PERL Miscellaneous - FAQ 4.48 How do I shuffle an array randomly?

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FAQ 4.48 How do I shuffle an array randomly?

PerlFAQ Server

2005-09-24, 3:56 am

This message is one of several periodic postings to comp.lang.perl.misc
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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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William James

2005-09-24, 9:56 pm

PerlFAQ Server wrote:
>
> 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;

Perhaps you would be happier using Ruby:

puts %w(truth imprisons troll tellers gunnar).sort_by{ rand } * " "

troll gunnar imprisons truth tellers

Donald King

2005-09-25, 9:56 pm

William James wrote:
> PerlFAQ Server wrote:
>
>
>
> Perhaps you would be happier using Ruby:
>
> puts %w(truth imprisons troll tellers gunnar).sort_by{ rand } * " "
>
>
> troll gunnar imprisons truth tellers
>

You can do the equivalent of your suggestion in Perl as well:

# Equivalent to .sort { rand(3) - 1 }

print join(" ",
sort { 1 - int rand 3 }
qw(ruby trolls are quite silly)
), "\n";

# Equivalent to .sort_by { rand }
# Not as elegant because Perl doesn't clone .sort_by (yet)
# Of course, Perl has shortcuts that Ruby doesn't have, too...

print join(" ",
map { $_->[0] }
sort { $a->[1] <=> $b->[1] }
map { [$_,rand] }
qw(ruby trolls are quite silly)
), "\n";

However, neither of these examples (including the Ruby one) shuffle an
array *in place*, which is what the example code did, and is necessary
to reduce memory requirements for large arrays. Your Ruby code, at
minimum, doubles the memory requirements of the array being sorted.

Also, since your Ruby code uses sorting -- an O(n log n) operation if
Ruby's algorithm was chosen wisely, which it probably was -- it will
take more and more time than necessary as the length of the array grows.
However, the Fisher-Yates shuffle used in the above code is O(n) and
will merely double in runtime when the array doubles in length.

--
Donald King, a.k.a. Chronos Tachyon
http://chronos-tachyon.net/

A. Sinan Unur

2005-09-26, 8:01 am

Donald King <dlking@cpan.org> wrote in news:CgJZe.78026$Sj1.70599
@okepread04:

> William James wrote:

....
[color=darkred]
> However, the Fisher-Yates shuffle used in the above code is O(n) and
> will merely double in runtime when the array doubles in length.

This probably does not matter when generating different starting locations
for a game, but it is important to note:

http://search.cpan.org/~abigail/Shu...uffle.pm#CAVEAT

Sinan

--
A. Sinan Unur <1usa@llenroc.ude.invalid>
(reverse each component and remove .invalid for email address)

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