Reversed Polish

Evaluate the Value of a Reversed Polish Notation String


#include <iostream>
#include <stack>
#include <sstream>
#include <cstdlib>
#include <cmath>
using namespace std ;


void pop(double &first, double &second, stack <double> &S) {
    second = S.top();
    S.pop();
    first = S.top();
    S.pop();
}

int main() {
    stack <double> S; //a double type stack
    double first, second;
    string line, str;

    while(true) {
        cout << "Enter the Reverse Polish Notation (as string): ";
        getline(cin, line);

        if(line[0] == '%') exit(1);

        istringstream inpstr(line);

        while(inpstr >> str) {
            if(str == "+") {//ADDITION +
                pop(first, second, S);
                S.push(first + second);
            }

            else if(str == "-") {//SUBSTRACTION -
                pop(first, second, S);
                S.push(first - second);
            }

            else if(str == "*") {//MULTIPLICATION *
                pop(first, second, S);
                S.push(first * second);
            }

            else if(str == "/") {//DIVISION /
                pop(first, second, S);
                S.push(first / second);
            }

            else if(str == "^") {//EXPONENT ^
                pop(first, second, S);
                S.push(pow(first, second));
            }

            else
                S.push(strtof(str.c_str(), NULL)); //str.c_str() returns a C-String eqv of the String type object
        }
        cout << "Value of the expression is: " << S.top() << endl;
        cout << "Enter '%' to stop" << endl << endl;
    }

    return 0;
}