Swift3 & Xcode8: ’subscript' is unavailable: cannot subscript String with a CountableClosedRange<Int>, - swift

Question

Error 1:
When I am trying get the stringValue from Metadata shows above error in Swift3:
let myMetadata: AVMetadataMachineReadableCodeObject = metadataObjects[0] as! AVMetadataMachineReadableCodeObject
// take out the system and check-digits
let myBarcode = myMetadata.stringValue[1...11] //error
Error 2:
In extensions of String I write these to get right(x) and left(x) function to get substring:
extension String {
// length of string
var length: Int {
return self.characters.count
}
// right(x) and left(x) function to get substring
func right(_ i: Int) -> String?
{
return self[self.length-i ... self.length-1 ] //error
}
func left(_ i: Int) -> String?
{
return self[0 ... i-1] //error
}
}

Answer 1

Use this extension for the countable closed range [0...4] subscripting
extension String {
subscript (r: CountableClosedRange<Int>) -> String {
get {
let startIndex = self.index(self.startIndex, offsetBy: r.lowerBound)
let endIndex = self.index(startIndex, offsetBy: r.upperBound - r.lowerBound)
return self[startIndex...endIndex]
}
}
}
or a safer version which checks the bounds and returns nil rather than an out-of-range exception:
extension String {
subscript (r: CountableClosedRange<Int>) -> String? {
get {
guard r.lowerBound >= 0, let startIndex = self.index(self.startIndex, offsetBy: r.lowerBound, limitedBy: self.endIndex),
let endIndex = self.index(startIndex, offsetBy: r.upperBound - r.lowerBound, limitedBy: self.endIndex) else { return nil }
return self[startIndex...endIndex]
}
}
}
Swift 4 change: You need to create a new string from the result
return String(self[startIndex...endIndex])

Answer 2

I took inspiration from #vadian's answer and created a set of (Swift 4) extensions that make pulling substrings trivially easy. These do not bounds check, which is generally my preference since I shouldn't be deferring sanity checking to lower level utilities like these.
extension String {
subscript (_ index: Int) -> String {
return String(self[self.index(startIndex, offsetBy: index)])
}
subscript (_ range: CountableRange<Int>) -> String {
let lowerBound = index(startIndex, offsetBy: range.lowerBound)
let upperBound = index(startIndex, offsetBy: range.upperBound)
return String(self[lowerBound..<upperBound])
}
subscript (_ range: CountableClosedRange<Int>) -> String {
let lowerBound = index(startIndex, offsetBy: range.lowerBound)
let upperBound = index(startIndex, offsetBy: range.upperBound)
return String(self[lowerBound...upperBound])
}
subscript (_ range: CountablePartialRangeFrom<Int>) -> String {
return String(self[index(startIndex, offsetBy: range.lowerBound)...])
}
subscript (_ range: PartialRangeUpTo<Int>) -> String {
return String(self[..<index(startIndex, offsetBy: range.upperBound)])
}
subscript (_ range: PartialRangeThrough<Int>) -> String {
return String(self[...index(startIndex, offsetBy: range.upperBound)])
}
}