This is an excerpt from the latest version perlfaq4.pod, which

comes with the standard Perl distribution. These postings aim to

reduce the number of repeated questions as well as allow the community

to review and update the answers. The latest version of the complete

perlfaq is at

http://faq.perl.org .

--------------------------------------------------------------------

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

Use the "List:

ermutor" module on CPAN. If the list is actually an

array, try the "Algorithm:

ermute" module (also on CPAN). It's written

in XS code and is very efficient:

use Algorithm:

ermute;

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

my $p_iterator = Algorithm:

ermute->new ( \@array );

while (my @perm = $p_iterator->next) {

print "next permutation: (@perm)\n";

}

For even faster execution, you could do:

use Algorithm:

ermute;

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

Algorithm:

ermute:

ermute {

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function is discussed in Volume 4 (still unpublished) of Knuth's *The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];

@idx[$p-1,$q]=@idx[$q,$p-1];

}

}

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;

--------------------------------------------------------------------

The perlfaq-workers, a group of volunteers, maintain the perlfaq. They

are not necessarily experts in every domain where Perl might show up,

so please include as much information as possible and relevant in any

corrections. The perlfaq-workers also don't have access to every

operating system or platform, so please include relevant details for

corrections to examples that do not work on particular platforms.

Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in

perlfaq.pod.