NAN as _FillValue/missing_value

[Chris Webster <cjw@xxxxxxxx>]

I went to convert my _FillValue to NAN; I found that upon reading data 
and doing comparisons a false is always returned:

  if (datum[i] == vp->attr->missingValue)  // is always false if nan's

At first I thought I was back in the world of round off hell until I was 
pointed to:

http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm

specifically these lines:

  "A NAN will never return true on a <= comparison," and

  "One other complication is that comparisons involving NANs are always 
supposed to return false, "



I would like to suggest that mention of this be made on the netCDF Best 
Practices page where you point people to use NAN as the _FillValue.

...then I'd like to whine that I have to retro-fit all my code and 
double the number of checks for every data point to determine if it's 
_Fill/missing (...using isnan(3)).  :)

--Chris