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From: Ian Clifton <ian.clifton@chem.ox.ac.uk>
Newsgroups: comp.lang.ada
Subject: Re: Fixed Point number mul, is it a bug?
Date: Thu, 18 Oct 2012 11:42:26 +0100
Organization: Oxford University
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Date: 2012-10-18T11:42:26+01:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> 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)

Didn’t some IBM machines have a floating‐point format where the exponent
was decimal rather than binary?
-- 
Ian ◎