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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.unix.wizards
Subject: Re: what does NaN mean out of printf?
Message-ID: <9639@alice.UUCP>
Date: 15 Aug 89 14:18:06 GMT
References: <20283@adm.BRL.MIL> <3876@phri.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 35

In article <3876@phri.UUCP>, roy@phri.UUCP (Roy Smith) writes:
> In article <20283@adm.BRL.MIL> Leisner.Henr@xerox.com (Marty) writes:
> > What does NaN mean out of printf?
> 
> 	You're on a machine which supports IEEE-754 arithmetic (for
> example, a Sun, but there are many, many modern machines which fit this
> description).  NaN means Not A Number.  NaN is the result of some illegal
> arithmetic operation (division by zero, square root of a negative number,
> etc).

You're almost right.

IEEE-754 defines two different kinds of exceptional values: infinity
and NaN.

Infinity has a sign and obeys several sensible rules.  For example:

	+inf + +inf = +inf
	-inf + -inf = -inf
	- (+inf) = -inf
	- (-inf) = +inf
	+1 / +0 = +inf	[with a possible trap]
	+1 / +inf = +0  [no trap]
	+1 / -0 = -inf	[with a possible trap]

and so on.  However,

	0 / 0 = NaN
	+inf + -inf = NaN

and so on.  NaN is extremely infectious: just about any operation on
a Nan gives Nan as a result.
-- 
				--Andrew Koenig
				  ark@europa.att.com