Use random number (arc4random_uniform) to display random shapes - swift

Question

The purpose for this code is to do random number to display random shapes but the simulator displays them in the same spot which is the bottom left corner of the screen, for some reason the random for the position which is a is not working.
Code:
import UIKit
import SpriteKit
import Darwin
class StartGame: SKScene {
var scoreLabel = SKLabelNode(fontNamed: "cholkDuster")
var square = SKSpriteNode(imageNamed: "square")
var circle = SKSpriteNode(imageNamed: "circle")
var rectangle = SKSpriteNode(imageNamed: "rectangle")
var triangle = SKSpriteNode(imageNamed: "triangle")
let bg = SKSpriteNode(imageNamed: "background.png")
var score = 0
override func didMoveToView(view: SKView) {
//random number for the shapes
var timecreatShapes = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("creatShapes"), userInfo: nil, repeats: true)
//background image
bg.position = CGPoint(x: CGRectGetMidX(self.frame), y:CGRectGetMidY(self.frame))
bg.size.width = self.frame.size.width
bg.size.height = self.frame.size.height
self.addChild(bg)
self.scoreLabel.text = "0"
self.scoreLabel.fontSize = 42
self.scoreLabel.position = CGPoint(x: CGRectGetMidX(self.frame) , y: CGRectGetMidY(self.frame))
self.addChild(scoreLabel)
scoreLabel.zPosition = 2
self.physicsWorld.gravity = CGVectorMake(0, -0.5)
//declaring a square image
square.size = CGSize(width: 150, height: 100)
square.color = SKColor.redColor()
square.physicsBody = SKPhysicsBody(circleOfRadius: square.size.height / 2)
square.physicsBody?.dynamic = true
square.physicsBody?.allowsRotation = false
//declaring a circle image
circle.size = CGSize(width: 150, height: 100)
circle.physicsBody = SKPhysicsBody(circleOfRadius: square.size.height / 2)
circle.physicsBody?.dynamic = true
circle.physicsBody?.allowsRotation = false
//declaring a triangle
triangle.size = CGSize(width: 150, height: 100)
triangle.physicsBody = SKPhysicsBody(circleOfRadius: square.size.height / 2)
triangle.physicsBody?.dynamic = true
triangle.physicsBody?.allowsRotation = false
//declaring rectangle
rectangle.size = CGSize(width: 150, height: 100)
rectangle.physicsBody = SKPhysicsBody(circleOfRadius: square.size.height / 2)
rectangle.physicsBody?.dynamic = true
rectangle.physicsBody?.allowsRotation = false
}
func creatShapes () {
var x = Int ( arc4random_uniform(4) + 1)
var a = CGFloat ( arc4random_uniform(1000) + 1)
switch(x){
case 1:
circle.position = CGPoint (x: a , y: self.frame.size.height)
self.addChild(SKSpriteNode(imageNamed:"circle"))
case 2:
square.position = CGPoint(x: a , y: self.frame.size.height)
self.addChild(SKSpriteNode(imageNamed:"square"))
case 3:
rectangle.position = CGPoint(x: a , y: self.frame.size.height)
self.addChild(SKSpriteNode(imageNamed:"rectangle"))
case 4:
triangle.position = CGPoint(x: a , y: self.frame.size.height)
self.addChild(SKSpriteNode(imageNamed:"triangle"))
default:
break
}
println(x)
println(a)
}
override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
/* Called when a touch begins */
for touch: AnyObject in touches {
let location = touch.locationInNode(self)
if (self.nodeAtPoint(location) == self.square || self.nodeAtPoint(location) == self.triangle || self.nodeAtPoint(location) == self.circle || self.nodeAtPoint(location) == self.rectangle){
self.score++
}
self.scoreLabel.text = String(self.score)
}
}