Dijkstra Two Stack Solution

twostack.cpp

#include <iostream>
#include <stack>
#include <stdexcept>
#include <cctype>
#include <cstdlib>

using namespace std;

int evaluate(string const &expression){
	stack<char> operators;
	stack<int> values;
	int result = 0;

	for(auto it = expression.begin(); it != expression.end(); ++it){
		if(*it == '(' || std::isspace(*it))
		{
			continue;
		}else if( std::isdigit(*it))
		{
			char value = *it;

			values.push(atoi(&value));
		}else if(*it == ')')
		{
			int value1 = values.top();
			values.pop();

			int value2 = values.top();
			values.pop();

			int op = operators.top();
			operators.pop();

			if(op == '+'){
				values.push(value1 + value2);
			}else if(op == '-'){
				values.push(value1 - value2);
			}else if(op == '/'){
				values.push(value1 / value2);
			}else if(op == '*'){
				values.push(value1 * value2);
			}
		}else if(*it == '+' || *it == '-' || *it == '*' || *it == '/')
		{
			operators.push(*it);
		}else
		{
			throw exception();
		}

	}
	result = values.top();
	values.pop();
	return result;
}

int main()
{
	cout << "The value of the expression is: " << evaluate(" ( 1 + ( ( 2 + 3 ) * ( 4 * 5 ) ) ) ") << endl;