Please analyze the code carefully, pull out your school books, and see if I have made a mistake.

Yes you have. You're not emulating a Fisher-Yates shuffle :-)

Consider the original code from perlfaq:

sub fisher_yates_shuffle {
    my $deck = shift;  # $deck is a reference to an array
    my $i = @$deck;
    while ($i--) {
        print $i, "\n";
        my $j = int rand ($i+1);
        @$deck[$i,$j] = @$deck[$j,$i];
    }
}

[download]

Note how $i is decremented on each iteration. Consider how that alters the sequence of possible indices.

Once you take that into account you get the textbook behaviour.

sub fixed_fisher_yates_shuffle {
    my ($deck, $rand) = @_;
    my $i = @$deck;
    while ($i--) {
        my $j = shift @$rand;
        @$deck[$i,$j] = @$deck[$j,$i];
    }
}

use Set::CrossProduct;
my $i = Set::CrossProduct->new([ [0..4], [0..3], [0..2], [0..1], [0] ]
+);
my %count;
while (my $a = $i->get) {
    print "@$a : ";
    my @foo = (1,2,3,4,5);
    fixed_fisher_yates_shuffle(\@foo, $a);
    print "@foo\n";
    $count{"@foo"}++;
};

foreach my $key (sort keys %count) {
    print $key, " = ", $count{$key}, "\n";
};

[download]

In reply to Re: Fisher-Yates theory... does this prove that it is invalid? by adrianh
in thread Fisher-Yates theory by Anonymous Monk

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