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

c++ double nan
hasen picture hasen · Feb 20, 2009 · Viewed 378.4k times · Source

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 #includeing at first.

Answer

jalf picture jalf · Feb 20, 2009

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.

Related questions

How do I convert a double into a string in C++?

I need to store a double as a string. I know I can use printf if I wanted to display it, but I just want to store it in a string variable so that I can store it in a …

Read More
C++ string to double conversion

Usually when I write anything in C++ and I need to convert a char into an int I simply make a new int equal to the char. I used the code(snippet) string word; openfile >> word; double lol=…

Read More
Convert double to string C++?

Possible Duplicate: How do I convert a double into a string in C++? I want to combine a string and a double and g++ is throwing this error: main.cpp: In function ‘int main()’: main.cpp:40: error: invalid operands of …

Read More