I would like to control an array in sequence for back and next buttons, right now it selects the items randomly "Int(arc4random_uniform)" how can I change this? (I have not added the else/if functions yet)

SwiftBackNext

import UIKit

class ViewController: UIViewController {

    
    @IBOutlet weak var slide: UIImageView!
    
    @IBOutlet weak var playNextButton: UIButton!
    
    var slideArray:[String] = ["slide1", "slide2", "slide3", "slide4"]
    
    
    
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    
    @IBAction func nextButton(sender: UIButton) {
        
    
        let slide = Int(arc4random_uniform(4))
        
        let slideString:String = self.slideArray [slide]
        
        self.slide.image = UIImage(named: slideString)
        
    }


}