Re: Random Number Generation

[stt@houdini.camb.inmet.com (Tucker Taft)]

Nigel J. Tracey (njt@minster.york.ac.uk) wrote:

: Can anyone point me in the direction of a routines to do
: random number generation. The requirements are as follows.

: 	A routine which will generate a integer between a given
: 		minimum and maximum value. Up to Integer'First and Integer'Last

: 	A routine which will do the same for floats i.e. a random
: 		value from Float'First to Float'Last but between two
: 		given values.

: The values of the min and max value for the above routines
: are not known at compile time, hence why I don't thing
: I can use Ada.Numerics.Discrete_Random

Discrete_Random does not require compile-time-known values.
However, it does require that the values be known at the time
of the instantiation.  Hence, the following is legal:

    subtype Random_Range is Integer range F(x) .. G(y);
    package Dyn_Random is new Ada.Numerics.Discrete_Random(Random_Range);

However, it may be want to change the range after the instantiation
takes place, or the instantiation needs to be at the library level,
and you won't know these values until you are inside the main
subprogram.

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.
Some care may be needed when converting from float back to integer,
to provide the desired distribution, if you want the end points to
be just as likely as any other value.  Conversion from
float to integer rounds, so you may want to use the following
trick (using "mod") to get the desired range, with no spillover to 
High_Bound+1 (this trick originates with Robert Eachus, I believe):

  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;
    ...

: Any help would be very much appreciated (please also e-mail
: a copy of relies as the news feed is a little unreliable here)

: Thanks a lot,

: 	Nigel

: --------------------
: Nigel J. Tracey MEng, Research Associate (High Integrity Systems Group).
: Department of Computer Science,   Office: X/D016
: University of York,               Tel   : [0|+44]1904 432769
: York, England.                    E-Mail: njt@minster.york.ac.uk
: YO1 5DD.                          URL   : http://sv1pc161.cs.york.ac.uk/~njt

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA