Question

I am quite a newbie to c. So when i writing a small game demo, i face a really strange problem.

 void testC()
 {
     float a = 825300160;
     float b = a + 0.1;
     assert(a != b);
 }

The above assert statement can’t passed. Very strange.
My environment is mac os ml. gcc 4.2.1

Answer 1

The fractional portion of a float consists of 23 bits. You need 30 bits to represent 825300160, so the less significant portion of the number is dropped. Adding .1 does not make a difference – you need to add roughly 32 for the number to change:

float a = 825300160;
float b = a + 31.5;
assert(a != b); // No change is detected
float c = a + 32;
assert(a != c); // Change is detected