Not A Number in IEEE Math

[James Salter]

In article <14266 at s.ms.uky.edu> simon at ms.uky.edu (G. Simon Gales) writes:
>I think that that behavior is correct.  
>
[... 0.0 ...]
>	NaN / NaN = 1.0
>
>Also a NaN/NaN situation is usually treated as 1, but this is definitely
>not intended to be a -correct- result.  If you end up with NaNs in your
>computation's results, you can't trust the answers you get.

No.  It is *explicitly* stated in IEEE 854, Sec. 6.2, Paragraph 4 (or so)

	"Every operation involving one or two input NaNs, ..., if a
	floating-point result is to be delivered, shall deliver as its result
	a quiet NaN, which should be one of the input NaNs."

where "operation" above is defined (5.1):

	"An implementation shall provide the add, subtract, multiply, divide
	and remainder operations for any two operands ..."

Thus, NaN/NaN != 1.0; in fact, it *can* not be a representable floating
point number.  It is NaNQ (maybe with an exception thrown in if one of
the NaNs is signalling).

If you want a controversy, ask about pow(0.0,0.0), but NaN behavior is
specified quite completely.

>Simon Gales at The University of Kentucky

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