如果你看到不能下标一个类型为'String'的值…使用这个扩展:

斯威夫特3

extension String {
    subscript (i: Int) -> Character {
        return self[self.characters.index(self.startIndex, offsetBy: i)]
    }

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

    subscript (r: Range<Int>) -> String {
        let start = index(startIndex, offsetBy: r.lowerBound)
        let end = index(startIndex, offsetBy: r.upperBound)
        return self[start..<end]
    }

    subscript (r: ClosedRange<Int>) -> String {
        let start = index(startIndex, offsetBy: r.lowerBound)
        let end = index(startIndex, offsetBy: r.upperBound)
        return self[start...end]
    }
}

斯威夫特2.3

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

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

来源:http://oleb.net/blog/2014/07/swift-strings/

斯威夫特4

let str = "My String"

索引处的字符串

let index = str.index(str.startIndex, offsetBy: 3)
String(str[index])    // "S"

子字符串

let startIndex = str.index(str.startIndex, offsetBy: 3)
let endIndex = str.index(str.startIndex, offsetBy: 7)
String(str[startIndex...endIndex])     // "Strin"

前n个字符

let startIndex = str.index(str.startIndex, offsetBy: 3)
String(str[..<startIndex])    // "My "

最后n个字符

let startIndex = str.index(str.startIndex, offsetBy: 3)
String(str[startIndex...])    // "String"

Swift 2和3

str = "My String"

**字符串索引**

斯威夫特2

let charAtIndex = String(str[str.startIndex.advancedBy(3)])  // charAtIndex = "S"

斯威夫特3

str[str.index(str.startIndex, offsetBy: 3)]

子字符串fromIndex toIndex

斯威夫特2

let subStr = str[str.startIndex.advancedBy(3)...str.startIndex.advancedBy(7)] // subStr = "Strin"

斯威夫特3

str[str.index(str.startIndex, offsetBy: 3)...str.index(str.startIndex, offsetBy: 7)]

前n个字符

let first2Chars = String(str.characters.prefix(2)) // first2Chars = "My"

最后n个字符

let last3Chars = String(str.characters.suffix(3)) // last3Chars = "ing"

顺便说一句，有几个函数可以直接应用于String的字符链表示，像这样:

var string = "Hello, playground"
let firstCharacter = string.characters.first // returns "H"
let lastCharacter = string.characters.last // returns "d"

结果类型为Character，但可以将其转换为String。

或:

let reversedString = String(string.characters.reverse())
// returns "dnuorgyalp ,olleH" 

:-)

我也有同样的问题。简单地这样做:

var aString: String = "test"
var aChar:unichar = (aString as NSString).characterAtIndex(0)

一个类似蟒蛇的解决方案，允许你使用负下标，

var str = "Hello world!"
str[-1]        // "!"

可能是:

extension String {
    subscript (var index:Int)->Character{
        get {
            let n = distance(self.startIndex, self.endIndex)
            index %= n
            if index < 0 { index += n }
            return self[advance(startIndex, index)]
        }
    }
}

顺便说一下，调换整个python的切片符号可能是值得的

你也可以像这样将字符串转换为字符数组:

let text = "My Text"
let index = 2
let charSequence = text.unicodeScalars.map{ Character($0) }
let char = charSequence[index]

这是在常数时间内在指定索引处获取char的方法。

下面的示例不是在常数时间内运行，而是需要线性时间。所以如果你有很多搜索字符串索引使用上述方法。

let char = text[text.startIndex.advancedBy(index)]