表达式求值以字符序列的形式从终端输入语法正确的、不含变量的数据表达式。利用教科书表3.1给出的算符优先关系，实现对算术四则混合运算表达式的求值，并仿照教科书的例子3-1演示在求值中运算符栈、运算数栈、输入字符和主要操作的变化过程.-Expressions evaluate to the form of a sequence of characters from the terminal input grammar correct, non-variable expression data. Table 3.1 using the textbook are given priority to the relationship between the operator, to achieve four mixed computing arithmetic expression evaluation and follow the textbook example of the value of 3-1 in the demonstration in order to stack operator, computing the number of stacks, enter the characters and the main operation The process of change.