Shunting Yard

gistfile1.java

package PhotoEditor;

import java.util.ArrayList;

public class Calculator {
	
	public int calculate( String expression ) {
		// Split it apart
		String[] parts = splitApart(expression);
		// Make two lists, one for numbers and another for operators
		ArrayList<Integer> numbers = new ArrayList<Integer> ();
		ArrayList <String> operators = new ArrayList<String> ();
		
		// Am I expecting a number or an operator?
		boolean isNumber = true;
		for (String s : parts) {
			// If we're expecting a number
			if (isNumber) {
				// Parse out the integer value of the string
				int i = Integer.parseInt(s);
				// Add it to the numbers list
				numbers.add(i);
				// Indicate that we are now looking for an operator
				isNumber = false;
			}
			// If we're expecting an operator
			else {
				// If I have no operators
				if (operators.size() == 0) {
					operators.add(s);
				}
				// I have stuff on the list
				else {
					// While the last operator has greater or equal precedence to this one
					while (operators.size() > 0 && // while there is an operator to check 
							precedence( operators.get(operators.size() - 1) ) >= precedence(s)) {
						int newNumber = 
								simplify( operators.get(operators.size() - 1), // first input - last operator 
								  numbers.get(numbers.size() - 2),     // second input - last number
								  numbers.get(numbers.size() - 1)	   // third input - second to last number
								);
						// Remove stuff from the list
						operators.remove(operators.size() - 1);
						numbers.remove(numbers.size() - 1);
						numbers.remove(numbers.size() - 1);
						// Then add on the new number
						numbers.add(newNumber);
					}
					// When you're done simplifying, add the operator on
					operators.add(s);
				}
				// Say I am now expecting a number
				isNumber = true;
			}
			//System.out.println(numbers);
			//System.out.println(operators);
		}

		// While I have an operator to simplify
		while (operators.size() > 0) {
			int newNumber = 
					simplify( operators.get(operators.size() - 1), // first input - last operator 
					  numbers.get(numbers.size() - 2),     // second input - last number
					  numbers.get(numbers.size() - 1)	   // third input - second to last number
					);
			// Remove stuff from the list
			operators.remove(operators.size() - 1);
			numbers.remove(numbers.size() - 1);
			numbers.remove(numbers.size() - 1);
			// Then add on the new number
			numbers.add(newNumber);
		}
		
		return numbers.get(0);
	}
	
	public String[] splitApart( String input ) {
		return input.split(" ");
	}
	
	public int precedence( String operator ) {
		if ( operator.equals("+") ) {
			return 1;
		}
		if ( operator.equals("-") ) {
			return 1;
		}
		if ( operator.equals("*") ) {
			return 2;
		}
		if ( operator.equals("/") ) {
			return 2;
		}
		return 0;
	}
	
	public int simplify(String op, int a, int b) {
		if (op.equals("+")) {
			return a + b;
		}
		if (op.equals("-")) {
			return a - b;
		}
		if (op.equals("*")) {
			return a * b;
		}
		if (op.equals("/")) {
			return a / b;
		}
		return 0;
	}
	
	
}