# 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

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?

PerlFAQ Server wrote:
> 4.48: How do I shuffle an array randomly?

<snip>

>     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.

That comment makes me think of this thread:

where it was shown that using splice for an occational shuffling is well
as efficient as the alternatives even for arrays with thousands of elements.

Is the above too categorical?

--
Email: http://www.gunnar.cc/cgi-bin/contact.pl

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

> PerlFAQ Server wrote:
> > 4.48: How do I shuffle an array randomly?
>
> <snip>
>
> >     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.
>
> That comment makes me think of this thread:
>
>
> where it was shown that using splice for an occational shuffling is well
> as efficient as the alternatives even for arrays with thousands of elements.
>
> Is the above too categorical?

No, Perl is too atypical :)

It is sometimes difficult to demonstrate N**2 behavior in Perl when one of
the loops is done in Perl and the other internally to Perl in C.  You need
large N to even see the quadratic behavior.

As general programming principles the statements in the FAQ are true.  I
wouldn't change them.

Anno