Evaluating an infix expression through postfix conversion

calculator.cpp

#include <iostream>
#include <stack>
using namespace std;

double Evaluate(string input);
int GetTokenPrecedence(char c);
double ApplyOperator(double a, double b, char op);

int main(int argc, char* argv[])
{
	for (int i = 1; i < argc; i++) {
		cout << argv[i] << " = " << round(Evaluate(argv[i]) * 10000) / 10000 << endl;
	}
}

double Evaluate(string input)
{
	stack<double> values;
	stack<char> operators;

	for (size_t i = 0; i < input.length(); ++i) {
		char c = input[i];

		if (isspace(c)) { // Token is whitespace - Check out documenation for isspace().
			continue;
		} else if (isdigit(c) || c == '.') { // Token is a number or a decimal point.
			double tempValue = 0;
			int tempDecimal = 0;

			while ((isdigit(input[i]) || input[i] == '.') && i < input.length()) {
				if (input[i] == '.') {
					i++;
					tempDecimal = 1;
				} else if (tempDecimal > 0) {
					tempDecimal *= 10;
					tempValue += ((double) input[i++] - (double) '0') / tempDecimal;
				} else {
					tempValue = tempValue * 10 + ((double) input[i++] - (double) '0');
				}
			}

			i--;
			values.push(tempValue);
		} else if (c == '(') { // Token is an opening parentheses.
			operators.push(c);
		} else if (c == ')') { // Token is a closing parentheses.
			while (operators.top() != '(') {
				char op = operators.top();
				operators.pop();
				double a = values.top();
				values.pop();
				double b = values.top();
				values.pop();

				values.push(ApplyOperator(a, b, op));
			}

			operators.pop();
		} else { // Token is an operator.
			while (!operators.empty() && GetTokenPrecedence(operators.top()) >= GetTokenPrecedence(c)) {
				char op = operators.top();
				operators.pop();
				double a = values.top();
				values.pop();
				double b = values.top();
				values.pop();

				values.push(ApplyOperator(a, b, op));
			}

			operators.push(c);
		}
	}

	while (!operators.empty()) {
		char op = operators.top();
		operators.pop();
		double a = values.top();
		values.pop();
		double b = values.top();
		values.pop();

		values.push(ApplyOperator(a, b, op));
	}

	return values.top();
}

int GetTokenPrecedence(char c)
{
	if (c == '*' || c == '/') {
		return 2;
	} else if (c == '+' || c == '-') {
		return 1;
	} else {
		return 0;
	}
}

double ApplyOperator(double a, double b, char op)
{
	switch (op) {
		case '+':
			return a + b;
		case '-':
			return a - b;
		case '*':
			return a * b;
		case '/':
			return a / b;
		default:
			return 0;
	}
}

See -> http://faculty.cs.niu.edu/~hutchins/csci241/eval.htm