我怎样才能得到字符串的第n个字符?我尝试了括号([])访问器,没有运气。

var string = "Hello, world!"

var firstChar = string[0] // Throws error

错误:'下标'是不可用的:不能下标String与Int,请参阅文档注释讨论


当前回答

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"

其他回答

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] }
}

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

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

获取和设置下标(字符串和子字符串)- Swift 4.2

Swift 4.2, Xcode 10

我的回答是基于@ alcarlson的回答。 唯一的区别是你可以返回一个Substring或String(在某些情况下,一个字符)。您还可以获取和设置下标。 最后,我的回答比@alecarlson的回答更麻烦,也更长,因此,我建议你把它放在源文件中。


扩展:

public extension String {
    public subscript (i: Int) -> Character {
        get {
            return self[index(startIndex, offsetBy: i)]
        }
        set (c) {
            let n = index(startIndex, offsetBy: i)
            replaceSubrange(n...n, with: "\(c)")
        }
    }
    public subscript (bounds: CountableRange<Int>) -> Substring {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[start ..< end]
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ..< end, with: s)
        }
    }
    public subscript (bounds: CountableClosedRange<Int>) -> Substring {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[start ... end]
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ... end, with: s)
        }
        
    }
    public subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            return self[start ... end]
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            replaceSubrange(start ... end, with: s)
        }
    }
    public subscript (bounds: PartialRangeThrough<Int>) -> Substring {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[startIndex ... end]
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ... end, with: s)
        }
    }
    public subscript (bounds: PartialRangeUpTo<Int>) -> Substring {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[startIndex ..< end]
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ..< end, with: s)
        }
    }
    
    public subscript (i: Int) -> String {
        get {
            return "\(self[index(startIndex, offsetBy: i)])"
        }
        set (c) {
            let n = index(startIndex, offsetBy: i)
            self.replaceSubrange(n...n, with: "\(c)")
        }
    }
    public subscript (bounds: CountableRange<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[start ..< end])"
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ..< end, with: s)
        }
    }
    public subscript (bounds: CountableClosedRange<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[start ... end])"
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ... end, with: s)
        }
        
    }
    public subscript (bounds: CountablePartialRangeFrom<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            return "\(self[start ... end])"
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            replaceSubrange(start ... end, with: s)
        }
    }
    public subscript (bounds: PartialRangeThrough<Int>) -> String {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[startIndex ... end])"
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ... end, with: s)
        }
    }
    public subscript (bounds: PartialRangeUpTo<Int>) -> String {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[startIndex ..< end])"
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ..< end, with: s)
        }
    }
    
    public subscript (i: Int) -> Substring {
        get {
            return Substring("\(self[index(startIndex, offsetBy: i)])")
        }
        set (c) {
            let n = index(startIndex, offsetBy: i)
            replaceSubrange(n...n, with: "\(c)")
        }
    }
}
public extension Substring {
    public subscript (i: Int) -> Character {
        get {
            return self[index(startIndex, offsetBy: i)]
        }
        set (c) {
            let n = index(startIndex, offsetBy: i)
            replaceSubrange(n...n, with: "\(c)")
        }
        
    }
    public subscript (bounds: CountableRange<Int>) -> Substring {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[start ..< end]
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ..< end, with: s)
        }
    }
    public subscript (bounds: CountableClosedRange<Int>) -> Substring {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[start ... end]
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ... end, with: s)
        }
    }
    public subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            return self[start ... end]
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            replaceSubrange(start ... end, with: s)
        }
        
    }
    public subscript (bounds: PartialRangeThrough<Int>) -> Substring {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[startIndex ... end]
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ..< end, with: s)
        }
    }
    public subscript (bounds: PartialRangeUpTo<Int>) -> Substring {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return self[startIndex ..< end]
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ..< end, with: s)
        }
    }
    public subscript (i: Int) -> String {
        get {
            return "\(self[index(startIndex, offsetBy: i)])"
        }
        set (c) {
            let n = index(startIndex, offsetBy: i)
            replaceSubrange(n...n, with: "\(c)")
        }
    }
    public subscript (bounds: CountableRange<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[start ..< end])"
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ..< end, with: s)
        }
    }
    public subscript (bounds: CountableClosedRange<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[start ... end])"
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(start ... end, with: s)
        }
        
    }
    public subscript (bounds: CountablePartialRangeFrom<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            return "\(self[start ... end])"
        }
        set (s) {
            let start = index(startIndex, offsetBy: bounds.lowerBound)
            let end = index(endIndex, offsetBy: -1)
            replaceSubrange(start ... end, with: s)
        }
    }
    public subscript (bounds: PartialRangeThrough<Int>) -> String {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[startIndex ... end])"
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ... end, with: s)
        }
    }
    public subscript (bounds: PartialRangeUpTo<Int>) -> String {
        get {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            return "\(self[startIndex ..< end])"
        }
        set (s) {
            let end = index(startIndex, offsetBy: bounds.upperBound)
            replaceSubrange(startIndex ..< end, with: s)
        }
    }
    
    public subscript (i: Int) -> Substring {
        get {
            return Substring("\(self[index(startIndex, offsetBy: i)])")
        }
        set (c) {
            let n = index(startIndex, offsetBy: i)
            replaceSubrange(n...n, with: "\(c)")
        }
    }
}

我的解决方法很简单:

斯威夫特4.1:

let myString = "Test string"
let index = 0
let firstCharacter = myString[String.Index(encodedOffset: index)]

斯威夫特5.1:

let firstCharacter = myString[String.Index.init(utf16Offset: index, in: myString)]