Why is x==x false if the value of x is NaN?

Asked Mar 17 '16 at 21:34

Active Mar 17 '16 at 21:37

Viewed 228 times

http://www.gnu.org/software/libc/manual/html_node/Infinity-and-NaN.html

This specifies that x==x is false if x is NaN. What would be the underlying reason? If the compiler detects stuff like 1.0/0.0 (NaN) wouldn't it automatically set the value to 0x7F800000 (IEEE754 Single Precision Floating Point representation for NaN/Inf). Then when the x==x comparison occurs, should it be true as it would be a comparison of the same hex value?

asked Mar 17 '16 at 21:34

Curious91

I believe `==` can only compare two "numbers". – user3528438 Mar 17 '16 at 21:36
2

The conceptual reason I've understood is that NaN is never equal to anything, even to itself. – Kyle Falconer Mar 17 '16 at 21:36
Dividing by zero results in floating point exception (FPE). You shuold use `isnan()` and `isinf()` unfctions instead. – Zaffy Mar 17 '16 at 21:37
6

`log(0) == NaN`. `0/0 == NaN`. That doesn't mean that `log(0) == 0/0`. – Oliver Charlesworth Mar 17 '16 at 21:38
This is not a C question in that C does not define what should happen with `x==x false if the value of x is NaN`. Many C platforms follow IEEE754 on this matter, so it is a IEEE754 one. – chux - Reinstate Monica Mar 17 '16 at 21:41
Just to enforce that once you have NaN as an intermediate result, you can never be able to return to the real side of things. Safety measure. And: THOU SHALT NEVER COMPARE FP VALUES USING == – tofro Mar 17 '16 at 21:45
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@tofro: Relax. Sometimes it's useful to compare floating point values with `==`. – Cornstalks Mar 17 '16 at 21:47
Really? Give me an example, please. – tofro Mar 17 '16 at 21:47
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@tofro: `if (x + K - K == x)`, where `K` is a properly chosen power of 2, to determine if `x` has any bits past a given precision. – R.. GitHub STOP HELPING ICE Mar 17 '16 at 21:50
Alright. Maybe also to find out whether two doubles were created using the very same calculations. Bought, but only accepted as *very sometimes* – tofro Mar 17 '16 at 21:54

Why is x==x false if the value of x is NaN?

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