Re: Truncating a fixed-point value

[Adam Beneschan <adam@irvine.com>]

On Jul 9, 1:39 pm, Gene <gene.ress...@gmail.com> wrote:
> On Jul 9, 3:06 pm, Adam Beneschan <a...@irvine.com> wrote:
>
> > I must be suffering from an embarrassing mental block, because I'm
> > sure there's a simple solution, but I can't see what it is.
>
> > I have a value X of a fixed-point type, X >= 0.0, and I need to
> > compute Floor(X) as an Integer.  'Truncation and 'Floor aren't defined
> > for fixed-point types; the type conversion Integer(X) rounds; and
> > Integer(X-0.5) fails if X=0.0.
>
> > How do others do this?  Or do I have to resort to an IF statement (or
> > conditional expression in Ada 2012) to handle the different cases?
>
> >                -- Adam
>
> There is a prior discussion of this question:
>
> http://coding.derkeiler.com/Archive/Ada/comp.lang.ada/2003-10/0729.html

Thanks for the link.  I was aware of AI95-60 but I hadn't been aware
that the problem had been brought up again.


> The problem is that fixed point types don't have to include all the
> integers.

Ah, I see.  This is a problem because 'Truncation and 'Floor are
defined to return the same fixed-point type.  It seems to me that the
language could use a way to do the truncation (or floor) and return an
integer, without having to worry about an intermediate result of the
same fixed-point type (which might not be a possible value of the
type).  I think someone mentioned that in the thread, but mentioned
that it would probably have to be generic.


> An ugly approach is to convert to e.g. a long float, take the floor,
> and convert back.  No doubt others will have something more elegant.

That will probably work for my purpose; but in general, it's easy to
define a fixed-point number with a 'Small of 2**(-N) that has more
bits to the right of the binary point than are available in the
mantissa part of the floating-point.  If that's the case, this
approach will fail on certain values (probably M - (K * T'Small) for
integer M and suitably small integer K).  Since this situation lends
itself to really efficient code (basically just a bit shift), it's
kind of a shame that the language doesn't give you a good way to do
this.

                           -- Adam