Keeping a decimal number within a range

If I have a decimal number representing an angle and I add it to another angle, how can I ensure that it remains between 0° and 359°? So if it's 120° + 270°, it will be 30° instead of 390°? I could check if they are above or below the limit and then add or subtract 360°, but what if it ends up more than 360° out? I would use modulo, but I'm not sure how that would work with decimals and negative numbers. I could also use a loop that adds/subtracts 360 until it is in the range, but I'm not sure if that's a good idea. (I'm actually doing this with radians, it's just easier to explain with degrees)

Solution

You can use the modulus operation. With integers this is the % operator, with floats you can use std::fmod

// \brief Calculates the sum of two angles
// \param[in]  a  First angle [degrees]
// \param[in]  b  Second angle [degrees]
// \return Sum of two angles [degrees, [0, 360)]
double AddAngles(double a, double b)
{
    const double totalAngle = a + b;
    if (totalAngle >= 0.0)
    {
        return std::fmod(totalAngle, 360.0);
    }
    else
    {
        return 360.0 - std::fmod(-totalAngle, 360.0);
    }
}

Demo

int main()
{
    std::cout << AddAngles(30.0, 450.0) << std::endl
              << AddAngles(50.0, 80.0) << std::endl
              << AddAngles(180.0, 180.0) << std::endl
              << AddAngles(-180.0, -270.0) << std::endl;
}

Output