The basic idea is, looking at each slot in the array, from the last to the first, swap it with one other before it in the array, chosen at random. That is, if you have an array of length 6, swap $a[5] (the last element) with one chosen at random from @a[0..4], then swap $a[4] with one from @a[0..3], and so on down. Obviously, when you get to $a[1], you can stop.

This is a very effective shuffling technique (assuming you've got a good random number generator). For a good explanation of why, you can read the documentation for Abigail's module.

jdporter
...porque es dificil estar guapo y blanco.

Basically the method works like this.

N is the number of items in the array.

Pick an element from 1 to N at random. Switch that element with the first element.

Now pick an element between 2 and N at random. Switch that element with the second element.

Then pick an element beween 3 and N at random. And switch that element with the third element.

Repeat those two step N-1 times and you have shuffled the array.

If that doesn't explain it for you, try getting a pack of card, put them in order and do the process manually.Even using cards 1 to 6 and a dice, and doing the process a couple of times should clarify things if my words don't help.

In Perl, the counting runs 0 to N-1 each time, but the logic remains the same.

Ah, ok. Make more sense now (yea, I got the N-1 stuff ;)). Didn't exactally know how the whole process worked. Thanks =)

Hi BrowserUk,

This doesn't give a fair shuffle. This is well documented, as it is in the FAQ, and is explained well in the Perl Cookbook. A recent node explains it. Check here.

cheers

thinker

Are you saying my description is inaccurate? If so, please clarify in what way you think that it is wrong.

My description is accurate. The link you give chooses a random value from 1 to n each time. My description reduces the range by 1 each time. This is the correct Fisher-Yates technique.

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Nah! You're thinking of Simon Templar, originally played (on UKTV) by Roger Moore and later by Ian Ogilvy

I'm suprised no one has mentioned it yet, but List::Util has a shuffle() method that does just that.

use List::Util qw( shuffle ) ;

my @arr = qw( a b c d e f A B C D E F ) ;

print( ( map { "[$_]" } @arr ), "\n" ) ;
for ( 1..5 )
{
    print( ( map { "[$_]" } shuffle @arr ), "\n" ) ;
}
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