/*****************************************************
  Author  : Yan Huang
  Date    : 02.18.07
  Synopsis: convert a prefix expression to postfix expression
**************************************************************/

#include <stack>
#include <iostream>

/************************************************ 
    Get the next token, symbol is the character
    representation, which is returned, the token is 
    represented by its enumerated value, which
    is returned in the function name 
***************************************************/

enum Precedence= {lparen,rparen,plus,minus,divide,times,mod,eos,operand};

Precedence get_token()
{
  char symbol;

  cin  >> symbol;

  switch (symbol)  {
     case '(' : return lparen;
     case ')' : return rparen;
     case '+': return plus;
     case '-' : return minus;
     case '/' :  return divide;
     case '*' : return times;
     case '%' : return mod;
     case '\n' : return eos;
     default  : cin.unget(); return operand; 
     /* no error checking, default is operand */
     }
}

/************************************************************
    Evaluate a postfix expression, expr, maintained as a 
    global variable, '\0' is the the end of the expression.
    The stack and top of the stack are global variables.
    get_token is used to return the token type and 
    the character symbol. Operands are assumed to be single
    character digits 
*****************************************************************/

int main(void)
{

  Precedence token;
  char symbol;  
  int op1, op2, op;
  int n = 0;  /* counter for the expression string */
  
  Stack<char> expression; 

  token = get_token(&symbol, &n);
  while (token != eos)  {
      if (token == operand)
           cin >> op; /* no error check */
           expression.push(op);   /* stack insert */
        else { 
               /* remove two operands, perform operation, and 
                return result to the stack */
       op2 = pop(&top);  /* stack delete */
       op1 = pop(&top);
       switch(token) {
            case plus: push(&top, op1+op2); break;
            case minus: push(&top, op1-op2); break;     
            case times: push(&top, op1*op2); break;     
            case divide: push(&top, op1/op2); break;     
            case mod: push(&top, op1%op2);
       }
    }
    token = get_token (&symbol, &n);
 }
 return pop(&top); /* return result */
}