How to position a Sprite randomly on the X axis in Swift 2

Question

I would like for enemies to spawn on the X axis, so far I got

enemy?.position = CGPoint(x: CGFloat(arc4random_uniform(UInt32(self.frame.width))), y: self.frame.maxY)

This works, howether I would like for them to spawn within the screen and not get cut out when spawning on x = 0 or the maximum x

I have also tried x: 100 + CGFloat(arc4random_uniform(Uint32(self.frame.width - 100)))

This give me an error, If the width of my screen is lets say 1000, how do I make a random number between 100 and 900, while also automatically finding the width of the screen.

I am also using SpriteKit

Ron Myschuk · Answer 1 · 2016-06-23T21:50:46.817

3

This example is assuming that your scene anchor point is 0,0

let buffer: UInt32 = 100
let randomX = randomizer(min: buffer, max: UInt32(self.size.width) - buffer)
enemy?.position = CGPoint(x: CGFloat(randomX), y: self.frame.maxY)

func randomizer(min min: UInt32, max: UInt32) -> CGFloat {

   assert(min < max)
   return CGFloat(arc4random_uniform(max - min)  + min)
}

edited Jun 23 '16 at 21:50

answered Jun 23 '16 at 21:42

Ron Myschuk

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I tried both print(self.size.width) and print(self.frame.width) when the game first runs they both print a value of 1 after that its always 1024, Im guessing that arc4random_uniform crashes at start because the initial value is a a negative if i subtract the buffer. Im running the function from viewDidLoad – Jackster Jun 23 '16 at 23:09
The scene won't have the information yet at the point. Try running the function in "override func didMoveToView(view: SKView) { }" instead – Ron Myschuk Jun 23 '16 at 23:17
When i run print(self.size.width) in didMoveToView, It first prints the value of 1 and once more for 1024. Which function will have the information ready in SpriteKit? – Jackster Jun 23 '16 at 23:43
somethings not right. It shouldn't print twice, do you still have the print statement in the viewDidLoad fund as well? – Ron Myschuk Jun 23 '16 at 23:50
I only have the print statement once, but I also have a title scene and I'm transitioning onto the GameScene – Jackster Jun 24 '16 at 01:05
On a blank project with one scene it only print 1024 right away, I guess its got something to do with the transition? – Jackster Jun 24 '16 at 01:12

score 0 · Answer 2 · edited May 23 '17 at 12:22

0

With the math below you can get a random number between 100 and the frames width minus 100:

var x = 0
var y = 0
var screenScale: CGFloat = UIScreen.mainScreen().scale()

let lower : UInt32 = 100 * Int(screenScale)
let upper : UInt32 = UInt32(self.frame.width - (100 * Int(screenScale)))

x = arc4random_uniform(upper - lower) + lower
y = arc4random_uniform(upper - lower) + lower

enemy?.position = CGPoint(x: x, y: y)

This should get your sprite to appear between 900 and 100.

Thanks to Brayen Chen for this answer.

edited May 23 '17 at 12:22

Community

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answered Jun 23 '16 at 21:55

Caleb Kleveter

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not a good approach, your code could be forever in an infinite loop if by chance every random number is < 100 or greater than 900 – Knight0fDragon Jun 24 '16 at 13:23
only issue is you are randomizing y based on screen width – Knight0fDragon Jun 24 '16 at 17:38
1) The question does not ask for y. 2) If you are not working in a 1:1 aspect ratio box, your sprite will go off screen – Knight0fDragon Jun 24 '16 at 17:41

score 0 · Answer 3 · answered Jun 29 '16 at 09:12

func randomRangeNumber(range: Range<Int> = 1...6) -> Int {
    let min = range.startIndex
    let max = range.endIndex
    return Int(arc4random_uniform(UInt32(max - min))) + min
}

let max = Int(self.frame.width - 100)
enemy?.position = CGPoint(x: CGFloat(randomRangeNumber(100...max)), y:self.frame.maxY)

How to position a Sprite randomly on the X axis in Swift 2

3 Answers3