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From: Brian Inglis <Brian DOT Inglis AT SystematicSW DOT Invalid>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: random() : What am I doing wrong?
Organization: Systematic Software
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Date: Sat, 08 Sep 2007 00:45:58 GMT
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To: djgpp AT delorie DOT com
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On Wed, 5 Sep 2007 11:34:39 -0400 in comp.os.msdos.djgpp, DJ Delorie
<dj AT delorie DOT com> wrote:

>
>> Well, okay - random() doesn't anything like guarantee
>> equidistribution.  But that said, wouldn't scaling the result set
>> at least not give you anything *worse* than what you started with?
>
>If you do the math, you see that there are 47 numbers left over when
>you mod MAX_INT % 50.  That means the last three numbers (47, 48, 49)
>are slightly less likely to be chosen.  The difference in probability?
>1 in 42,949,672 (0.9999999767169356).
>
>So, for a range this small, it really doesn't matter, and if it *does*
>matter, you probably already know more about random number generation
>than any of us here.
>
>However, integer modulus is a LOT cheaper than floating point
>division.

IIRC the best distributed approach is to take a sufficient number of
bits b around the middle of the random integer, shift right to give you
a set 0..2^b-1 and if the number is bigger than you can use, try again
with a new random integer. 

-- 
Thanks. Take care, Brian Inglis 	Calgary, Alberta, Canada

Brian DOT Inglis AT CSi DOT com 	(Brian[dot]Inglis{at}SystematicSW[dot]ab[dot]ca)
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