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Re: Re: Rolling a biased die

by ferrency (Deacon)
on Apr 12, 2002 at 13:39 UTC ( #158567=note: print w/replies, xml ) Need Help??


in reply to Re: Rolling a biased die
in thread Rolling a biased die

Update:I'm totally wrong! Please ignore me! And take my advice: try the code before you claim it's broken :) Sorry, IO.

I don't think you want to put the rand() inside the loop. I think to get the correct results, you need to choose one random number outside the loop, and test it vs. the sum as you go along.

Try out a simple case by hand:

my %bias = (1 => 1, 2 => 1);
Assuming your first iteration picks up (1 => 1), you have:
$rand = $k if rand($sum += $v) <= $v; # This simplifies to: # $rand = 1 if rand(1) <= 1 # this is always true. That's wrong.
What you really want is something like this:
my $sum = 0; $sum += $_ foreach (values %bias); my $target = rand($sum); $sum = 0; while (my ($k, $v) = each %bias) { if ($target <= ($sum += $v)) { $rand = $k; last; } }
I'm sure there's a golfier way to do it, but this demonstrates the basic idea.

Update: Okay, after actually trying the code, I believe I'm totally wrong. I think that what I thought was a combination of two bugs may actually be a clever solution. Though I'm still not sure I believe it produces the correct result distribution. IO, would you care to describe how it works? Sorry about that.

Alan

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