Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl Monk, Perl Meditation
 
PerlMonks  

How do I shuffle an array randomly?

by faq_monk (Initiate)
on Oct 08, 1999 at 00:20 UTC ( #615=perlfaq nodetype: print w/ replies, xml ) Need Help??

Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

Use this:

    # fisher_yates_shuffle( \@array ) : 
    # generate a random permutation of @array in place
    sub fisher_yates_shuffle {
        my $array = shift;
        my $i;
        for ($i = @$array; --$i; ) {
            my $j = int rand ($i+1);
            next if $i == $j;
            @$array[$i,$j] = @$array[$j,$i];
        }
    }

    fisher_yates_shuffle( \@array );    # permutes @array in place

You've probably seen shuffling algorithms that works 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.

Log In?
Username:
Password:

What's my password?
Create A New User
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others cooling their heels in the Monastery: (19)
As of 2015-07-07 18:10 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









    Results (93 votes), past polls