a3-4.swift

func tableView(_ tableView: UITableView, willSelectRowAt indexPath: IndexPath) -> IndexPath? {
    let photo = self.photos[indexPath.row]
    if photo.for_sale { // If item is for sale 
        self.selectedIndexPath = indexPath
        return indexPath
    }else { // If item is not for sale 
        let alert = UIAlertController(title: "Not for sale", message: "This item is not for sale", preferredStyle: .alert)
        alert.addAction( UIAlertAction(title: "Ok", style: .cancel, handler: nil))
        self.present(alert, animated: true, completion: nil)
        return nil
    }
}