From: Martin Stromberg <Martin DOT Stromberg AT lu DOT erisoft DOT se>
Message-Id: <200005191159.NAA08580@lws256.lu.erisoft.se>
Subject: Re: rand() in libc
To: djgpp-workers AT delorie DOT com
Date: Fri, 19 May 2000 13:59:06 +0200 (MET DST)
In-Reply-To: <200004271315.JAA03708@delorie.com> from "Dieter Buerssner" at Apr 27, 2000 04:20:23 PM
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Dieter said:
> On 27 Apr 00, at 14:05, Martin Stromberg wrote:
> > In particular I wonder how the ones generating doubles
> > (drand48(), erand48()) behave, as I improvised most on them.
> 
> My manpages don't document the method, that should be
> used, to create a [0,1[ double. If I understand your code correctly, 
> you "mix" the bits of the three state variables.
> 
> return
> (b16s2,b16s1,b16s0,b15s2,b15s1,b15s0, ..., b1s2,b1s1,b1s0)/2^48.
> 
> (b16s2 is the most significant bit of s2, and so on ...)
> 
> A more natural approach seems to me to just do
> 
>   s2*2^-16 + s1*2^-32 + s0*2^-48;
> 
> Doing this with floating point arithmetics will always yield "exact" 
> results, no rounding will be involved with binary floating point 
> representation.
> 
> Reason for my suggestion: s0 is not very random at all (period 2^16). 
> With the "mixing method", information of s0 will creep into the most 
> significant digits of the returned double.

Ok, how doI  easily say 2^-16, 2^-32 and 2^-48 to the compiler? Is it
ok to use pow(2, -16) etc. (efficiency)?


Right,

							MartinS