表达式求值是程序设计语言编译中的一个最基本的问题。它的实现是栈应用的又一个典型例子。要把一个表达式翻译成正确求值的一个机器指令序列，或者直接对比表达式求值，首先要能够正确解释表达式。

-Expression evaluation is a fundamental question in the programming language compiler. Its realization is a typical example of the stack applications.Should be an expression translated into the correct evaluation of a machine instruction sequence, or a direct comparison of the expression is evaluated, first to be able to correct interpretation of expression.