Permutations: Difference between revisions

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 =={{header|Racket}}==
-<lang racket>#lang racket
+<lang Racket>
+#lang racket
+;; using a builtin
-(define (permutations ℓ)
+(permutations '(A B C))
-  (if (empty? ℓ) (list ℓ)
-      (append-map (λ (e) (map (curry cons e)
+;; -> '((A B C) (B A C) (A C B) (C A B) (B C A) (C B A))
-                              (permutations (remove e ℓ))))
+;; a random simple version (which is actually pretty good for a simple version)
-                  ℓ)))</lang>
+(define (perms l)
-Try it:
+  (let loop ([l l] [tail '()])
-<lang racket>(printf "Perumutations of ~a:\n~a\n\n" '(r o c) (permutations '(r o c)))
+    (if (null? l) (list tail)
-(printf "Time [in milliseconds] to generate permutations of 9 numbers:\n")
+        (append-map ( (x) (loop (remq x l) (cons x tail))) l))))
-(time (void (length (permutations (range 0 9)))))</lang>
+(perms '(A B C))
-Output:
+;; -> '((C B A) (B C A) (C A B) (A C B) (B A C) (A B C))
-<pre>Perumutations of (r o c):
+</lang>
-((r o c) (r c o) (o r c) (o c r) (c r o) (c o r))
-Time [in milliseconds] to generate permutations of 9 numbers:
-cpu time: 1672 real time: 1719 gc time: 816</pre>
 =={{header|REXX}}==
 ===names===