Negative number modulo in swift

by

The Swift remainder operator % computes the remainder of
the integer division:

a % b = a - (a/b) * b

where / is the truncating integer division. In your case

(-1) % 3 = (-1) - ((-1)/3) * 3 = (-1) - 0 * 3 = -1

So the remainder has always the same sign as the dividend (unless
the remainder is zero).

This is the same definition as required e.g. in the C99 standard,
see for example
Does either ANSI C or ISO C specify what -5 % 10 should be?. See also
Wikipedia: Modulo operation for an overview
how this is handled in different programming languages.

A “true” modulus function could be defined in Swift like this:

func mod(_ a: Int, _ n: Int) -> Int {
    precondition(n > 0, "modulus must be positive")
    let r = a % n
    return r >= 0 ? r : r + n
}

print(mod(-1, 3)) // 2

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