本演示程序中，当用户输入一个合法的表达式后，能够返回正确的结果。能够计算的运算符包括：加、减、乘、除、括号；能够计算的数要求在实数范围内。对于异常表达式给出错误提示。-The demo program, when the user enters a legitimate expression, they can return the correct results. Operators can be calculated include: add, subtract, multiply, divide, square brackets　able to calculate the number of requests in the real number range. Abnormal expression is given for the error.