Why does trunc() return 0?

Question

• Unit test fails even though values are equal

I was stuck in the above problem.

While trying to solve that, I wrote the following listing.

I expect it to keep two digits after the decimal point.

#include "simulation.hpp"
#include <cstdio>

typedef double real;

double truncate_(double f)
{
    return std::trunc(f * 100.0);
}

int main()
{
    //Simulation sim;
    //sim.run();
    LennardJones lj(Constants::EPSILON, Constants::SIGMA, Constants::KB);

    Vec3 diatance(4.0, 4.0, 4.0);

    real attractive = lj.getPotentialAttractive(diatance);
    real repulsive = lj.getPotentialRepulsive(diatance);

    std::cout << attractive << std::endl;
    std::cout << repulsive << std::endl;

    real attractive2 = truncate_(attractive);
    real repulsive2 = truncate_(repulsive);

    std::cout << attractive2 << std::endl;
    std::cout << repulsive2 << std::endl;

    getchar(); // Wait for a keypress
}

Output:

-4.84292e-06
6.82474e-08
-0
0

Answer 1

-4.84292e-06 and 6.82474e-08 are still zero within two digits after the decimal point, so as expected, you're getting zero when truncating them.

If you wrote

std::cout << std::fixed << std::setprecision(2);

... then you would print 0.00 instead of 0. It keeps two digits after the decimal point, and both those digits are zero.