calculator.hpp (cpp-lecture/stack/calculator.hpp)

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#ifndef CALCULATOR_HPP
#define CALCULATOR_HPP

#include <string>
#include <istream>
#include <sstream>
#include "stack.hpp"

class Calculator
{
    public:

        double calculate(const std::string& expr)
        {
            // PRE: expr in RPN (reversed polish notation) syntax
            std::istringstream in(expr);
            std::string token;
            double result;

            while (in >> token) {
                if (token == "+" || token == "-" ||
                    token == "*" || token == "/")
                {
                    double op2{opstack.pop()};
                    double op1{opstack.pop()};
                    double result;
                    if (token == "+") {
                        result = op1 + op2;
                    } else if (token == "-") {
                        result = op1 - op2;
                    } else if (token == "*") {
                        result = op1 * op2;
                    } else {
                        result = op1 / op2;
                    }
                    opstack.push(result);
                } else {
                    std::istringstream vin{token};
                    double value;
                    if (vin >> value) {
                        opstack.push(value);
                    } else {
                        std::cerr << "syntax error" << std::endl;
                    }
                }
            }
            result = opstack.pop();
            if (!opstack.empty()) {
                std::cerr << "bad expression" << std::endl;
            }
            return result;
        }

    private:
        Stack opstack;
};

#endif // CALCULATOR_HPP