How can I create coordinates that almost border my GameScene?

I am trying to generate circles that nearly border the edge of the screen. I tried creating my own coordinate generator, my issue is that the circles that I am randomly generating are only appearing at the top and bottom of the screen. Here is a screenshot of what it is doing: http://ift.tt/1FB77au

I have no idea why this is happening because when I print the x and y coordinate of the circle's position, it says that both points are less than the frame's width and height. In my GameScene.swift I call this function.

    private func generateRandomCoorindates() -> CGPoint {

    let randomNumber = arc4random_uniform(2)
    var xCoordinate: Double
    var yCoordinate: Double
    if randomNumber == 0 {
        var _xCoordinate: Double {
           let _randomNumber = arc4random_uniform(2)
            //x-corrdinate either 50 or width-50
            if _randomNumber == 0 {
                return 50
            } else {
                return Double(self.frame.width - 50)
            }
        }
        xCoordinate = _xCoordinate
        //random y-coordinate from 50 to height-50
        yCoordinate = Double.random(lower: 50, upper: Double(self.frame.height) - 50)
    }
    else {
        //random x-coordinate from 50 to width-50
        xCoordinate = Double.random(lower: 50, upper: Double(self.frame.width) - 50)
        var _yCoordinate: Double {
            //y-coordinate either 50 or height - 50
            let _randomNumber = arc4random_uniform(2)
            if _randomNumber == 0 {
                return 50
            } else {
                return Double(self.frame.height - 50)
            }
        }
        yCoordinate = _yCoordinate
        }
    return CGPoint(x: CGFloat(xCoordinate), y: CGFloat(yCoordinate))
    }

My extensions are:

public func arc4random <T: IntegerLiteralConvertible> (type: T.Type) -> T {
var r: T = 0
arc4random_buf(&r, Int(sizeof(T)))
return r
}

public extension Double {
    public static func random(lower lower: Double, upper: Double) -> Double {
        let r = Double(arc4random(UInt64)) / Double(UInt64.max)
        return (r * (upper - lower)) + lower
    }
 }