iOS Swift Type Conversation Math causing app to Crash

Question

I have code that is meant to place a small UIView as a subview at a random X coordinate.

If I place the view at a fixed spot it works but when I calculate the random position my app crashes. Here is the code:

    @IBOutlet var gameView : UIView
    let DROP_SIZE = CGSize(width: 40, height: 40)

    @IBAction func tap(sender : AnyObject) {
        self.drop()
    }

    func drop() {
        var frame = CGRect()
        frame.origin = CGPointZero
        frame.size = DROP_SIZE
        var x = Int(arc4random()) % Int(self.gameView.bounds.size.width) / Int(DROP_SIZE.width)
        frame.origin.x = CGFloat(x) * CGFloat(DROP_SIZE.width)
        var dropView: UIView = UIView(frame: frame)
        dropView.backgroundColor = randomColor()
        self.gameView.addSubview(dropView)

    }

whenever self.drop() is called my app crashes

If I comment out var x & frame.origin.x my app does not crash

So I know the issue is with these two lines of code:

var x = Int(arc4random()) % Int(self.gameView.bounds.size.width) / Int(DROP_SIZE.width)
frame.origin.x = CGFloat(x) * CGFloat(DROP_SIZE.width)

Something about the way these variables are casted is causing the app to crash because if I use a fixed x origin for example frame.origin.x = 100, then the app works as expected.

UPDATE:

because arc4random return UInt32 it will overflow when trying to convert it to an Int

I used this instead

var x = UInt32(arc4random()) % UInt32(self.gameView.bounds.size.width) / UInt32(DROP_SIZE.width)
frame.origin.x = CGFloat(x) * CGFloat(DROP_SIZE.width)

The error I receive highlights self.drop() with the text: Thread 1: EXC_BAD_INSTRUCTION(code=EXC+l386_INVOP, subcode=0x0) — Nearpoint, Jun 15 '14 at 07:10
Interesting thing is sometimes it crashes right away, and sometimes it works for the first few taps then crashes — Nearpoint, Jun 15 '14 at 07:11
possible duplicate of [Crash when casting the result of arc4random() to Int](http://stackoverflow.com/questions/24087518/crash-when-casting-the-result-of-arc4random-to-int) — David Berry, Jun 15 '14 at 07:15
arc4random returns a UInt32. Half the time it will overflow an Int32 and crash on the mandatory bounds checking. Seethe linked duplicate. — David Berry, Jun 15 '14 at 07:16
That exactly it thanks! It is now working if I convert all the numbers that are operating with the arc4random to UInt32 — Nearpoint, Jun 15 '14 at 07:20
Better yet, follow the duplicate link and use arc4random_uniform. It'll avid the problem *and* give you better results. — David Berry, Jun 15 '14 at 07:44
Ah got it working, I think the example has an extra ) in there but it works great thanks — Nearpoint, Jun 15 '14 at 08:15

score 0 · Accepted Answer · edited May 23 '17 at 10:26

0

because arc4random return UInt32 it will overflow when trying to convert it to an Int

I used this instead

var x = UInt32(arc4random()) % UInt32(self.gameView.bounds.size.width) / UInt32(DROP_SIZE.width)
frame.origin.x = CGFloat(x) * CGFloat(DROP_SIZE.width)

This issue is similar: Crash when casting the result of arc4random() to Int

edited May 23 '17 at 10:26

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answered Jun 30 '14 at 19:06

Nearpoint

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iOS Swift Type Conversation Math causing app to Crash

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