int a = 9/5;
performs integer division and ignores the remainder, so a is set to 1. Attempting to print that using %f gives undefined behavior, but by chance you got 0.000000 out of it.
Do
double a = 9./5.;
instead, or print with %d if integer division was the desired behavior. (float would also work, but a will be promoted to double when passed to printf, so there's no reason not to use double.)
