使用递归子程序法设计一个语法分析程序，自顶向下分析方法.

1、使用递归下降分析算法分析表达式文法：

exp ::= exp addop term | term

addop ::= + | -

term ::= term mulop factor | factor

mulop ::= * | /

factor ::= (exp) | number

其中number可以是多位的十进制数字串（整数即可），因此这里还需要一个小的词法分析器来得到number的值。

2、该词法分析器以子程序形式出现，当需要进行词法分析时进行调用；

3、能够识别正确和错误的表达式；

4、在进行语法分析的过程中，计算输入表达式的值。

-Recursive subroutine method to design a syntax analysis program, top-down analysis. 1, using recursive descent parsing algorithm analysis of the expression grammar: exp:: = exp addop term | term addop:: =+ |- term:: = term mulop factor | factor mulop:: =* |/factor:: = (exp) | number of which is the number of decimal number can number string (integer can be), so here also need a small lexical analyzer to get the number value. 2, the lexical analyzer in subroutine form, when the need for lexical analysis of a call　3, able to recognize right and wrong expression　4, during the course of parsing, calculating import value of the expression.