)
(mapcan #'(lambda (length-m-set)
(insert length-m-set))
length-m-sets))
(power-set-aux (length-m-sets)
;; n = m + 1
(unless (null length-m-sets)
(let ((length-n-sets (power-set-length-n length-m-sets)))
(setf power-set (append length-n-sets power-set))
(power-set-aux length-n-sets)))))
(power-set-aux power-set))
power-set))
(power-set '(1 2 3))((1 2 3) (1 2) (1 3) (2 3) (1) (2) (3) NIL)
Scheme
;; Adapted from a CL version.
(define (powerset List)
(if (null? List)
'(())
(let* ((x (car List))
(p (powerset (cdr List))))
(append p
(map (lambda(xs) (cons x xs)) p)))))
(powerset '(a b c))
===>
(() (c) (b) (b c) (a) (a c) (a b) (a b c))
(powerset '(a b c d))
===>
(() (d) (c) (c d) (b) (b d) (b c) (b c d) (a) (a d) (a c) (a c d)
(a b) (a b d) (a b c) (a b c d))
Gauche Scheme
(use util.match)
(define powerset
(match-lambda
[() '(())]
[(h . t) (let1 p (powerset t) `(,@p ,@(map. cons h p)))]))
Given:
(define (map. func obj seq)
(map (lambda(x) (func obj x)) seq))
Another way:
(use util.combinations)
(define (powerset L)
(append-map (^n (combinations L n)) (iota (+ 1 (length L)))))
| Date | Sujet | # | Auteur | |
| 7 Jul 25 |  Re: Homework Help! Subsets | 3 | B. Pym | |
| 31 Jul 25 |  Re: Homework Help! Subsets | 2 | B. Pym | |
| 31 Jul 25 |   Re: Homework Help! Subsets | 1 | B. Pym |
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