# FAQ 4.48 How do I shuffle an array randomly?

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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 .

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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

srand;
@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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## Re: FAQ 4.48 How do I shuffle an array randomly?

This gives an "Modification of non-creatable array value attempted,
subscript -1" exception if the @\$deck is empty.

## Re: FAQ 4.48 How do I shuffle an array randomly?

Yes it does.

Did you mean to make some point regarding that?

--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.noitatibaher0cmdat/"

## Re: FAQ 4.48 How do I shuffle an array randomly?

Tad J McClellan wrote:

I thought it was rather obvious that he was pointing out a problem with
that example in that FAQ, as it's customary to fish out any bugs that
end up in such examples, is it not?

--
szr

## Re: FAQ 4.48 How do I shuffle an array randomly?

In article

Thanks for finding that. I'll adjust the answer to fix that problem.