如何在Swift 4中使用String切片下标？

本文翻译自：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

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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)
    }
}