View objects make up the UI, so developers typically create, configure, and connect view objects using Interface Builder. The parts of the model layer, on the other hand, are typically set up in code.

In the project navigator, select ViewController.swift. Add the following code that declares two arrays of strings and an integer.

class ViewController: UIViewController {
    @IBOutlet var questionLabel: UILabel!
    @IBOutlet var answerLabel: UILabel!

    let questions: [String] = [
        "What is 7+7?",
        "What is the capital of Vermont?",
        "What is cognac made from?"
    ]
    let answers: [String] = [
        "14",
        "Montpelier",
        "Grapes"
    ]
    var currentQuestionIndex: Int = 0
    ...
}

The arrays are ordered lists containing questions and answers. The integer will keep track of what question the user is on.

Notice that the arrays are declared using the let keyword, whereas the integer is declared using the var keyword. A constant is denoted with the let keyword; its value cannot change. The questions and answers arrays are constants. The questions and answers in this quiz will not change and, in fact, cannot be changed from their initial values.

A variable, on the other hand, is denoted by the var keyword; its value is allowed to change. You made the currentQuestionIndex property a variable because its value must be able to change as the user cycles through the questions and answers.

...
@IBAction func showNextQuestion(_ sender: UIButton) {
    currentQuestionIndex += 1
    if currentQuestionIndex == questions.count {
        currentQuestionIndex = 0
    }

    let question: String = questions[currentQuestionIndex]
    questionLabel.text = question
    answerLabel.text = "???"
}

@IBAction func showAnswer(_ sender: UIButton) {
    let answer: String = answers[currentQuestionIndex]
    answerLabel.text = answer
}
...

class ViewController: UIViewController {
    ...
    override func viewDidLoad() {
        super.viewDidLoad()
        questionLabel.text = questions[currentQuestionIndex]
    }
}