An extension of a string to provide subscription functionality.

StringExtensionSubscript.swift

extension String {
    subscript (i: Int) -> Character {
        get {
            assert(i < self.characters.count && i >= 0, "Index out of range.")
            return self[self.index(self.startIndex, offsetBy: String.IndexDistance(i))]
        }
        set {
            assert(i < self.characters.count && i >= 0, "Index out of range.")
            let index = self.index(self.startIndex, offsetBy: String.IndexDistance(i))
            let endIndex = self.index(self.startIndex, offsetBy: String.IndexDistance(i+1))
            let str = self.replacingCharacters(in: index..<endIndex, with: String(newValue))
            self.characters = str.characters
        }
    }
    subscript (i: UInt) -> Character {
        get {
            return self[Int(i)]
        }
        set {
            self[Int(i)] = newValue
        }
    }
}