floating point - How to produce a NaN float in c?

Question

float f = (float)'a';
if(f < 0){ 
}   
else if(f == 0){ 
}   
else if(f > 0){ 
}   
else{
    printf("NaN
");                                                          
}   

f won't be greater/equal/less than 0 if it's a NaN.

But how to produce such a f in the first place?

I tried various ways to produce a NaN,but none work..

Answer 1

To produce a nan, there are a few ways:

1) generate it manually (read ieee754 to set up the bits properly)

2) use a macro. GCC exposes a macro NAN. It's defined in math.h

The general way to check for a nan is to check if (f == f) (which should fail for nan values)

For nan, the exponent bits in the float representation should all be set to 1 (float consists of a signed bit, a set of exponent bits and a set of mantissa bits)