Newsgroups: comp.os.msdos.djgpp
From: cigna AT helios DOT phy DOT OhioU DOT Edu (Dave Cigna)
Subject: Re: a randomize function for DJGPP?
Message-ID: <E6so1o.AI@boss.cs.ohiou.edu>
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Organization: Ohio University Physics and Astronomy
References: <5flbpp$m74 AT nr1 DOT ottawa DOT istar DOT net> <331FC0D1 DOT 1A26 AT cs DOT com> <E6oqJs DOT K2n AT boss DOT cs DOT ohiou DOT edu> <5fps67$pc3 AT news DOT ox DOT ac DOT uk>
Date: Sun, 9 Mar 1997 21:08:12 GMT
Lines: 38
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

George Foot <gfoot AT mc31 DOT merton DOT ox DOT ac DOT uk> wrote:
>Dave Cigna (cigna AT helios DOT phy DOT OhioU DOT Edu) wrote:
>
>Admittedly, using '%' with a number which is not a power of two will 
>give slightly more low numbers than high, but...
>
>: The best way to get get random numbers uniformly distributed from
>: 0 to X-1 is to use:
>
>:   n = ((double)random() / RAND_MAX) * X;
>
>: If you want to cast n to an int, that's your business.
>
>... this is no better. For example, suppose that MAXINT were 4, and you 
>set X to be 3. Then n would be one of {0.0,0.75,1.5,2.25}, and when cast
>to int this becomes {0,0,1,2} which is the same result as random()%3.

Yes, you're right. After thinking about it more carefully I realize
that there will always be (RAND_MAX % X) values that are favored. 

>This is getting pedantic, but you cannot (simply) get a truly random 
>distribution over X numbers from a random function returning one of
>2^n numbers. I say simply; one solution is to keep taking random() until
>the value is in the range you want. A more efficient technique is to keep
>taking random()&Y until it is in the required range, with Y=2^n-1 such
>that Y>=X-1.
>
>In practise, though, MAXINT and RAND_MAX are so high that it doesn't 
>really matter.

Well, whether on not it's pedantic depends on what you're using the 
numbers for. There are applications (for example Monte Carlo type
calculations) where non-uniformity of even this magnitude is a real 
problem. But anyone using rand() or random() for this sort of thing 
deserves what they get. So you are right, in practise it doesn't really 
matter.

 -- Dave Cigna