Re: Random Number Generation

[jp3@rl.ac.uk ( Dr J Parker)]

Tucker Taft suggests: 

"In any case, Ada.Numerics.Float_Random is the easiest one to adapt to your
use, since it always generates values between 0.0 and 1.0.  You
can then multiply that value by an appropriate integer or float
and add the low bound to produce the desired dynamic range.

  declare
    Result : Integer range Low_Bound .. High_Bound;
    Range_Length : constant Integer := High_Bound - Low_Bound + 1;
  begin
    Result := (Integer(Float(Range_Length) *
      Ada.Numerics.Float_Random.Random(Gen)) mod Range_Length) + Low_Bound;
    ...
"


Careful, there are some limits in which this fails. Simple example:
Suppose you have a high quality random number generator R1 in the range
0 .. 127
and you want to turn it into a high quality random number generator R2
in the range 0 .. 82.
Only the rejection method works: you must throw out at least
45 of the values returned by R1.  eg, if R1 returns a number in the range
83..127, then reject it and keep calling R1 until you get
a number in the range 0 .. 82.  You'll get a stream of high quality random
numbers in the range 0 .. 82.  In general, you reject just enough to make
the number of elements (think bins) in range of R1 an integer multiple of
the number of elements in the range of R2 (you reject never more than 50% of
the number of elements in range of R1).  Then you
can use "*"'s, etc to transform R1 into R2. Floating point doesn't
change the problem.