title: How to I equate double values to string label? (This is for a caculator app I'm building in xcode Swift)
body: <pre><code>//
  //  ViewController.swift
  //  Arithmetic Calculator
  //
  //  Created by Mapondera, Tanaka on 10/7/15.
  //  Copyright (c) 2015 Mapondera, Tanaka. All rights reserved.
  //
  
  import UIKit
  
  class ViewController: UIViewController {
  
      @IBOutlet weak var calculatorDisplay: UILabel!
  
      var isEnteredNumber = false
      var firstNumber:Double = 0.0
      var secondNumber:Double = 0.0
      var operation = ""
      var result:Double = 0.0
  
      @IBAction func numberPunched(sender: AnyObject) {
          // number
          var number = sender.currentTitle
  
          if isEnteredNumber {
              calculatorDisplay.text = calculatorDisplay.text! + number!!
          } else {
              calculatorDisplay.text = number
              isEnteredNumber = true
          }
      }
  
      @IBAction func calculationPunched(sender: AnyObject) {
          // operation
          isEnteredNumber = false
  
  
  firstNumber = calculatorDisplay.text?
  </code></pre>
  
  <p>// There is an error code for this line above that reads "cannot assign a value of type string to value of type double"...?</p>
  
  <pre><code> operation = sender.currentTitle!!
      }
  
      @IBAction func equalsPunched(sender: AnyObject) {
          // equals
          isEnteredNumber = false
          var result = 0.0
  
  
  secondNumber = calculatorDisplay.text?
  </code></pre>
  
  <p>// Error code for line above that reads "? must be followed by call, member lookup or subcrpit"...?</p>
  
  <pre><code>        if operation == "+" {
              result = firstNumber + secondNumber
          } else if operation == "-" {
              result = firstNumber - secondNumber
          } else if operation == "x" {
              result = firstNumber * secondNumber
          } else if operation == "/" {
              result = firstNumber / secondNumber
          } /*else if operation == "^" {
              result = pow(firstNumber, secondNumber)
          }*/
          calculatorDisplay.text = "\(result)"
      }
  
      @IBAction func changeSignPunched(sender: AnyObject) {
          isEnteredNumber = false
  
  
  firstNumber = calculatorDisplay.text?
  </code></pre>
  
  <p>// There is an Error code for the line above that reads "? must be followed by call, member lookup or subscrpit"...?</p>
  
  <pre><code>result = firstNumber * (-1)
          calculatorDisplay.text = "\(result)"
      }
  
      @IBAction func clearPunched(sender: AnyObject) {
          firstNumber = 0
          secondNumber = 0
          isEnteredNumber = false
          result = 0
          calculatorDisplay.text = "\(result)"
      }
  
      override func viewDidLoad() {
          super.viewDidLoad()
          // Do any additional setup after loading the view, typically from a nib.
      }
  
      override func didReceiveMemoryWarning() {
          super.didReceiveMemoryWarning()
          // Dispose of any resources that can be recreated.
      }
  }
  </code></pre>