var str = "12345678"
        let length = str.characters.count

        //截取前四位
        var sub1 = str.substring(to: str.index(str.startIndex, offsetBy: 4))
        
        //截取后2位（两种方法）
        var sub2 = str.substring(from: str.index(str.startIndex, offsetBy: length-2))
        var sub3 = str.substring(from: str.index(str.endIndex, offsetBy: -2))
        

        
        //截取中间4位，从第2位开始(二种方法)
        let start1 = str.index(str.startIndex, offsetBy: 2)
        str = str.substring(from: start1)  //取到从第2位开始的所有字符
        let end1 = str.index(str.startIndex, offsetBy: 4)
        var sub5 = str.substring(to: end1)   //从新的字符开始取4位

        //截取中间4位，从第2位开始
        let start = str.index(str.startIndex, offsetBy: 2)
        let end = str.index(str.endIndex,offsetBy:-(length - 6))
        var sub4 = str.substring(with: start..<end)