From:	"A. Sinan Unur" <asu1 AT cornell DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: One method for small random nuimbers
Date:	Mon, 09 Jun 1997 07:54:57 -0400
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Bill Currie wrote:
> 
> Francois Charton wrote:
> >
> > Bill Currie wrote:
> > >
> > > Basicaly, I treat the number received from the main RNG as a
> > > bit stream and pull off the bits as needed.
> > >
> > I read about such a method being a bad thing in several books 
> > (Numerical Recipes warns a lot against this, and I remember reading 
> > something about it in Knuth, I'll dig it out tonight).
> 
> If you can send me a quote, that would be apreciated.
> 

i realize i am not the person to whom this request was directed, but in
light of the recent discussions, i think a few quotes can be useful.
(you can download chapters from 'merical recipes' at www.std.com/~nr/)

Numerical Recipes in C, 2nd ed. p.277:

(Linear congruential) generators have their low order (least
significant) bits much less random than their high order bits. If you
want to generatea random integer between 1 and 10, you should always do
it using high order bits as in:

  j = (int)(10.0*rand()/(RAND_MAX+1.0));

and never by anything resembling

  j = 1 + (rand() % 10);

more related to what bill currie was proposing ("I treat the number
received from the main RNG as a bit stream and pull off the bits as
needed.") is the remark on p.299, but their warning concerns the
opposite of what he wants to do:

A word of caution is: Don't use sequential bits from these routines as
the bits of a large, supposedly random, integer ...

pp.296-300 of their book is devoted to generation of random sequences of
bits.

  -- Sinan

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