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From: Dirk Herrmann <fight_spam@invalid.invalid>
Newsgroups: comp.lang.ada
Subject: Re: conversions between fixed-point types
Date: Sat, 19 Sep 2009 14:41:49 +0200
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Date: 2009-09-19T14:41:49+02:00
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Dirk Herrmann wrote:

> I am currently trying to learn about Ada's fixed-point types, and there
> are some points that I couldn't find any statement about, or which I
> simply could not understand.

And, while experimenting and trying out Adam Beneschan's solution for
rounding (thanks for your answer, Alan) I got totally confused because
of the following code (I am using GNAT 4.3.4 with the following
command line options: gnatmake -f -gnatVa -gnata -gnatwadhl.o
-save-temps conversion.adb):  (For your convenience, I added the results
as comments within the code.)

    with Ada.Text_IO;

    procedure Conversion is

       package TIO renames Ada.Text_IO;
       package FIO is new Ada.Text_IO.Float_IO(Float);

       type FpA is delta 0.5 range -10.0 .. +10.0; for FpA'Small use 0.5;
       type FpB is delta 0.4 range -10.0 .. +10.0; for FpB'Small use 0.4;

       function MakeB(F: in Float) return FpB is
       begin
          return FpB(F);
       end MakeB;

       function MakeBFromA(F: in FpA) return FpB is
       begin
          return FpB(F);
       end MakeBFromA;

       function MakeBViaA(F: in Float) return FpB is
       begin
          return FpB(FpA(F));
       end MakeBViaA;

    begin

       FIO.Put(Float(FpA(-1.5))); TIO.Put(" ");             --> -1.50000E+00
       FIO.Put(Float(FpB(-1.5))); TIO.Put(" ");             --> -1.20000E+00
       FIO.Put(Float(FpB(Float(-1.5)))); TIO.Put(" ");      --> -1.20000E+00
       FIO.Put(Float(MakeB(-1.5))); TIO.Put(" ");           --> -1.60000E+00
       FIO.Put(Float(FpB(FpA(-1.5)))); TIO.Put(" ");        --> -1.20000E+00
       FIO.Put(Float(FpB(FpA(Float(-1.5))))); TIO.Put(" "); --> -1.20000E+00
       FIO.Put(Float(MakeBFromA(FpA(-1.5)))); TIO.Put(" "); --> -1.60000E+00
       FIO.Put(Float(MakeBViaA(-1.5))); TIO.Put(" ");       --> -1.60000E+00

    end Conversion;

Is this confusing behaviour allowed?  I will submit a bug report if some 
expert confirms it is a GNAT bug.

Thanks in advance,

Dirk Herrmann
(email address gladly given on request)