这是一个简单的演示如何评价文本的数学表达式

代码说明：

This a simple demo of how to evaluate mathematical expressions in text format, including provision for variables and functions. The code consists of three simple classes: 1) Calc - which does the main calculations, 2) Stack - which is used to push and pop intermediate operators and numbers and 3) Symbol Table. The symbol table is a collection of calc symbols, a type structure consisting of various elements, including the expression name, function name, and the final value of the expression The evaluation procedure is done in three steps: 1) simplify the expression by removing all the elements within () and storing them in subexpressions within the symbol table. 2) Each of these sub expressions are then converted to RPN format (Reverse polar notation) and evaluated 3) Finally, the main expression is converted to RPN and evaluated-This is a simple demo of how to evaluate mathematical expressions in text format, including provision for variables and functi