#include <iostream>
#include <string>
#include <vector>
#include <sstream>

float string2float(const std::string &str) {
	float num;
	std::stringstream ss;
	ss << str;
	if (ss >> num) {
		return num;
	}
	std::cout << "Error: Could not convert string to float. Exiting..." << std::endl;
	return 0;
}

std::string float2string(const float i) {
	std::string str;
	std::stringstream ss;
	ss << i;
	return ss.str();
}

std::string printvector(const std::vector<std::string> &vector, const std::string &str = "\n") {
	std::string string;
	for (unsigned int i = 0; i < vector.size(); i++) {
		string += vector[i];
		string += str;
	}
	return string;
}

int find_op(const std::vector<std::string> &vector, const std::string &str) {
	for (unsigned int i = 0; i < vector.size(); i++) {
		if (vector[i] == str) {
			return i;
		}
	}
	return -1;
}

int find_ops(const std::vector<std::string> &vector, const std::string &alphabet) {
	for (unsigned int i = 0; i < alphabet.length(); i++) {
		int j = find_op(vector, alphabet.substr(i, 1));
		if (j > -1) {
			return  j;
		}
	}
	return -1;
}

std::string trim(const std::string& str) {
	int start = str.find_first_not_of(" ");
	if (start == std::string::npos) {
		return "";
	}
	return str.substr(start, str.find_last_not_of(" ")-start+1);
}

int main(void) {
	
	std::string in;
	std::vector<std::string> equation;

	//Take input until done
	for(int i = 0; in != "="; i++) {
		std::cout << "Your expression: \"" << trim(printvector(equation, " ")) << "\". ";
		if (equation.size() % 2 == 0) {
			std::cout << "Expecting a number: ";
		}
		else {
			std::cout << "Expecting an operator (+, -, *, /, or =): ";
		}
		std::cin >> in;
		//Push input onto vector if acceptable
		if (equation.size() % 2 == 0) {
			std::stringstream ss(in);
			float fnum;
			if (ss >> fnum) {
				equation.push_back(in);
			}
		}
		else {
			if (in == "*" || in == "/" || in == "+" || in == "-") {
				equation.push_back(in);
			}
		}
	}
	
	//Calculate in order: *, /, +, -
	for (int strpos = find_ops(equation, "*/+-"); strpos > 0; strpos = find_ops(equation, "*/+-")) {
		if (equation[strpos] == "*") {
			equation.insert(equation.begin()+strpos+2, float2string(string2float(equation[strpos-1]) * string2float(equation[strpos+1])));
		}
		else if (equation[strpos] == "/") {
			if (equation[strpos+1] == "0") {
				std::cout << "Error: Can't divide by zero. Exiting..." << std::endl;
				return 0;
			}
			equation.insert(equation.begin()+strpos+2, float2string(string2float(equation[strpos-1]) / string2float(equation[strpos+1])));
		}
		else if (equation[strpos] == "+") {
			equation.insert(equation.begin()+strpos+2, float2string(string2float(equation[strpos-1]) + string2float(equation[strpos+1])));
		}
		else if (equation[strpos] == "-") {
			equation.insert(equation.begin()+strpos+2, float2string(string2float(equation[strpos-1]) - string2float(equation[strpos+1])));
		}
		equation.erase(equation.begin()+strpos-1, equation.begin()+strpos+2);
	}

	//Print what we have
	std::cout << "Result: " << printvector(equation, " ") << std::endl;

	return 0;
}