Algorithm to evaluate a postfix expression

Algorithm

Suppose P is an expression written in postfix notation. This algorithm finds VALUE of postfix expression P.

1) Add ‘)’ to the end of P.
2) Scan P from left to right and repeat step3 and step4 for each symbol of P until sentinel ‘)’ is encountered.

3) If an operand is encountered, push it onto stack
4) If an operator  encountered, then
a) Pop two elements from the stack, where A is topmost element and B is next to top element.

b) Evaluate BA.
c) Push above result onto stack.
5) VALUE ←top element on the stack.
6) Exit.