FAQ 4.48 How do I shuffle an array randomly?

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4.48: How do I shuffle an array randomly?

    If you either have Perl 5.8.0 or later installed, or if you have
    Scalar-List-Utils 1.03 or later installed, you can say:

            use List::Util 'shuffle';

            @shuffled = shuffle(@list);

    If not, you can use a Fisher-Yates shuffle.

            sub fisher_yates_shuffle {
                    my $deck = shift;  # $deck is a reference to an array
                    my $i = @$deck;
                    while (--$i) {
                            my $j = int rand ($i+1);
                            @$deck[$i,$j] = @$deck[$j,$i];

            # shuffle my mpeg collection
            my @mpeg = <audio/*/*.mp3>;
            fisher_yates_shuffle( \@mpeg );    # randomize @mpeg in place
            print @mpeg;

    Note that the above implementation shuffles an array in place, unlike
    the "List::Util::shuffle()" which takes a list and returns a new
    shuffled list.

    You've probably seen shuffling algorithms that work using splice,
    randomly picking another element to swap the current element with

            @new = ();
            @old = 1 .. 10;  # just a demo
            while (@old) {
                    push(@new, splice(@old, rand @old, 1));

    This is bad because splice is already O(N), and since you do it N times,
    you just invented a quadratic algorithm; that is, O(N**2). This does not
    scale, although Perl is so efficient that you probably won't notice this
    until you have rather largish arrays.


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