如何在Swift 4中使用String切片下标？

港控/mmm° 2022-11-27 02:00 178阅读 0赞

本文翻译自：[How can I use String slicing subscripts in Swift 4?][How can I use String slicing subscripts in Swift 4]

*I have the following simple code written in Swift 3:* **我有以下用Swift 3编写的简单代码：**

let str = "Hello, playground"
    let index = str.index(of: ",")!
    let newStr = str.substring(to: index)

*From Xcode 9 beta 5, I get the following warning:* **从Xcode 9 beta 5，我收到以下警告：**

> *' `substring(to:)` ' is deprecated: Please use `String` slicing subscript with a 'partial range from' operator.* **不推荐使用' `substring(to:)` '：请使用带有'partial range from'运算符的`String`切片下标。**

*How can this **slicing subscript with partial range from** be used in Swift 4?* **如何在Swift 4中使用部分范围的切片下标 ？**

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### \#1楼 ###

参考：[https://stackoom.com/question/35aVE/如何在Swift-中使用String切片下标][https_stackoom.com_question_35aVE_Swift-_String]

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### \#2楼 ###

*You should **leave one side empty** , hence the name "partial range".* **您应将一侧留空 ，因此命名为“ partial range”。**

let newStr = str[..<index]

*The same stands for **partial range from** operators, just leave the other side empty:* **运算符的部分范围相同，只是将另一侧留空：**

let newStr = str[index...]

*Keep in mind that these range operators return a `Substring` .* **请记住，这些范围运算符返回`Substring` 。** *If you want to convert it to a string, use `String` 's initialization function:* **如果要将其转换为字符串，请使用`String`的初始化函数：**

let newStr = String(str[..<index])

*You can read more about the new substrings [here][] .* **您可以[在此处][here]阅读有关新子字符串的更多信息。**

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### \#3楼 ###

*Example of `uppercasedFirstCharacter` convenience property in Swift3 and Swift4.* **Swift3和Swift4中的`uppercasedFirstCharacter`便利属性的示例。**

*Property `uppercasedFirstCharacterNew` demonstrates how to use String slicing subscript in Swift4.* **属性`uppercasedFirstCharacterNew`演示了如何在Swift4中使用字符串切片下标。**

extension String {
    
       public var uppercasedFirstCharacterOld: String {
          if characters.count > 0 {
             let splitIndex = index(after: startIndex)
             let firstCharacter = substring(to: splitIndex).uppercased()
             let sentence = substring(from: splitIndex)
             return firstCharacter + sentence
          } else {
             return self
          }
       }
    
       public var uppercasedFirstCharacterNew: String {
          if characters.count > 0 {
             let splitIndex = index(after: startIndex)
             let firstCharacter = self[..<splitIndex].uppercased()
             let sentence = self[splitIndex...]
             return firstCharacter + sentence
          } else {
             return self
          }
       }
    }
    
    let lorem = "lorem".uppercasedFirstCharacterOld
    print(lorem) // Prints "Lorem"
    
    let ipsum = "ipsum".uppercasedFirstCharacterNew
    print(ipsum) // Prints "Ipsum"

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### \#4楼 ###

*The conversion of your code to Swift 4 can also be done this way:* **您的代码到Swift 4的转换也可以通过以下方式完成：**

let str = "Hello, playground"
    let index = str.index(of: ",")!
    let substr = str.prefix(upTo: index)

*You can use the code below to have a new string:* **您可以使用下面的代码来创建新字符串：**

let newString = String(str.prefix(upTo: index))

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### \#5楼 ###

*substring(from: index) **Converted to** \[index...\]* **substring（from：index） *转换为* \[index ...\]**

*Check the sample* **检查样品**

let text = "1234567890"
    let index = text.index(text.startIndex, offsetBy: 3)
    
    text.substring(from: index) // "4567890"   [Swift 3]
    String(text[index...])      // "4567890"   [Swift 4]

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### \#6楼 ###

*Some useful extensions:* **一些有用的扩展：**

extension String {
        func substring(from: Int, to: Int) -> String {
            let start = index(startIndex, offsetBy: from)
            let end = index(start, offsetBy: to - from)
            return String(self[start ..< end])
        }
    
        func substring(range: NSRange) -> String {
            return substring(from: range.lowerBound, to: range.upperBound)
        }
    }

[How can I use String slicing subscripts in Swift 4]: https://oldbug.net/q/35aVE/How-can-I-use-String-slicing-subscripts-in-Swift-4
[https_stackoom.com_question_35aVE_Swift-_String]: https://stackoom.com/question/35aVE/%E5%A6%82%E4%BD%95%E5%9C%A8Swift-%E4%B8%AD%E4%BD%BF%E7%94%A8String%E5%88%87%E7%89%87%E4%B8%8B%E6%A0%87
[here]: https://developer.apple.com/documentation/swift/substring