'subscript' is unavailable: cannot subscript String with a CountableClosedRange, see the documentation comment for discussion

Question

In Swift 4, I\'m getting this error when I try to take a Substring of a String using subscript syntax.

\'subscript\' is unavaila

Answer 1

Building on both p-sun's and Justin Oroz's answers, here are two extensions that protect against invalid indexes beyond the start and end of a string (these extensions also avoid rescanning the string from the beginning just to find the index at the end of the range):

extension String {

    subscript(bounds: CountableClosedRange) -> String {
        let lowerBound = max(0, bounds.lowerBound)
        guard lowerBound < self.count else { return "" }

        let upperBound = min(bounds.upperBound, self.count-1)
        guard upperBound >= 0 else { return "" }

        let i = index(startIndex, offsetBy: lowerBound)
        let j = index(i, offsetBy: upperBound-lowerBound)

        return String(self[i...j])
    }

    subscript(bounds: CountableRange) -> String {
        let lowerBound = max(0, bounds.lowerBound)
        guard lowerBound < self.count else { return "" }

        let upperBound = min(bounds.upperBound, self.count)
        guard upperBound >= 0 else { return "" }

        let i = index(startIndex, offsetBy: lowerBound)
        let j = index(i, offsetBy: upperBound-lowerBound)

        return String(self[i..