Swift iOS. Program crashes when i try to mutate an array of buttons

Question

I have about 20 buttons linked to allKeys . I want the turnRed button to change the color of their text to red. I tried the following code:

    @IBOutlet var allKeys: [UIButton]!


    @IBAction func turnRed(sender: UIButton) {
        var i = allKeys.count

        repeat {
            allKeys[i].setTitleColor(UIColor.redColor(), forState: UIControlState.Normal)
        i -= 1
        } while i != -1
    }

When I press the turnRed button, my program crashes and xcode jumps to the appDelegate file and highlights the AppDelegate class with the error Thread1: signal SIGABRT.

The console says "Terminating app due to uncaught exception 'NSRangeException Reason: NSArrayI objectAtIndex

Answer 1

The issue here is that you are setting the variable i to allKeys.count , but since arrays in swift start with zero, the twentieth element should actually be allKeys[19] . Thus, when the loop is at i = 20 , it tries to access allKeys[20] , which crashes the app.

One way to solve this is to set i to allKeys.count - 1 instead, but a better way would be to use a for-in loop instead of repeat .

for key in allKeys {
    key.setTitleColor(UIColor.redColor(), forState: UIControlState.Normal)
}

There are multiple ways to loop in Swift, and you should use the options to your advantage. You can find Apple's documentation for Control Flow here .