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)
    }
}

使用Swift 4从字符串创建子字符串(前缀和后缀):

let str : String = "ilike"
for i in 0...str.count {
    let index = str.index(str.startIndex, offsetBy: i) // String.Index
    let prefix = str[..<index] // String.SubSequence
    let suffix = str[index...] // String.SubSequence
    print("prefix \(prefix), suffix : \(suffix)")
}

输出

prefix , suffix : ilike
prefix i, suffix : like
prefix il, suffix : ike
prefix ili, suffix : ke
prefix ilik, suffix : e
prefix ilike, suffix : 

如果你想在两个索引之间生成一个子字符串，使用:

let substring1 = string[startIndex...endIndex] // including endIndex
let subString2 = string[startIndex..<endIndex] // excluding endIndex

Swift3和Swift4中的uppercasedFirstCharacter便利属性示例。

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"

var str = "Hello, playground"
let indexcut = str.firstIndex(of: ",")
print(String(str[..<indexcut!]))
print(String(str[indexcut!...]))

你可以试着用这种方法，并会得到适当的结果。

编程时，我经常用简单的A-Za-z和0-9组成的字符串。不需要困难的索引操作。这个扩展是基于普通的老左/中/右函数。

extension String {

    // LEFT
    // Returns the specified number of chars from the left of the string
    // let str = "Hello"
    // print(str.left(3))         // Hel
    func left(_ to: Int) -> String {
        return "\(self[..<self.index(startIndex, offsetBy: to)])"
    }

    // RIGHT
    // Returns the specified number of chars from the right of the string
    // let str = "Hello"
    // print(str.left(3))         // llo
    func right(_ from: Int) -> String {
        return "\(self[self.index(startIndex, offsetBy: self.length-from)...])"
    }

    // MID
    // Returns the specified number of chars from the startpoint of the string
    // let str = "Hello"
    // print(str.left(2,amount: 2))         // ll
    func mid(_ from: Int, amount: Int) -> String {
        let x = "\(self[self.index(startIndex, offsetBy: from)...])"
        return x.left(amount)
    }
}

Swift 4/5更短:

let string = "123456"
let firstThree = String(string.prefix(3)) //"123"
let lastThree = String(string.suffix(3)) //"456"