swift better work with strings

stringSubscript.swift

extension String {

    subscript (i: Int) -> String? {
        if characters.count > i && i >= 0 {
            return String(Array(self.characters)[i])
        }
        return nil
    }

    subscript (r: Range<Int>) -> String? {
        guard r.lowerBound >= 0 && r.upperBound <= self.characters.count && r.upperBound >= r.lowerBound else { return nil }
        let start = self.index(startIndex, offsetBy: r.lowerBound)
        let end = self.index(startIndex, offsetBy: r.upperBound)
        return self[Range(start ..< end)]
    }
    
    func indexAt(int: Int) -> String.Index? {
        if characters.count > int {
            return index(self.startIndex, offsetBy: int)
        }
        return nil
    }
}