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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: (no subject)
Date: 1998/11/13
Message-ID: <EACHUS.98Nov13170739@spectre.mitre.org>#1/1
X-Deja-AN: 411596580
References: <364C0C41.6CC14A8C@virgin.net> <72hgcq$5d4$1@nnrp1.dejanews.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-11-13T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <72hgcq$5d4$1@nnrp1.dejanews.com> dennison@telepath.com writes:

  > Ada.Numerics.Discrete_Random

  Good answer, but there is a typo/bug in gnat 3.10p and some earlier
versions that causes it to generate biased results. This program will
show you if you have the problem, if the values for the three doors
are not approximately equal.  (The fix is trivial, correct the sign in
the line below, but cause confusion by broadcasting a non-CM version
of the package. Of course, if you fix the installed version you have
to recompile it with -gnatg.)


  if TF > Flt (Rst'Pos (Rst'Last)) + 0.5 then
         return Rst'First;


----------------------------------------------------------------------------

with Discrete_Random;
with Ada.Numerics.Float_Random;
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Float_Text_IO; use Ada.Float_Text_IO;
procedure Random_Bug is
   Tries: constant := 10000;
   Wins, Switch_Wins, Offers: Integer := 0;
   type Doors is (Door1, Door2, Door3);
   package Select_Door is new Discrete_Random(Doors);
   Door_Gen: Select_Door.Generator;
   function Random_Door(Gen: in Select_Door.Generator := Door_Gen)
           return Doors renames Select_Door.Random;
   Float_Gen: Ada.Numerics.Float_Random.Generator;
   function Rand(Gen: in Ada.Numerics.Float_Random.Generator :=
Float_Gen)
           return Float renames Ada.Numerics.Float_Random.Random;
   Door_Counts: array(Doors) of Integer := (0,0,0);
   Old_Door, New_Door: Doors;
begin

   Select_Door.Reset(Door_Gen);
   Old_Door := Random_Door;
   for Test in 1..8 loop
     Door_Counts := (0,0,0);
     Wins := 0;
     for I in 1..Tries loop
       New_Door := Random_Door;
       Door_Counts(New_Door) := Door_Counts(New_Door) + 1;
       if New_Door = Old_Door then Wins := Wins + 1; end if;
       Old_Door := New_Door;
     end loop;
     Put (" Door 1: " & Integer'Image(Door_Counts(Door1)));
     Put (" Door 2: " & Integer'Image(Door_Counts(Door2)));
     Put (" Door 3: " & Integer'Image(Door_Counts(Door3)));
     New_Line;
     Put (" Wins: " & Integer'Image(Wins));
     New_Line(2);
   end loop;
  
end Random_Bug;

--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...