Re: How to check a Float for NaN

[Adam Beneschan <adam@irvine.com>]

On Apr 30, 1:36 pm, Jerry <lancebo...@qwest.net> wrote:

> Check this out:
>
> function Is_NaN(x : Long_Float) return Boolean is
> begin
>     return x /= x;
> end Is_NaN;

Maybe that will work, but I wouldn't count on it.  First of all, you
don't know that the compiler will actually do anything.  It may get
clever and decide that this is trivially False, and thus generate code
that doesn't even look at x.  Second, if it generates a "floating-
point comparison" instruction, you may not get the behavior you think
you're getting.  My Pentium manual, for instance, says of the FCOM*
(Compare Real) instructions: "If either operand is a NaN or is in an
undefined format, ... the invalid-operation exception is raised, and
the condition bits are set to 'unordered'".  Which means that (on that
processor) the comparison attempt will probably fault, and if it
doesn't because the fault is masked, the condition-code bits will be
set to an "unordered" relation that your compiler may not be
expecting, which means that it may not be translated to "inequality"
the way you think it will.


> A couple of minutes on Wikipedia saves the day. Fromhttp://en.wikipedia.org/wiki/NaN#NaN_encodings:
>
> "A NaN does not compare equal to any floating-point number or NaN,
> even if the latter has an identical representation. One can therefore
> test whether a variable has a NaN value by comparing it to itself."

But if you read further, you'll find that signaling NaNs "should raise
an invalid exception".  (Why the Pentium FCOM instructions appear to
raise exceptions for quiet NaN's as well as signed NaN's, I don't
know.)

Anyway, Wikipedia describes the IEC 559/IEEE 754 standard, but as
we've already discussed, (1) Ada doesn't fully support this standard
and (2) NaN's aren't valid values in Ada, so you can't reliably use
the IEC/IEEE standard to predict what will happen if you do this in
Ada.  You're welcome to try it, of course, but don't be surprised or
disappointed if it doesn't work.

                                 -- Adam