c - Why the control goes in “else” part?

int main() 
{
  float a = 0.8;
  if (a == 0.8)
    printf("x\n");
  else 
    printf("y\n");


  return 0;
}

Though a is equal to 0.8, it outputs y.

Answer

0.8 cannot be represented accurately in binary floating-point. Your code if (a == 0.8) basically compares single-precision 0.8 with double-precision 0.8, which are not equal.

To see this for yourself, try the following code:

int main()
{
    double a = 0.8f;
    double b = 0.8;

    printf("%lX\n", *(long *)&a);
    printf("%lX\n", *(long *)&b);
}

It outputs:

3FE99999A0000000
3FE999999999999A