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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.ada
Subject: Re: Fixed point constants issue
Date: Tue, 14 Sep 2010 12:44:29 -0700
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> On Mon, 13 Sep 2010 19:27:30 +0200, Vinzent Hoefler wrote:
>> Is this a GNAT bug or am I just stupid?
>> 
>> Suppose we have a fixed point type (with a rather large delta):
>> 
>> -- 8< --
>> with Ada.Text_IO; use Ada.Text_IO;
>> 
>> procedure Fixed_Point
>> is
>>     FEET_PER_METER : constant := 0.3048;
>>     HEIGHT         : constant := 10; -- Feet
>>
>>    type Altitude is delta (2.0**15 / 50_000.0) range 0.0 .. 50_000.0;
>
> [...]  
>> Can anyone shed some light on this? This has driven us crazy for weeks now.
>> 
>> [1] Of course, "Altitude'(HEIGHT * FEET_PER_METER)" yields "0.0" again.
>>      At least this is consistent with my understanding so far.
>
> Hmm, what did you expect? 0.3048 (FEET_PER_METER) is 0 when Altitude. That
> is because 2.0**15/50_000.0=0.65536 > 0.3048.

Close, but actually it's because 2.0**15/50_000.0=0.65536 > 0.5.

Altitude'Delta = 0.65536.  Altitude'Small = 0.5.  If you want the
'Small to be the same as the 'Delta, you have to specify it (or
use a power of 2.0).

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"