Re: Random Number Generation

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <DyD3DF.Exo@thomsoft.com> kst@thomsoft.com (Keith Thompson) writes:

  > It might have been nice if the Random function had been declared like
  > this:

  >     function Random
  >	  ( Gen   : Generator; 
  >	    First : Result_Subtype := Result_Subtype'First;
  >	    Last  : Result_Subtype := Result_Subtype'Last );

  > allowing successive calls to Random with different specified bounds to
  > share the same generator, but this wasn't done.

   This wasn't done because defining the required behavior of such a
generator is beyond the state of the art.  (Not defining the
generator, THAT part is easy.)  We could have tried to extent the art
by defining tests for randomness for such a generator, but that is not
a job for a programming language standards committee.

  > Probably the best way to handle this is to use Ada.Numerics.Float_Random
  > and scale the result to the desired discrete range.  I think someone
  > else has posted code that does this.  Watch for rounding errors near
  > the endpoints!

  Yes, this can usually be done as a "one-liner," but if you think you
need the capability, think the issue through carefully.  There are
some cases where you need such behavior, but precious few.  (The bias
that can be introduced by rounding from float to integer is why I
don't want to post code without knowing the hardware, and the RNG
used.  But as long as there are hundreds of values from the generator
mapped to each possible integer value, the bias shouldn't cause a
problem.)

--

					Robert I. Eachus

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