Re: How to check a Float for NaN

[Jerry <lanceboyle@qwest.net>]

On May 1, 1:04 am, "Stuart" <stu...@0.0> wrote:
> "Adam Beneschan" <a...@irvine.com> wrote in message
>
> news:93b0d930-102a-4ac4-8b85-48e87d9d3df1@j33g2000pri.googlegroups.com...> On Apr 30, 3:50 am, christoph.gr...@eurocopter.com wrote:
> >> On 30 Apr., 12:47, christoph.gr...@eurocopter.com wrote:
>
> >> > Try Float'Valid (X), see RM 13.9.2
>
> >> Ahem, I meant X'Valid for X of type Float
>
> <snip>
>
> > The problem here is that if you're dealing with NaN's on purpose,
> > you're not really dealing with Ada, as far as I can tell, unless your
> > purpose is to test for an uninitialized variable (in which case
> > X'Valid makes sense).  Otherwise, though, NaN's and infinities are not
> > possible values of floating-point types, and therefore they can't be
> > returned by operations that return floating-point values, even if
> > Float'Machine_Overflows is False.  (There are certain passages in the
> > RM that say a result is implementation-defined if the
> > Machine_Overflows attribute is False; however, I do not think
> > assigning a variable to an invalid value, as a NaN would be, is an
> > allowed "implementation-defined" result.  13.9.2(4-11) lists the ways
> > that invalid data could be created, and the "result of a floating-
> > point operation that cannot return a valid result" is not one of those
> > listed.  Yes, I know that this is a NOTE and is not normative.)
>
> Would not 13.9.2(10) cover this:
>    "disrupting an assignment due to the failure of a language-defined check
> (see 11.6)"
>
> Thus if a floating point operation creates a NaN, which would indicate an
> ill-conditioned computation that should violate some language-defined check,
> the object (X) that was to receive the result would be considered invalid.
>
> As noted elsewhere, 'valid encompasses more than NaNs - which may or may not
> suit the OPs purpose.

Not sure. Apparently, the plotting example I'm translating from C
needs to check for NaN when the underlying surface plot machinery is
working. It seems odd that this heavy a responsibility should appear
to the user in an example, but that's out of my control. I suppose
'valid would work since I doubt that other pathologies would arise.

Jerry
>
> Regards
> --
> Stuart