NA in C/C++

A few general comments.

The IEEE 754 floating-point standard is one of the more striking
successes in getting computer hardware to be more useful for those who
program.  There are, of course, glitches, both in non-compliance and
in holes in the standard, but if we can work within the standard as
far as possible, while complaining about the glitches, we'll be better
off, and C/C++ software produced will be more likely to port
gracefully to other environments.

From that view, using C routines that are part of the standard (while
perhaps overriding them on machines that don't conform) has advantages
inside your own C/C++ code, IF that code is not intrinsically R/S
dependent.  So isnan() would be better in that case, R_IsNaNorNA()
better for code that is R-dependent.

Since John may not have read *all* the R code yet, that is precisely what
R_IsNaNorNA is: a wrapper for isnan on machines which have a working
isnan, and something else otherwise. Namely:

#ifdef IEEE_754

int R_IsNaNorNA(double x)
{
/* NOTE: some systems do not return 1 for TRUE. */
    return (isnan(x) != 0);
}
#else
....

where IEEE_754 is only set after testing (somewhat) functionality.

For finite, which is buggier, the 1997 draft revision to the 
ANSI C standard promised isfinite and defined it tightly.  It's just that
neither that revision not isfinite seem to be making any progress.

One thing the R project keeps on teaching me is the importance of
pragmatism here: a large proportion of the bug-fixing time is actually
bug-avoidance over all of an increasing range of machines.  Even so. I was
unprepared for the truth of Inf < 3 on Visual C++ ! I would prefer to be
pragmatic with correct answers than purist with wrong ones.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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