Re: How to check a Float for NaN

[Jerry <lanceboyle@qwest.net>]

On May 5, 1:49 pm, Adam Beneschan <a...@irvine.com> wrote:
> On May 5, 11:23 am, Martin Krischik <krisc...@users.sourceforge.net>
> wrote:
>
> > Jerry wrote:
> > > How would one check a Float or Long_Float if it has value NaN? The
> > > only ways that I can come up with are to import a C function (isnan, I
> > > think) or to write Long_Float'image(Some_Float) to a string and
> > > examine the first three characters to see if they are "NaN" (and that
> > > seems to be a GNAT implementation choice so might not be portable,
> > > which is OK for my use).
>
> > How about an Unchecked Conversion to an Unsigned_64 and comparing
>
> > >2#0111_1111_1111_0000_0000_0000_0000_0000#
>
> > Note that it's '>' as 2#0111_1111_1111_0000_0000_0000_0000_0000# is positive
> > infinity.
>
> Any negative float (including negative infinity) would be > than the
> binary number you give, since the leftmost bit (sign bit) would be 1.
> I think you need to clear the sign bit first, then your comparison
> will work:
>
>   Is_NaN := (To_Unsigned_64(X) and not
>               2#1000_0000_0000_0000_0000_0000_0000_0000#)
>              > 2#0111_1111_1111_0000_0000_0000_0000_0000#;
>
> But I haven't tried it.
>
>                                   -- Adam

I thought about doing something like this once I saw the bit patterns
on the Wikipedia page. However, I would guess that the bit pattern for
quiet and signaling NaNs is different, so the test would be somewhat
more complicated if one wanted to detect both types.

Jerry