在Swift编程语言中获取字符串的第n个字符

如何获取字符串的第n个字符？我试过括号([])存取器没有运气。

var string = "Hello,world!"

var firstChar = string[0] // Throws error

extension String {

  subscript (i: Int) -> Character {
    return self[self.startIndex.advancedBy(i)]
  }

  subscript (i: Int) -> String {
    return String(self[i] as Character)
  }

  subscript (r: Range<Int>) -> String {
    let start = startIndex.advancedBy(r.startIndex)
    let end = start.advancedBy(r.endindex - r.startIndex)
    return self[Range(start ..< end)]
  }
}

现在，这些都等于true。

"abcde"[0] == "a"
"abcde"[0...2] == "abc"
"abcde"[2..<4] == "cd"