Re: Fixed point constants issue

[Adam Beneschan <adam@irvine.com>]

On Sep 24, 2:38 pm, Jeffrey Carter
<spam.jrcarter....@spam.not.acm.org> wrote:
> On 09/24/2010 07:43 AM, Markus Schöpflin 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 from
> > delta /= 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 := 0.3048;
> > HEIGHT : constant := 10; -- Feet
>
> > TEN_FEET_1 : constant := HEIGHT * FEET_PER_METER;
> > TEN_FEET_2 : constant ALTITUDE := 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 that
> static universal (sub)expressions were evaluated exactly by the compiler, using
> universal operations. For example, I was taught that in
>
> Pi : constant := 3.14159;
>
> X : Float;
> ...
> Reduce : loop
>     exit Reduce when X < Pi / 2;
>
>     X := 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maybe that AI got superseded by some other AI.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