这是最好最简单的方法之一

        let yourString = "thisString"
        print(Array(yourString)[8])

将字符串中的每个字母放入数组 然后选择第9个

不使用整数进行索引，只使用String.Index。主要是线性复杂度。您还可以从String中创建范围。索引并使用它们获取子字符串。

斯威夫特3.0

let firstChar = someString[someString.startIndex]
let lastChar = someString[someString.index(before: someString.endIndex)]
let charAtIndex = someString[someString.index(someString.startIndex, offsetBy: 10)]

let range = someString.startIndex..<someString.index(someString.startIndex, offsetBy: 10)
let substring = someString[range]

快2.倍

let firstChar = someString[someString.startIndex]
let lastChar = someString[someString.endIndex.predecessor()]
let charAtIndex = someString[someString.startIndex.advanceBy(10)]

let range = someString.startIndex..<someString.startIndex.advanceBy(10)
let subtring = someString[range]

请注意，不能使用从一个字符串到另一个字符串创建的索引(或范围)

let index10 = someString.startIndex.advanceBy(10)

//will compile
//sometimes it will work but sometimes it will crash or result in undefined behaviour
let charFromAnotherString = anotherString[index10]

我的解决方案是在一行中，假设cadena是字符串，4是你想要的第n个位置:

let character = cadena[advance(cadena.startIndex, 4)]

简单的…我想Swift在未来的版本中会包含更多关于子字符串的内容。

还有另一种选择，在String声明中解释过

extension String : BidirectionalCollection {
    subscript(i: Index) -> Character { return characters[i] }
}

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4.2

这个答案是理想的，因为它在一个扩展中扩展了String及其所有子序列(Substring)

public extension StringProtocol {
    
    public subscript (i: Int) -> Element {
        return self[index(startIndex, offsetBy: i)]
    }

    public subscript (bounds: CountableClosedRange<Int>) -> SubSequence {
        let start = index(startIndex, offsetBy: bounds.lowerBound)
        let end = index(startIndex, offsetBy: bounds.upperBound)
        return self[start...end]
    }
    
    public subscript (bounds: CountableRange<Int>) -> SubSequence {
        let start = index(startIndex, offsetBy: bounds.lowerBound)
        let end = index(startIndex, offsetBy: bounds.upperBound)
        return self[start..<end]
    }
    
    public subscript (bounds: PartialRangeUpTo<Int>) -> SubSequence {
        let end = index(startIndex, offsetBy: bounds.upperBound)
        return self[startIndex..<end]
    }
    
    public subscript (bounds: PartialRangeThrough<Int>) -> SubSequence {
        let end = index(startIndex, offsetBy: bounds.upperBound)
        return self[startIndex...end]
    }
    
    public subscript (bounds: CountablePartialRangeFrom<Int>) -> SubSequence {
        let start = index(startIndex, offsetBy: bounds.lowerBound)
        return self[start..<endIndex]
    }
}

使用

var str = "Hello, playground"

print(str[5...][...5][0])
// Prints ","