Swift Learning-演算子&文字列

        // 1.    
        //         
        //               ，     ，             。           ，             
        let x = (3,"c");
        let y = (3,"bfffsdfsdfsdfsdsdfsdfsdasdfasfffffdvcxcxvc");
        if x > y {
            print("x > y");
        } else if x == y {
            print("x == y");
        } else if x < y{
            print("x < y");
        }

        let a : Int? =  Int("123a");
        print(a ?? 2); // a != nil ? a! : 2

        // 3.1    1...5
        for index in 1...5 {
            print(index,terminator: "，");
        }
        print("");
        // 3.2    
        for index in 1..<5 {
            print(index,terminator:"，");
        }
        print("");
        // 3.3     
        let names = ["Anna", "Alex", "Brian", "Jack"];
        for index in names[1...2] {
            print(index,terminator:"，");
        }
        print("");
        for index in names[0..

       // 4.1      - """
        let quotation = """
        The White Rabbit put on his spectacles.  "Where shall I begin,
        please your Majesty?" he asked.
        "Begin at the beginning," the King said gravely, "and go on
        till you come to the end; then stop."
        """
        print(quotation);

        // 4.3  \     
        let softWrappedQuotation = """
        The White Rabbit put on his spectacles.  "Where shall I begin, \
        please your Majesty?" he asked.
        "Begin at the beginning," the King said gravely, "and go on \
        till you come to the end; then stop."
        """
        print(softWrappedQuotation);

let spec = ["
","\0","\\","\t","\"","\r","\'","\u{1F496}"];
        for item in spec {
            print(item);
        }
        print("");

      print(#"hahahahha\#nhhheehhehehe"#);
        
        let string1 = "aaaaaa";
        let string2 = string1;
        print(Unmanaged.passUnretained(string1 as AnyObject).toOpaque());
        print(Unmanaged.passUnretained(string2 as AnyObject).toOpaque());
        
        
        let characterArr : [Character] = ["1","2","!","
",""];
        let charaString : String? = String(characterArr);
        print(charaString ?? "");
        
        let multiplier = 3;
        print("\(multiplier) times 2.5 is \(Double(multiplier) * 2.5) ")

      //      
        var welcome = "hello";
        welcome.insert("!", at: welcome.endIndex);
        print(welcome);
        welcome.insert(contentsOf : " world", at: welcome.index(before: welcome.endIndex));
        print(welcome);
//
        //      
        welcome = welcome.substring(to: welcome.firstIndex(of: " ")!);
        print(welcome);
        
        let index = welcome.firstIndex(of: "l") ?? welcome.endIndex;
        let subWelcome = welcome[..