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Newsgroups: comp.lang.ada
Subject: Re: Fixed Point number mul, is it a bug?
Date: Thu, 18 Oct 2012 09:35:21 +0200
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Date: 2012-10-18T09:35:21+02:00
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kylix <likai3g@gmail.com> 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

This is expected. The definition requests a fixed point type with a 
delta of at least 0.01, most compilers would choose 2**-7 then, which 
is 0.0078125 in decimal notation. Hence the results you see.

> Why not: 0.02 0.04 0.08 0.16 0.32 ?

Because 0.01 is not representable in a finite, binary number.

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

Yes. In that case you're requesting a decimal type which is required 
to be exact.

So, no, this is not a bug.


Vinzent.