/*
利用棧實現簡單計算器的例子(Calculator)     
選擇自 YangJq 的 Blog  
關鍵字   利用棧實現簡單計算器的例子(Calculator) 

NOTE：只接受 逆波蘭 表達式 -- by szh

*/
 
 /* Calculator.h */
//#ifndef __CALCULATOR_H__
//#define __CALCULATOR_H__

#include <iostream>
#include <stack>

using namespace std;

class Calculator 
{
private:
    //存放操作數的棧
    stack<double>   s;
    //將一個double型操作數壓入棧中
    void Enter(double operand) 
	{
        s.push(operand);
    }

    //從棧頂讀取兩個操作數
    int GetTwoOperands(double &operand1, double &operand2)
    {
        if (s.empty()) 
		{
            cerr << "No operand to pop!" << endl;
            //s.ClearStack();
			while (!s.empty()) s.pop();
            return 0;
        }

        operand2 = s.top(); s.pop();  //NOT operand1 !!
        if (s.empty()) 
		{
            cerr << "No operand to pop!" << endl;
            //s.ClearStack();
			while (!s.empty()) s.pop();
            return 0;
        }
        operand1 = s.top();s.pop(); //NOT operand2 !!
        return 1;
    }

    //將調用GetTwoOperands讀取的兩個操作數做運算op
    void Compute(char op)
    {
        double  operand1, operand2, result;
        if (!GetTwoOperands(operand1, operand2)) 
			return;

        switch(op) 
		{
            case '+': 
				result = operand1 + operand2; break;
            case '-': 
				result = operand1 - operand2; break;
            case '*': 
				result = operand1 * operand2; break;
            case '/':
                if (operand2 == 0) 
				{
                    cerr << "Divided by 0!" << endl;
                    //s.ClearStack();
					while (!s.empty()) s.pop();
                    return;
                }
                else 
					result = operand1 / operand2;
                break;
        }
        s.push(result);
    }

    //清空操作數棧
    void Clear() 
	{ 
		while (!s.empty()) s.pop();
		//s.ClearStack(); 
	}

public:
    //構造函數，建立一空棧
    Calculator(void) {}

    //計算表達式的值
    void Run() 
	{
        char    op;
        double  operand;

        while (1) 
		{
            cin >> op;
            if (cin.eof()) 
				return;

            while (op != '=') 
			{
                switch(op) 
				{
                    case '+': 
					case '-': 
					case '*': 
					case '/':
                        Compute(op);
                        break;

                    default:
                        cin.putback(op);
                        cin >> operand;
                        Enter(operand);
                        break;
                }

                cin >> op;
                if (cin.eof()) 
					return;
            }

            cout << s.top() << endl; s.pop();

            Clear();
        }
    }
};

//#endif

//////////////////////////////////////////////////////////////////
// Calc.cpp
//#include "calc.h"

void main()
{
    Calculator  c;
    c.Run();
}

/*
1 2 -
=
-1
1 2 /
=
0.5
1 2 + 3 * =
9

1 2 + 3 / =
1

123 3 /
=
41

*/