good chemistry is complicated,and a little bit messy -LW PerlMonks

### comment on

 Need Help??
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

In reply to Re: Re: Rolling a biased die by ferrency
in thread Rolling a biased die by MeowChow

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