Negative indices in Swift arrays

Question

Is it possible and easy to have the first index in an array to start on a negative number instead of 0?

If so, how would I define the following so these start on -1 instead of 0? And how do I access the value?

var myIntArray:[Int] = [4, 2, 1]
var myDoubleArray:[Double] = [28, 28, 14]
var myCGFloatArray:[CGFloat] = [0.14, 0.14, 0.1]

Answer 1

You can define a custom subscript for that:

extension Array {
    private var customIndexOffset: Int { return -1 }

    subscript(customIndex index: Int) -> Element {
        get {
            return self[index - customIndexOffset]
        }
        set {
            self[index - customIndexOffset] = newValue
        }
    }
}

Now myIntArray[customIndex: -1] == myIntArray[0]

- More generic, wider coverage:

extension MutableCollection where Index == Int {
    private var customIndexOffset: Int { -2 }

    subscript(customIndex index: Int) -> Element {
        get { self[index - customIndexOffset] }
        set { self[index - customIndexOffset] = newValue }
    }
}


extension Collection where Index == Int {
    private var customIndexOffset: Int { -2 }

    subscript(customIndex index: Int) -> Element { self[index - customIndexOffset] }
}