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From: geert@fozzie.sun3.iaf.nl (Geert Bosch)
Subject: Re: Random Number Generation
Date: 1996/10/12
Message-ID: <53o3ot$han@fozzie.sun3.iaf.nl>#1/1
X-Deja-AN: 189102407
references: <53jgrh$r20@newton.cc.rl.ac.uk>
organization: La Calandre Infortunee
newsgroups: comp.lang.ada
Date: 1996-10-12T00:00:00+00:00
List-Id: <comp.lang.ada>


I wrote:
`` This is not a simple example. It is not possible to write an
   Ada program that does convert R1 to R2 and is guaranteed to
   finish in finite time. ''

Dr J Parker (jp3@rl.ac.uk) responded:
`` Nah... the rejection method is rigorously correct, and so is
   the subsequent use of x rem 3 in the code fragment below. ''

I don't say it is not possible to present a correct algorithm
to convert R1 to R2. Such a correct algorithm just is not guaranteed to
finish in finite time.

Your algorithm:
`` x := random;
   while x > R1'Last - 1024 rem 3 loop
      x := random;
   end loop;
   return x rem 3; ''

You can't give an upper boundary for the number of iterations
of the while loop if R1 is a truly random generator.

Of course in practise you're only interested in having an
algorithm that has a good probability to finish in a certain
amount of time.

I just pointed out that the example wasn't that simple.
Somebody using this code should be aware of that.

Regards,
   Geert
-- 
E-Mail: geert@sun3.iaf.nl    
      ``I think there is a world market for maybe five computers.''
        Thomas Watson,  chairman of IBM, 1943