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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Fixed Point number mul, is it a bug?
Date: Thu, 18 Oct 2012 09:57:45 +0200
Organization: cbb software GmbH
Message-ID: <wac2fm8hf0h7$.du8cji1kw1bw$.dlg@40tude.net>
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Date: 2012-10-18T09:57:45+02:00
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On Thu, 18 Oct 2012 00:11:32 -0700 (PDT), kylix wrote:

> -- GNAT GPL 2012 
> with Ada.Text_IO;
> procedure FixPoint is
>    type FP is delta 0.01 range 0.00 .. 99.99;
>    -- type FP is delta 0.01 digits 4;
>    x : FP := 0.01;
> begin
>    for i in 1 .. 5 loop
>       x := x * 2;
>       Ada.Text_IO.Put_Line("x =>" & FP'Image(x)); 
>    end loop;
> end Fixpoint;
> 
> In my machine, it yield results:
> 
> x => 0.02
> x => 0.03
> x => 0.06
> x => 0.13
> x => 0.25
> 
> Why not: 0.02 0.04 0.08 0.16 0.32 ?

If you try this one:

   Ada.Text_IO.Put_Line("List:");
   loop
      Ada.Text_IO.Put_Line("x =>" & FP'Image(x));
      exit when x = FP'Last;
      x := FP'Succ (x);
   end loop;

you will see what is going on. A binary fixed point type has values which
are not necessarily exact decimal ones. In RM's language it would read as
"the small" is not a power of 10. The small is the difference between two
adjacent values.

> If FP declared as "type FP is delta 0.01 digits 4",
> it yield expected results.

Because this makes it decimal = the small is a power of 10 = values are
exactly decimal. See 3.5.9(9)

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de