1
 
 extension String {        
 2     //subscript函數可以檢索數組中的值
 3     //直接按照索引方式截取指定索引的字符
 4     subscript (_ i: Int) -> Character {
 5         //讀取字符
 6         get {return self[index(startIndex, offsetBy: i)]}
 7         
 8         //修改字符
 9         set
10         {
11             var str:String = self
 
12             var index = str.index(startIndex, offsetBy: i)
13             str.remove(at: index)
14             str.insert(newValue, at: index)
15             self = str
16         }
17     }
18 }
19 
20 //Character擴展方法  
21 extension Character  
22 {  
23   //屬性：ASCII整數值(定義小寫為整數值)
24    var
 
 ascii: Int {
25         get {
26             let s = String(self).unicodeScalars
27             return Int(s[s.startIndex].value)
28         }
29     }
30 }
31 
32 //Int擴展方法  
33 extension Int
34 {
35     //屬性：ASCII值(定義大寫為字符值)
36     var ASCII:Character 
37     {
38         get {return Character(UnicodeScalar(self)!)}
39     }
40 }

調用擴展方法，實現功能。示例：

 1 //字符串
 2 let str:String = "Strengthen"
 3 //根據索引獲取字符
 4 let char:Character = str[0]
 5 //根據字符獲取ASCII對應的字符整數值
 6 let num:Int = char.ascii
 7 print("\(char) ", terminator: "")
 8 print("\(num) ", terminator: "")
 9 //Print: S 83     
10       
11 //表示整數的字符串
12 let strNum:String = "88"
13 //先轉換為整數，在根據ASCII表轉換為對應字符
14 let charNum:Character = Int(strNum)!.ASCII
15 print("\(charNum) ", terminator: "")
16 //Print: X

[Swift]字符串根據索引獲取指定字符，依據ASCII編碼實現字符和整數的快速轉換