Testing if Float value is NaN [duplicate]

Question

Possible Duplicate:
Checking if a double (or float) is nan in C++

I have a requirement to check if float is NaN. By going through some of the links I found the most common check.

FLOAT32 f32_test_NaN = (some_value);
if (f32_test_NaN == f32_test_NaN)
{
    //do something;
}
else
{
    // do something;
}

But this does not seem to work for me. My code is as follows:

FLOAT32 test_NaN = 0x414570A3;//some value - is this ok?

Debugging on GDB:

(gdb) p test_NaN
$1 = 1.09506982e+09

(gdb) p/x test_NaN
$2 = 0x41457080 // Hex is not same as init value - What is compiler doing?

So in my case test_NaN is equal to test_NaN.

Please let me know if any compiler setting has to be done. I am running on solaris. Or is there any other way to check the same.

Thanks in advance.

Answer 1

The problem is maybe in your initialization (at least that explains the value you see in gdb) :

FLOAT32 test_NaN = 0x414570A3;

The hex value given is considered as an integer and converted to float (with exponent and value) meaning it is stored in a different format.

If you want to force the bits inside the float, then you need to memcpy:

FLOAT32 test_NaN;
memcpy(&test_NaN, 0x414570A3, 4);

Answer 2

Include math.h and use int isnan(x). Don't forget to link with -lm

Answer 3

If <math.h> is not available, then do this:

if (x != x)
{
    // x is NaN
}

Answer 4

if (x != x)

For x = 0x7FBFFFFF (sign bit 0, a = 0, rest of bits 1)

http://en.wikipedia.org/wiki/NaN

A bit-wise example of a IEEE floating-point standard single precision (32-bit) NaN: s111 1111 1axx xxxx xxxx xxxx xxxx xxxx where s is the sign, x is the payload, and a determines the type of NaN. If a = 1, it is a quiet NaN; if a is zero and the payload is nonzero, then it is a signaling NaN