Negative number modulo in swift

Question

How does modulo of negative numbers work in swift ? When i did (-1 % 3) it is giving -1 but the remainder is 2. What is the catch in it?

Answer 1

If what you are really after is capturing a number between 0 and b, try using this:

infix operator %%

extension Int {
    static  func %% (_ left: Int, _ right: Int) -> Int {
        if left >= 0 { return left % right }
        if left >= -right { return (left+right) }
        return ((left % right)+right)%right
    }
}

print(-1 %% 3) //prints 2

This will work for all value of a, unlike the the previous answer while will only work if a > -b.

I prefer the %% operator over just overloading %, as it will be very clear that you are not doing a true mod function.

The reason for the if statements, instead of just using the final return line, is for speed, as a mod function requires a division, and divisions are more costly that a conditional.