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

Discussion in 'Perl Misc' started by PerlFAQ Server, Mar 6, 2011.

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    function is discussed in Volume 4 (still unpublished) of Knuth's *The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