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=-0.3 required=5.0 tests=BAYES_00,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
X-Google-Thread: 103376,548c38bb2d3e1bb6
X-Google-NewGroupId: yes
X-Google-Attributes: gida07f3367d7,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII
Path: 
 g2news1.google.com!news4.google.com!proxad.net!feeder1-2.proxad.net!194.25.134.126.MISMATCH!newsfeed01.sul.t-online.de!t-online.de!newsfeed.arcor.de!newsspool3.arcor-online.net!news.arcor.de.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: understanding floating point types
Newsgroups: comp.lang.ada
User-Agent: 40tude_Dialog/2.0.15.1
MIME-Version: 1.0
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 8bit
Reply-To: mailbox@dmitry-kazakov.de
Organization: cbb software GmbH
References: 
 <74406fc1-f64b-4a3e-9dd6-301f1ed467ab@w30g2000yqw.googlegroups.com>
 <mt3els2pgx78$.1dwj1p44vk16d.dlg@40tude.net>
 <7b0ca24f-4a5e-43a9-9f71-e4adffb98694@q1g2000yqg.googlegroups.com>
Date: Sun, 22 Aug 2010 15:57:42 +0200
Message-ID: <ghi6wajpwx2t.1u5g5erb7i1i6$.dlg@40tude.net>
NNTP-Posting-Date: 22 Aug 2010 15:57:40 CEST
NNTP-Posting-Host: 38a07c4d.newsspool4.arcor-online.net
X-Trace: 
 DXC=G1JPO4m^1C0lIh70@<QIE74IUK<Cl32<14Fo<]lROoR18kF<OcfhCO;72S`E2=fYY<[6LHn;2LCV>[<mhadbfdU;E]K_QhhSli03ANOJUhMcN4
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news1.google.com comp.lang.ada:13607
Date: 2010-08-22T15:57:40+02:00
List-Id: <comp.lang.ada>

On Sun, 22 Aug 2010 03:37:35 -0700 (PDT), Ada novice wrote:

> On Aug 22, 11:51�am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> Compare:
>>
>> � �type My_Float is digits 1;
>>
>> My_Float'Machine_Mantissa = 24 (32-bit IEEE 754 is used to model it)
>> My_Float'Mantissa = 5 (= 4+1)
>>
>> � �type My_Float is digits 2;
>>
>> My_Float'Machine_Mantissa = 24 (32-bit IEEE 754 is used to model it)
>> My_Float'Mantissa = 8 (= 7+1)
> 
> Thanks. The Float'Machine_Mantissa remains constant in your examples
> with 1 and 2 precision digits. Is the Float'Machine_Mantissa a
> predefined value that is constant for any precision digit requested?

No, it is the is best fitting machine number available (compiler's choice):

 � type My_Float is digits 7;

   My_Float'Machine_Mantissa = 53 (64-bit IEEE 754 is used to model it)
   My_Float'Mantissa = 25

> I have a second query: Say we consider the type My_Float is digits 2.
> This means that between the model numbers, the resolution is 1/(2^8)
> with the 8 in the expression obtained as 4 X 2. Am I right? Now if we
> have to multiply two numbers: say a = 2.33 and b = 3.45. Does Ada
> first convert each operation to its nearest model number and then do
> the multiplication and represent the result to its nearest model
> number? Or does Ada do the multiplication first and then represent the
> result to to its nearest model number?

I would expect that all computations performed in machine numbers, not
because it is mandated, I believe it is not, but because it is simpler to
implement. At least with GNAT it is so. Try this:

   type My_Float is digits 2;
   X : My_Float := 1.0 / 3.0;
   ...
   Put_Line (Float'Image (Float (X)));

The output will be: 3.33333E-01

If you can pick any number from the interval [0.33, 0.34[ to represent 1/3
in with 2 decimal digits. Why not 0.333333?

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