Infix to Postfix

Infix to Postfix.cpp

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


// Simply determine if character is one of the four standard operators.
bool isOperator(char character) {
	if (character == '+' || character == '-' || character == '*' || character == '/') {
		return true;
	}
	return false;
}


// If the character is not an operator or a parenthesis, then it is assumed to be an operand.
bool isOperand(char character) {
	if (!isOperator(character) && character != '(' && character != ')') {
		return true;
	}
	return false;
}


// Compare operator precedence of main operators.
// Return 0 if equal, -1 if op2 is less than op1, and 1 if op2 is greater than op1.
int compareOperators(char op1, char op2) {
	if ((op1 == '*' || op1 == '/') && (op2 == '+' || op2 == '-')) { return -1; }
	else if ((op1 == '+' || op1 == '-') && (op2 == '*' || op2 == '/')) { return 1; }
	return 0;
}


void main()
{
	// Empty character stack and blank postfix string.
	stack<char> opStack;
	string postFixString = "";

	char input[100];

	// Collect input
	cout << "Enter an expression: ";
	cin >> input;

	// Get a pointer to our character array.
	char *cPtr = input;

	// Loop through the array (one character at a time) until we reach the end of the string.
	while (*cPtr != '\0') {
		// If operand, simply add it to our postfix string.
		// If it is an operator, pop operators off our stack until it is empty, an open parenthesis or an operator with less than or equal precedence.
		if (isOperand(*cPtr)) { postFixString += *cPtr; }
		else if (isOperator(*cPtr)) {
			while (!opStack.empty() && opStack.top() != '(' && compareOperators(opStack.top(), *cPtr) <= 0) {
				postFixString += opStack.top();
				opStack.pop();
			}
			opStack.push(*cPtr);
		}
		// Simply push all open parenthesis onto our stack
		// When we reach a closing one, start popping off operators until we run into the opening parenthesis.
		else if (*cPtr == '(') { opStack.push(*cPtr); }
		else if (*cPtr == ')') {
			while (!opStack.empty()) {
				if (opStack.top() == '(') { opStack.pop(); break; }
				postFixString += opStack.top();
				opStack.pop();
			}
		}

		// Advance our pointer to next character in string.
		cPtr++;
	}

	// After the input expression has been ran through, if there is any remaining operators left on the stack
	// pop them off and put them onto the postfix string.
	while (!opStack.empty()) {
		postFixString += opStack.top();
		opStack.pop();
	}


	// Show the postfix string at the end.
	cout << "Postfix is: " << postFixString << endl;
	
	cin.get();
	cin.ignore();
}