Permutations: Difference between revisions

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 =={{header|Erlang}}==
-<lang Erlang>module(permute).
+<lang Erlang>-module(permute).
 -export([permute/1]).
--import(lists, [append/1, append/2, delete/2, map/2]).
+-import(lists, [append/1, map/2, seq/2, split/2]).
+%% insert element E into list L immediately after the item at position N
-permute([]) ->
+%% (N=0 prepends E to the list)
-  [[]];
-permute(L)  ->
+insert( L, N, E ) ->
+  {H, T} = split( N, L ),
-  append(map(fun(X) ->
+  append( [H, [E], T] ).
-               map(fun(P) -> append([X],P) end, permute(delete(X, L)))
-             end, L)).</lang>
+% Return a list of all the permutations of the given list
+permute( [] ) -> [[]];
+permute( [H|T] )  ->
+  append( map( fun(P) ->
+                 map( fun(N) ->
+                        insert(P, N, H)
+                      end,
+                      seq( 0, length(P) ))
+               end,
+               permute( T ))).
+</lang>
 Demonstration (escript):
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 <pre>[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]</pre>
 =={{header|Factor}}==
 The all-permutations word is part of factor's standard library. See http://docs.factorcode.org/content/word-all-permutations,math.combinatorics.html