hasen hasen - 2 months ago 22
C++ Question

Checking if a double (or float) is NaN in C++

Is there an isnan() function?

PS.: I'm in MinGW (if that makes a difference).

I had this solved by using isnan() from

<math.h>
, which doesn't exist in
<cmath>
, which I was
#include
ing at first.

Answer

According to the IEEE standard, NaN values have the odd property that comparisons involving them are always false. That is, for a float f, f != f will be true only if f is NaN.

Note that, as some comments below have pointed out, not all compilers respect this when optimizing code.

For any compiler which claims to use IEEE floating point, this trick should work. But I can't guarantee that it will work in practice. Check with your compiler, if in doubt.