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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Fixed point constants issue
Date: Mon, 27 Sep 2010 10:58:17 -0700 (PDT)
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Date: 2010-09-27T10:58:17-07:00
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On Sep 24, 2:38=A0pm, Jeffrey Carter
<spam.jrcarter....@spam.not.acm.org> wrote:
> On 09/24/2010 07:43 AM, Markus Sch=F6pflin wrote:
>
>
>
>
>
>
>
> > Are you sure about that? Because I compiled and ran the following
> > program with an Ada 83 compiler (DEC Ada) and I got a different result.
> > Note that I modified the delta to 0.5 to avoid any issues resulting fro=
m
> > delta /=3D small.
>
> > ---%<---
> > with TEXT_IO; use TEXT_IO;
>
> > procedure FIXED_POINT
> > is
> > type ALTITUDE is delta 0.5 range 0.0 .. 50_000.0;
>
> > package ALTITUDE_IO is new FIXED_IO (ALTITUDE);
>
> > FEET_PER_METER : constant :=3D 0.3048;
> > HEIGHT : constant :=3D 10; -- Feet
>
> > TEN_FEET_1 : constant :=3D HEIGHT * FEET_PER_METER;
> > TEN_FEET_2 : constant ALTITUDE :=3D HEIGHT * FEET_PER_METER;
> > begin
> > ALTITUDE_IO.PUT (TEN_FEET_1); NEW_LINE; -- gives 3.0
> > ALTITUDE_IO.PUT (TEN_FEET_2); NEW_LINE; -- gives 5.0 (*)
> > end FIXED_POINT;
> > --->%---
>
> > (*) This certainly looks like FEET_PER_METER is converted to ALTITUDE
> > before doing the multiplication.
>
> That is interesting. I learned Ada 83 in 1984; I was explicitly taught th=
at
> static universal (sub)expressions were evaluated exactly by the compiler,=
 using
> universal operations. For example, I was taught that in
>
> Pi : constant :=3D 3.14159;
>
> X : Float;
> ...
> Reduce : loop
> =A0 =A0 exit Reduce when X < Pi / 2;
>
> =A0 =A0 X :=3D X - Pi / 2;
> end loop Reduce;
>
> both occurrences of "Pi / 2" would be evaluated exactly by the compiler. =
Perhaps
> I was taught wrong.

Hmmm...  in looking into this, I bumped into AI83-165 which would make
it appear that the use of Pi/2 in both occurrences is illegal.  But
maybe that AI got superseded by some other AI.  I don't know.

                                   -- Adam