# 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is discussed in Volume 4 (still unpublished) of Knuth's *The Art of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-Kause 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];
@idx[\$p-1,\$q]=@idx[\$q,\$p-1];
}
}

permute split;

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