invalid floating point value

From: Allin Cottrell (cottrell_at_wfu.edu)
Date: 09/29/04 

Date: Tue, 28 Sep 2004 23:56:02 -0400

OK, I realize that what I am asking here is not likely to have a
answer within the C standard. Nonetheless, it is not specific
to any particular platform, so I'll hazard the question anyway.

A C double can be aliased by an array of unsigned char, to which
any desired byte pattern may be written. Is there any somewhat
portable way of assigning a byte pattern such that the result
will, with very high probability, _not_ count as a valid double?

I ask because I'm in search of a mechanism for flagging "missing
values" in an array of doubles. I'm aware that the "best" (fully
portable) way probably involves setting up an ancillary array, or
making the basic data-type not a straight double but a struct of
some sort. But in context this would be a royal pain. One
common solution is to flag "missing" with a specific value such
as -999.0. I'm wondering if that can be improved upon without
resorting to a wider data type.

Allin Cottrell
Wake Forest University

Quantcast