How do I shuffle an array?

[vroom]

How do I shuffle an array?

Originally posted as a Categorized Question.

[vroom]

The Fisher-Yates shuffle is easy to code up:

# randomly permutate @array in place
sub fisher_yates_shuffle
{
    my $array = shift;
    my $i = @$array;
    while ( --$i )
    {
        my $j = int rand( $i+1 );
        @$array[$i,$j] = @$array[$j,$i];
    }
}

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

[runrig]

And mentioned numerous times in numerous places, the 'next if $i == $j' is unnecessary, as it rarely saves an element swap (except on the smallest of arrays), so it actually costs more to do the comparison every time rather than just swapping unconditionally.

BTW, in case anyone was wondering who these guys Fisher and Yates were, here's some links. They're hard to find when almost every reference to Fisher and Yates (and especially with the term 'shuffle') on Google is a perl reference :-)

[chromatic]

In older Perl versions (before 5.004) calling srand was necessary. Now it's called implicitly when rand is first used. See the second paragraph of srand for details. :)

[rajib]

shuffle( \@array );

sub shuffle
{
  my $array = shift;

  for ( my $i = 0; $i <= $#array; ++$i )
  {
    my $rand = rand($#array);
    @$array[$i,$rand] = @$array[$rand,$i];
  }
}

{Editor's note: This solution is flawed. Read Abigail-II's reply, Re: Answer: How do I shuffle an array?, for an explanation.}

[Abigail-II]

This "shuffles" the array, but not in a fair way. In fact, the algorithm contains two mistakes, a minor one and a severe one. Let's start with the minor one - in your loop you set $rand to rand $#array. But that means $rand could never be the last element of the array. However, fixing it that you take rand @array will not do you any good.

Let the size of the array be N. Then at each iteration, you select from N elements (after the fix given above), and change it with the one on position $i. You do this N times. Hence, you will get N^N different outcomes. There are however, only N! different permutations of the array. And since N! isn't a divisor of N^N for N > 2, some outcomes will be favoured over others by your algorithm.

Please use the Fisher-Yates shuffle as described in the FAQ. That one is fair.

Abigail

[hossman]

The List::Util module provides a shuffle function which implements the Fisher-Yates shuffle.

use List::Util qw(shuffle);

my @data = (0..51);

my @cards = shuffle @data;

[gmpassos]

Independet way:

  my @data = 0..51;
  my @cards = sort { (-1,1)[rand(2)] } @data ;

Graciliano M. P.
"The creativity is the expression of the liberty".

[wufnik]

I prefer japhy's one-liner, which achieves shuffling via fisher-yates:



@entry[-$i,$j] = @entry[$j,-$i] while $j = rand(@entry - $i), ++$i < @
+entry;

as an aside - this is in the snippets section. would it not be nice to use snippets, or grinder's fabled categorized snippets library, as a source of data for categorized q&a?

ttfn,

...wufnik

-- in the world of the mules there are no rules --

[jdhedden]

Math::Random::MT::Auto has a shuffle function, as well:

  use Math::Random::MT::Auto 'shuffle';

  my @cards = 0..51;
  shuffle(\@cards);

[perlplayer]

i have solved the issue in my own way i am newbie so if there is any inefficiency in my code please point out...as "there is more than one way to do it"

my @array=('one','two','three','four','five','six');

 &shuffle(\@array);







 sub shuffle{

 my $array=shift;
 my $max=@$array;
 #print $max;
 
 foreach(@$array)
 {
   my $random=int rand($max+1);
   
   unless($random eq $max)
   { 
     @$array[$random,$max]=@$array[$max,$random];
     $max--;
     }
    
    }
 
 }
 
 print "@array\n";

** i also have a querry if i omit the $max-- what wrong will happen

[moritz]

   my $random=int rand($max+1);
   unless($random eq $max)

The correct way to compare numbers is ==, not eq.

And when you want to exclude $max from the range of random numbers, write my $random = int rand($max); in the first place.