Allow CollectionView cells to have different identifiers each time

import UIKit

classViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate{
    // An identifier of a cell is made constant by an array.
    let identifiers: [String] = ["CellA", "CellB", "CellC", "CellD", "CellE", "CellF", "CellG" ]
    // An integer variable that indicates the number of the array identifiers.
    var num —Int=0

    override func viewDidLoad(){
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from anib.
    }

    override funcdidReceiveMemoryWarning(){
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    // Collection View Datasource and Delete
    func numberOfSectionsInCollectionView (collectionView:UICollectionView) - > Int {
        return1
    }

    functioncollectionView (collectionView:UICollectionView, numberOfItemsInSection section:Int) - > Int {
        return 30 // The number of cells is fixed at 30
    }

    functioncollectionView (collectionView:UICollectionView, cellForItemAtIndexPathindexPath:NSIndexPath) - >UICollectionViewCell{
        // Place the numth Identifier cell.
        letcell=collectionView.dueReusableCellWithReuseIdentifier (identifiers [num], forIndexPath:indexPath) as!UICollectionViewCell
        return cell
    }

    functioncollectionView(collectionView:UICollectionView, didSelectItemAtIndexPathindexPath:NSIndexPath){
        // Each time a cell is tapped, it switches to CellA→CellB→ through→CellF→CellG→CellA→CellB→ through.
        num=(num+1)%7
        collectionView.reloadData()
    }

}