FAQ 4.51 How do I permute N elements of a list?


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4.51: How do I permute N elements of a list?

Use the "List::permutor" module on CPAN. If the list is actually an
array, try the "Algorithm::permute" module (also on CPAN). It's written
in XS code and is very efficient:

use Algorithm::permute;

my @array = 'a'..'d';
my $p_iterator = Algorithm::permute->new ( \@array );

while (my @perm = $p_iterator->next) {
print "next permutation: (@perm)\n";

For even faster execution, you could do:

use Algorithm::permute;

my @array = 'a'..'d';

Algorithm::permute::permute {
print "next permutation: (@array)\n";
} @array;

Here's a little program that generates all permutations of all the words
on each line of input. The algorithm embodied in the "permute()"
function is discussed in Volume 4 (still unpublished) of Knuth's *The
Art of Computer Programming* and will work on any list:

#!/usr/bin/perl -n
# Fischer-Krause ordered permutation generator

sub permute (&@) {
my $code = shift;
my @idx = 0..$#_;
while ( $code->(@_[@idx]) ) {
my $p = $#idx;
--$p while $idx[$p-1] > $idx[$p];
my $q = $p or return;
push @idx, reverse splice @idx, $p;
++$q while $idx[$p-1] > $idx[$q];

permute { print "@_\n" } split;

The "Algorithm::Loops" module also provides the "NextPermute" and
"NextPermuteNum" functions which efficiently find all unique
permutations of an array, even if it contains duplicate values,
modifying it in-place: if its elements are in reverse-sorted order then
the array is reversed, making it sorted, and it returns false; otherwise
the next permutation is returned.

"NextPermute" uses string order and "NextPermuteNum" numeric order, so
you can enumerate all the permutations of 0..9 like this:

use Algorithm::Loops qw(NextPermuteNum);

my @list= 0..9;
do { print "@list\n" } while NextPermuteNum @list;


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