Re: [Bug-gnubg] The dice are (slightly) rigged

[Michael Petch]

On 22/11/2011 3:11 PM, Philippe Michel wrote:

> Are there list members who are knowledgeable on rng robustness and could 
> confirm that, if one starts with a "good" rng for integers uniformly 
> distributed between 0 and n and discards any occurence of the top p 
> values, one gets a good rng for the [0, n-p] interval ?
> 
> Intuitively, it is tempting to say : "yes, and it is a condition for the 
> original generator being any good" but I'm afraid it may be a treacherous 
> domain.
> 

I brought this up a good 8 years ago. Previously it was using straight
modulo. My original post was:

http://lists.gnu.org/archive/html/bug-gnubg/2003-08/msg00410.html

Joern Thyssen
http://lists.gnu.org/archive/html/bug-gnubg/2003-08/msg00438.html

The fix he put in was based on this information which you will note in
his response:

-------

I've modified the code to use:

        anDice[ 0 ] = 1+(int) (6.0*rand()/(RAND_MAX+1.0));

instead. This is suggested on the rand(3) man page:

       In Numerical Recipes in C: The Art of Scientific Computing
       (William  H.  Press, Brian P. Flannery, Saul A. Teukolsky,
       William T.  Vetterling;  New  York:  Cambridge  University
       Press, 1992 (2nd ed., p. 277)), the following comments are
       made:
              "If you want to generate a random integer between 1
              and 10, you should always do it by using high-order
              bits, as in

                     j=1+(int) (10.0*rand()/(RAND_MAX+1.0));

              and never by anything resembling

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

              (which uses lower-order bits)."

-- 
Michael Petch
CApp::Sysware Consulting Ltd.
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