Re: conversions between fixed-point types

["John B. Matthews" <nospam@nospam.invalid>]

In article <h92jie$q4g$1@news.albasani.net>,
 Dirk Herrmann <fight_spam@invalid.invalid> wrote:

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

I'm no expert, and I had some trouble following the conversions in your 
example. I tried a more direct test, and I don't see anything unusual; 
rounding appears consistent. Rounding away from zero is required for 
decimal fixed point types; but rounding toward zero is permitted for 
ordinary fixed point types such as FpA and FpB.

<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;
    
    package AIO is new Ada.Text_IO.Fixed_IO(FpA);
    package BIO is new Ada.Text_IO.Fixed_IO(FpB);
    
    V : Float := -2.0;

 begin

    while V < 0.0 loop
       FIO.Put(V, 1, 1, 0); TIO.Put(" ");
       AIO.Put(FpA(V)); TIO.Put(" ");
       BIO.Put(FpB(V)); TIO.New_Line;
       V := V + 0.1;
    end loop;

 end Conversion;
</code>

<console>
$ make run
Darwin: gcc 4.3.4
gnatmake conversion -cargs -O2 -bargs -shared -largs -dead_strip
gcc -c -O2 conversion.adb
gnatbind -shared -x conversion.ali
gnatlink conversion.ali -shared-libgcc -dead_strip
./conversion
-2.0  -2.0  -2.0
-1.9  -2.0  -2.0
-1.8  -2.0  -1.6
-1.7  -1.5  -1.6
-1.6  -1.5  -1.6
-1.5  -1.5  -1.6
-1.4  -1.5  -1.2
-1.3  -1.5  -1.2
-1.2  -1.0  -1.2
-1.1  -1.0  -1.2
-1.0  -1.0  -0.8
-0.9  -1.0  -0.8
-0.8  -1.0  -0.8
-0.7  -0.5  -0.8
-0.6  -0.5  -0.4
-0.5  -0.5  -0.4
-0.4  -0.5  -0.4
-0.3  -0.5  -0.4
-0.2   0.0   0.0
-0.1   0.0   0.0
</console>

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>