分数计算器及测试程序。

1.该程序是分数计算器，程序的有些功能没有完成，请编写函数，将程序完成，要求界面提示清晰，便于操作。

2.增加输入分数四则运算的功能。

3.增加输出文件的功能，使输入的字符串及输出结果可以保存在文件中。

4.增加判别分母是否为零的功能，无论任何运算，分母一旦为零，则退出当前运算并输出提示消息。-Score Calculator and testing procedures. 1. The program is fractional calculator, some features did not complete the program, please write function, the process is complete, the interface is clear and prompt, easy to operate. 2. Increase the input fraction arithmetic functions. 3. To increase the function of the output file, so that the input string and output results can be saved in the file. 4. Increase the discriminant function of the denominator is zero, regardless of any operation, if the denominator is zero, then quit the current operation and output prompt message.