From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,388fc53f584bd695
X-Google-NewGroupId: yes
X-Google-Attributes: gida07f3367d7,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news3.google.com!feeder1.cambriumusenet.nl!feed.tweaknews.nl!85.158.31.10.MISMATCH!newsfeed-0.progon.net!progon.net!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: "Vinzent Hoefler" <nntp-2010-09@t-domaingrabbing.de>
Newsgroups: comp.lang.ada
Subject: Re: Fixed point constants issue
Date: Mon, 13 Sep 2010 20:25:43 +0200
Message-ID: <op.vizhg5ygd20q5n@jellix.jlfencey.com>
References: <op.vizer4qnd20q5n@jellix.jlfencey.com>
 <1f0l0baynq1x7$.phl9cqpwcr37.dlg@40tude.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes
Content-Transfer-Encoding: 7bit
X-Trace: individual.net X4/8kX1Migb3G9C4nlHEnA5q6ObexvpWH7MroqWYX80XlvDDdi
Cancel-Lock: sha1:ewmDf7usEexFQFdzWQMocMNm8t0=
User-Agent: Opera Mail/10.62 (Win32)
Xref: g2news1.google.com comp.lang.ada:14049
Date: 2010-09-13T20:25:43+02:00
List-Id: <comp.lang.ada>

On Mon, 13 Sep 2010 20:04:32 +0200, Dmitry A. Kazakov  
<mailbox@dmitry-kazakov.de> wrote:

> 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.

Yes, of course. Still, it's not quite intuitive why

  TEN_FEET_1 : constant          := HEIGHT * FEET_PER_METER;
  TEN_FEET_2 : constant Altitude := HEIGHT * FEET_PER_METER;

just because a type is given in the second case.

I would expect the compiler to evaluate the expression "HEIGHT *  
FEET_PER_METER"
_before_ converting it to the appropriate fixed point type. And, obviously  
it
does this that when evaluating

   TEN_FEET_4 : constant Altitude := Altitude (HEIGHT * FEET_PER_METER);

In our opinion, this type conversion on the right hand side shouldn't be
necessary, yet it is (with GNAT).


Vinzent.

-- 
There is no signature.