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From: doucet@laas.fr (Jean-Etienne Doucet)
Newsgroups: comp.lang.ada
Subject: Re: Ada2005 random
Date: 7 Apr 2003 07:26:28 GMT
Organization: LAAS/CNRS, Toulouse, France
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Randy Brukardt wrote:
|
| I needed that for a 'choosing' algorithm, say drawing balls one by one
| out of a bowl. If you have 8 balls originally, you're choosing 1 out of
| 7, then 1 out of 6, etc. If you try to use 1 out of 8 the whole time and
| discarding the useless ones, the second last draw can take a very long
| time (only 2 out of 8 choices are meaningful). I'm pretty sure that this
| is a valid technique; certainly the underlying float generator is still
| random (at least as much as it ever was!), and the use of that result is
| unbiased.
|

For this kind of problems, I use the Ada.Numerics.Float_Random generator,
along with the function:

  function Rand (N : Positive) return Positive is
     -- gives a random integer in the range 1 .. N
  begin
     return Integer(Float(N) * Random(Gen) + 0.5);
  end Rand;


It can be used for ranges other than 1 .. N:

  function Rand (Min, Max : Integer) return Integer is
     -- gives a random integer in the range Min .. Max
  begin
     return Rand(Max - Min + 1) + Min - 1;
  end Rand;


It works as well for enum types:

  type Enum is (...);
  
  function Random_Enum return Enum is 
     -- gives a random Enum value
  begin
     return Enum'Val(Rand(Enum'Pos(Enum'Last) + 1) - 1);
  end Random_Enum;


I've never had a problem with this.

_______________________________________________________________________________
Jean-Etienne Doucet / LAAS-CNRS / Toulouse, France       E-mail: doucet@laas.fr
"S'il n'y a pas de solution, c'est qu'il n'y a pas de probl�me."  (Les Shadoks)