Swift 2.2解决方案:

下面的扩展在Xcode 7中工作，这是这个解决方案和Swift 2.0语法转换的组合。

extension String {
    subscript(integerIndex: Int) -> Character {
        let index = startIndex.advancedBy(integerIndex)
        return self[index]
    }

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

斯威夫特3:

extension String {
    func substring(fromPosition: UInt, toPosition: UInt) -> String? {
        guard fromPosition <= toPosition else {
            return nil
        }

        guard toPosition < UInt(characters.count) else {
            return nil
        }

        let start = index(startIndex, offsetBy: String.IndexDistance(fromPosition))
        let end   = index(startIndex, offsetBy: String.IndexDistance(toPosition) + 1)
        let range = start..<end

        return substring(with: range)
    }
}

"ffaabbcc".substring(fromPosition: 2, toPosition: 5) // return "aabb"

你可以使用SwiftString (https://github.com/amayne/SwiftString)来做这件事。

"Hello, world!"[0] // H
"Hello, world!"[0...4] // Hello

免责声明:我写了这个扩展

到目前为止，下标(_:)不可用。就像我们不能这么做一样

str[0] 

与字符串。我们必须提供"String "。指数” 但是，我们如何给出我们自己的索引号呢，我们可以用，

string[str.index(str.startIndex, offsetBy: 0)]

斯威夫特5.1.3:

添加一个String扩展名:

extension String {

 func stringAt(_ i: Int) -> String { 
   return String(Array(self)[i]) 
 } 

 func charAt(_ i: Int) -> Character { 
  return Array(self)[i] 
 } 
}

let str = "Teja Kumar"
let str1: String = str.stringAt(2)  //"j"
let str2: Character = str.charAt(5)  //"k"

Swift 3:另一个解决方案(在操场测试)

extension String {
    func substr(_ start:Int, length:Int=0) -> String? {
        guard start > -1 else {
            return nil
        }

        let count = self.characters.count - 1

        guard start <= count else {
            return nil
        }

        let startOffset = max(0, start)
        let endOffset = length > 0 ? min(count, startOffset + length - 1) : count

        return self[self.index(self.startIndex, offsetBy: startOffset)...self.index(self.startIndex, offsetBy: endOffset)]
    }
}

用法:

let txt = "12345"

txt.substr(-1) //nil
txt.substr(0) //"12345"
txt.substr(0, length: 0) //"12345"
txt.substr(1) //"2345"
txt.substr(2) //"345"
txt.substr(3) //"45"
txt.substr(4) //"5"
txt.substr(6) //nil
txt.substr(0, length: 1) //"1"
txt.substr(1, length: 1) //"2"
txt.substr(2, length: 1) //"3"
txt.substr(3, length: 1) //"4"
txt.substr(3, length: 2) //"45"
txt.substr(3, length: 3) //"45"
txt.substr(4, length: 1) //"5"
txt.substr(4, length: 2) //"5"
txt.substr(5, length: 1) //nil
txt.substr(5, length: -1) //nil
txt.substr(-1, length: -1) //nil