FAQ 4.51 How do I permute N elements of a list?
- From: PerlFAQ Server <comdog@xxxxxxxxx>
- Date: Thu, 21 Jul 2005 10:03:01 +0000 (UTC)
This message is one of several periodic postings to comp.lang.perl.misc
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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is discussed in Volume 4 (still unpublished) of Knuth's *The Art of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 {print"@_\n"} split;
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