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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: Not A Number in IEEE Math
Keywords: IEEE floating point
Message-ID: <10515@alice.UUCP>
Date: 21 Feb 90 18:07:26 GMT
References: <44@newave.UUCP> <14266@s.ms.uky.edu>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 31

In article <14266@s.ms.uky.edu>, simon@ms.uky.edu (G. Simon Gales) writes:

> I think that that behavior is correct.  

> 	0.0 * (anything) = 0.0
> and
> 	NaN / NaN = 1.0

Sorry, but that's not what IEEE says.

NaN is infinitely infectious.  That is: any operation that gives a
floating point result is supposed to return NaN if any of its operands
is NaN.  Relational operators all return `false' if either operand
is NaN.  Thus (x == x) is false if and only if x is NaN (unless,
of course, your implementation is broken).

Incidentally,

	0.0 * infinity = NaN (+ or - infinity -- I think it's implementation
		defined whether or not NaN has a sign)
	0.0 * NaN = NaN (as mentioned before)
	0.0 * anything else = 0.0

	NaN / NaN = NaN (as mentioned before)

	infinity / infinity = NaN
	infinity / NaN = Nan (as mentioned before)
	infinity / anything else = infinity
-- 
				--Andrew Koenig
				  ark@europa.att.com