1.将菜单的显示放在一个单独的函数中，随用随调。

　　2.将接收用户输入和校验输入的功能放在一个独立的函数中。

　　3.将四种运算分别放在四个函数中。-1. To display the menu in a separate function, as used with the transfer. 2. To receive user input and input validation function on an independent function. 3. Will be four on four types of computing functions, respectively.