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I'm following a tutorial to learn swift and one of the apps is tic tac toe. The code uses lots of arrays and arrays within arrays, and it became confusing pretty quickly. I don't want to move on to the next tutorial until I'm confident in my understand of how this code is working. My questions about this code are as follows:

  • How does combo[] know which space it is referring to when checking for a win?
  • In for combo in winCombos, what is combo's function?
  • How does winCombos work with gameState to tell if there is a winner?

Here is the code:

import UIKit

class ViewController: UIViewController {

    var turnNumber = 1

    var winner = 0

    @IBOutlet weak var button0: UIButton!

    var gameState = [0, 0, 0, 0, 0, 0, 0, 0, 0]

    let winCombos = [[0,1,2], [3,4,5], [6,7,8], [0,3,6], [1,4,7], [2,5,8], [0,4,8], [2,4,6]]

    @IBAction func buttonPressed(sender: AnyObject) {

        if(gameState[sender.tag] == 0) {

            var oImage = UIImage(named: "Tic Tac Toe O.png")
            var xImage = UIImage(named: "Tic Tac Toe X.png")



            if(turnNumber%2 == 0) {

                sender.setImage(oImage, forState: .Normal)
                gameState[sender.tag] = 1
            }else{
                sender.setImage(xImage, forState: .Normal)
                gameState[sender.tag] = 2
            }

            for combo in winCombos {
                if(gameState[combo[0]] == gameState[combo[1]] && gameState[combo[1]] == gameState[combo[2]] && gameState[combo[0]] != 0) {

                    println(combo[0])
                    println(combo[1])
                    println(combo[2])

                    winner = gameState[combo[0]]
                }
            }

            if( winner != 0){
                println("Winner!")
            }

            turnNumber++

        }

    }

    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.
    }


}
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The gameState array is simply the cells of the Tic Tac Toe game, with indexes as follows:

 0 | 1 | 2
---+---+---
 3 | 4 | 5
---+---+---
 6 | 7 | 8

From that, it's easy to discern what winCombo is, it's the list of possible wins, such as 0,1,2 (top row) and 2,4,6 (bottom-left to top-right diagonal).

The statement:

for combo in winCombos

is just a loop, setting combo to each of the values in winCombos and using that to see if there's been a win.

So ther first time through the loop, combo is set to [0,1,2] and you can check if all those cells are non-zero and equal to each other - if so, you have a win.

Ditto for subsequent iterations of that loop, where combo takes on the subsequent values in the winCombos array.

Let's say you have a win on the final check 2,4,6 with all the values being X (2 in this case). The checking statement can be gradually narrowed down from:

if(gameState[combo[0]] == gameState[combo[1]] &&
   gameState[combo[1]] == gameState[combo[2]] &&
   gameState[combo[0]] != 0)

to:

if(gameState[2] == gameState[4] && gameState[4] == gameState[6] && gameState[2] != 0)

and, from there:

if(2 == 2 && 2 == 2 && 2 != 0)

to get your winner.

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