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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Not intended for use in medical,
Date: 1997/05/19
Message-ID: <EACHUS.97May19155433@spectre.mitre.org>#1/1
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References: 
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Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1997-05-19T00:00:00+00:00
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In article <mheaney-ya023680001805971550190001@news.ni.net> mheaney@ni.net (Matthew Heaney) writes:

  > Funny you should bring that up.  We just wrote a small application
  > with 10 tasks, where each one wakes up every 5 seconds to check a
  > randomly-generated temperature.  In spite of the fact that each
  > task had its own generator, each one yielded the same value.  This
  > was undoubtedly caused by the fact the each generator had been
  > (automatically) seeded with the same time.

  No.  First read A.5.2(28).  There is a single state which all
generators are implicitly initialized to.  This was really chosen on
the assumption that any non-trivial user would use one of the Reset
functions.  An alternative would have been to have each new generator
start in a state uncorrelated with any other, but as my previous
message pointed out that is next to impossible for some generators.
At this point, I think that the best idea would have been to require
that the generators be set before being called by calling Reset and
otherwise raising Program_Error.

  Read what I wrote in the previous article for one solution to this
problem.  Generate a seed from the clock--you can use the explicit
generator that does this, but:

   Seed: Integer := Integer(Float(Seconds(Calendar.Clock) * 1000))

   is probably enough.  Now pass Seed, Seed+1, Seed+2, etc. to the
appropriate version of Reset.  Another solution is below:

  > Here's what we did to solve the problem, but it didn't work.
  > Instead of each task having its own generator, we made one
  > generator, and wrapped it in a protected object.  When each task
  > woke up, it called a protected function that returned the random
  > value:

  > protected Generator is
  >    function Random return Temperature;
  > private
  >    G : Random_Temperatures.Generator;
  > end;

  > protected body Generator is
  >    function Random return Temperature is
  >    begin
  >	 return Random_Temperatures.Random (G);
  >    end;
  > end;

  > I assumed each invokation of Generator.Random would return a
  > different random number (or at least random numbers evenly
  > distributed about the range of temperature), but this isn't so.
  > Every 5 seconds, all 10 tasks would wake up and call the protected
  > object.  I thought that these 10 invokations would generate
  > different random values, but all 10 tasks received the same random
  > value.  The value would change every 5 seconds, but all 10 tasks
  > would get that same value.

    Burned by the side effects of Random!  The problem is that you
have to access the generator from inside a protected procedure or
entry since you can't afford to have several tasks accessing a
function with side effects at the same time.

   Try:

  function Generator return Temperature;
  ...

  and in the body:

  protected Generator is
    procudure Protected_Random(Temp: out Temperature);
  private
    G : Random_Temperatures.Generator;
  end;

  function Random return Temperature is
    Temp: Temperature;
  begin
    Generator.Random(Temp);
    return Temp;
  end Random;

  protected body Generator is
    procedure Protected_Random(Temp: out Temperature) is
    begin
      Temp := Random_Temperatures.Random (G);
      return;
    end;
  end;



--

					Robert I. Eachus

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