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Date: Sun, 15 Aug 2010 17:43:43 -0400
From: "Peter C. Chapin" <chapinp@acm.org>
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Subject: Question about ordinary fixed point types.
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Date: 2010-08-15T17:43:43-04:00
List-Id: <comp.lang.ada>

I have a need to work with ordinary fixed point types. I've read the in
the reference manual about them (section 3.5.9, for example), and some
other things. I still have a few questions. I thought I'd start with one
here.

Consider this example:


type Angle_Type is delta 0.0005 range -3.1416 .. 3.1416;
Angle : Angle_Type;
...
Angle := Angle_Type'First;
while Angle < Angle_Type'Last loop
  -- Work with Angle here.
  Angle := Angle + Angle_Type'Small;
end loop;


Aside from Angle_Type'Last does the loop above reliably visit every
possible value of Angle_Type? By "reliably" I mean "has the desired
effect on all possible correct Ada implementations." I guess my question
is: does Angle + Angle_Type'Small always advance Angle to the next
machine number in the type?

I'm using GNAT GPL 2010 and in the example above GNAT appears to be
using 2**(-11) for Angle_Type'Small. It thus uses something over 12,000
values for the type. The code above is for a test program; it is my
intention to exercise every possible value.

As an aside it appears as if Angle_Type'First is actually slightly less
than the -3.1416 mentioned in the type definition. My reading of the Ada
standard is that this is okay. Specifically I should be getting a
multiple of Angle_Type'Small that is closest to the mathematical value
of -3.1416. Apparently the closest multiple is just "to the left" of the
number I specified. This actually works out to be slightly awkward in my
case, but that's a problem I can probably figure out.

Thanks!

Peter