计算器，功能要求：

<1>用户输入算式文本，确定后程序给出计算结果，支持两个操作数的加减乘除四则运算即可。例如

用户输入“1234+5678”，敲回车键后，屏幕显示“1234+5678=6912”,然后等待用户输入下一个算式文本；

又如用户输入“123/78”，敲回车键后，屏幕显示“123/78=1.577”， 然后等待用户输入下一个算式文本；

乘法符号用“*”表示

<2>操作数最多不超过四位,可以是整数或小数；结果如果是小数，保留到小数点后3位。

<3>容错功能要求:

当用户输入不合法的文本表达式，程序应提示用户输入错误。例如：用户输入“12f5+123”时，程序应报错，并指出错误内容。

发挥部分：

<4>如果程序还可支持其它运算符（例如指数、开方、sin、log、ln等-Calculator, function requirements:

< 1 > user input factor text, determine the program after calculation results, support two operands arithmetic can add, subtract, multiply and divide. For example

User input "1234+ 5678", press enter key, and the screen shows "1234+ 5678 = 6912", then wait for users to enter the next formula text

And as the user input "123/78", press enter key, the screen display "123/78 = 1.577" and then wait for the users to enter the next formula text

Multiplication sign with "*" said

< 2 > operator at most four, can be an integer or a decimal　Results if the decimal, keep to three decimal point.

< 3 > fault-tolerant function requirements:

When the user input illegal text expression, the program should prompt the user input error. For example: the user input "12 f5+ 123", the program should be error, and pointed out the mistake content.

Play a part:

< 4 > if the program can also support other operator (such as index, prescribing,