Re: Random Number Generation

[njt@minster.york.ac.uk (Nigel J. Tracey)]

Robert I. Eachus (eachus@spectre.mitre.org) wrote:
: In article <52ten0$ish@netty.york.ac.uk> njt@minster.york.ac.uk (Nigel J. Tracey) writes:
: 
:   > I really need
:   > to be able to generate random values in the range of
:   > Float'Safe_First..Float'Safe_Last. Or perhaps a variable
:   > sub-range of these. Any idea...
: 
:   With what distribution?  If you are using this for testing
: arithmetic units, I suggest unchecked converting a string of random
: bits of the right length.  This will be a distribution which covers
: all representable numbers equally. (To save Robert Dewar the
: trouble...Certain bit patterns do not correspond to legal floating
: point values.  This a legitimate use for 'VALID).  However if you want
: a numerically uniform distribution:
: 
:    2.0 * Float'Safe_Last * (Ada.Numerics.Float_Random.Random(Gen) - 0.5);
: 
:    Actually you may want to add more bits to the typical floating
: point generator value.
: 
: 
: --
: 
: 					Robert I. Eachus

I have managed to write something which works with some limitations.
The problem this has is that if the range includes 0.0 and is not
sufficiently large then there is a stong bias towards 0.0. e.g
numbers in the range -1.0..1.0 are most often 0.0. However for
larger ranges or ranges not including 0.0 it seem okay. An ideas
on an improvement to the 0.0 bias...

The code is as follows (Random_Seed is declared in the package
body, it is initialised to a random integer on ellaboration
and incremented after each reset so that repeated calls to
the routine do no give non random results, due to granularity
of the reset(Gen) function):

with  Ada.Numerics.Discrete_Random,
      Ada.Numerics.Float_Random;

   function Random(Min, Max : Float) return Float is

      subtype Exponent_Type is Integer range
         Float'Machine_EMin..Float'Machine_EMax;

      package Exp_Random is new
         Ada.Numerics.Discrete_Random(Exponent_Type);

      Gen1 		: Ada.Numerics.Float_Random.Generator;
      Gen2 		: Exp_Random.Generator;
      Mantissa : Float range -1.0..1.0;
      Result   : Float;

   begin
      Ada.Numerics.Float_Random.Reset(Gen1, Random_Seed);
      Random_Seed := Random_Seed + 1;
      Exp_Random.Reset(Gen2, Random_Seed);
      Random_Seed := Random_Seed + 1;

      loop
         Mantissa := -1.0;
         Mantissa := Mantissa + Ada.Numerics.Float_Random.Random(Gen1);
         Mantissa := Mantissa + Ada.Numerics.Float_Random.Random(Gen1);

         Result := Float'Compose(Mantissa, Exp_Random.Random(Gen2));
      exit when Result > Min and Result < Max;
      end loop;

      return Result;
   end Random;