Question

I have the following source code in a simple main:

int main(int argc, char** argv)
{
    double x = atof(argv[1]);
    double y = atof(argv[2]);

    double res = x + std::floor((y - x) * .5 * 100 + .5)*0.01;

    std::cout << res << std::endl;
}

If I run the above with 75.21 75.22 it gives me 75.22 but if i run it with 7.21 and 7.22 it gives me 7.21. Both these number differ by 0.01 so I don’t understand why this is happening?

Answer 1

The short answer: floating-point values are imprecise.

The long answer: What Every Computer Scientist Should Know About Floating-Point Arithmetic