我是斯威夫特4号

let myString = "LOVE"

self.textField1.text = String(Array(myString)[0])
self.textField2.text = String(Array(myString)[1])
self.textField3.text = String(Array(myString)[2])
self.textField4.text = String(Array(myString)[3])

Swift 2.0的Xcode 7通用种子

var text = "Hello, world!"

let firstChar = text[text.startIndex.advancedBy(0)] // "H"

对于第n个字符，将0替换为n-1。

编辑:Swift 3.0

text[text.index(text.startIndex, offsetBy: 0)]

注意，有更简单的方法来获取字符串中的某些字符

例如:let firstChar = text.characters.first

Swift 4.2或更高版本

使用String的indexes属性进行范围和部分范围下标

作为@LeoDabus nice answer的变体，我们可以为DefaultIndices添加一个额外的扩展，目的是允许我们在为后者实现自定义下标(通过Int专用范围和部分范围)时使用String的indexes属性。

extension DefaultIndices {
    subscript(at: Int) -> Elements.Index { index(startIndex, offsetBy: at) }
}

// Moving the index(_:offsetBy:) to an extension yields slightly
// briefer implementations for these String extensions.
extension String {
    subscript(range: Range<Int>) -> SubSequence {
        let start = indices[range.lowerBound]
        return self[start..<indices[start...][range.count]]
    }
    subscript(range: ClosedRange<Int>) -> SubSequence {
        let start = indices[range.lowerBound]
        return self[start...indices[start...][range.count]]
    }
    subscript(range: PartialRangeFrom<Int>) -> SubSequence {
        self[indices[range.lowerBound]...]
    }
    subscript(range: PartialRangeThrough<Int>) -> SubSequence {
        self[...indices[range.upperBound]]
    }
    subscript(range: PartialRangeUpTo<Int>) -> SubSequence {
        self[..<indices[range.upperBound]]
    }
}

let str = "foo bar baz bax"
print(str[4..<6]) // "ba"
print(str[4...6]) // "bar"
print(str[4...])  // "bar baz bax"
print(str[...6])  // "foo bar"
print(str[..<6])  // "foo ba"

感谢@LeoDabus指出我在使用索引属性作为字符串下标的(其他)替代方案的方向!

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 ","

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]

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"