I've been asked to come up with an algorithm to do a biased weighted dice roll function, I couldn't do it from the top of my head and after searching how to do it I've stumbled upon this piece of code from O'Reilly's Perl Cookbook: 
sub biased_die_roll {
    my %dist = (1=>0.01,2=>0.01,3=>0.01,4=>0.01,5=>0.48,6=>0.48);
    my ($key, $weight);

    my $rand = rand;
    while ( ($key, $weight) = each %dist ) {
             return $key if ($rand -= $weight) < 0;
    }
}

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As I've verified, this function indeed does what it's supposed to but I can't get my head around the inner while... For each dice face value ($key), if $rand = $rand - $weight is below zero, return $key. From what I understand, it will always produce less than zero output at the return line statement.

What's going on?

 Update: I was misinterpreting the weighted distribution hash, using actual weights instead of probabilities, once I realized that, it was easy to understand the algorithm.

In reply to weighted biased dice roll by sys0p

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