AdTableViewController.swift

//
//  ViewController.swift
//  AdTableTest
//
//  Created by Paul Wilkinson on 15/06/2016.
//  Copyright © 2016 Paul Wilkinson. All rights reserved.
//

import UIKit

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
    
    enum CellType: String {
        case contentCell
        case advertisingCell
    }
    
    @IBOutlet var tableView: UITableView!
    
    let kAdFrequency = 3
    let kNumberRows = 25
    
    var tableContent = [String]()
    

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        for i in 1...kNumberRows {
            self.tableContent.append("Row \(i)")
        }
        
        
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    func numberOfSectionsInTableView(tableView: UITableView) -> Int {
        return 1
    }
    
    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        
        let adCount = self.adCountForRowCount(self.tableContent.count, frequency: kAdFrequency)
       
        return self.tableContent.count + adCount
    }

    
    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        
        let cellType = self.cellTypeForRow(indexPath.row, baseRowCount: self.tableContent.count, frequency: self.kAdFrequency)
        
        let cell = tableView.dequeueReusableCellWithIdentifier(cellType.rawValue)
        
        switch cellType {
        case .contentCell:
            
            let contentRow = contentRowsWithAds(indexPath.row, frequency: kAdFrequency)
            
            cell!.textLabel!.text = self.tableContent[contentRow]
            
            
        case .advertisingCell:
            cell!.textLabel!.text = "Advertisement"
        }

        
        return cell!
    }
    
    func cellTypeForRow(row: Int, baseRowCount: Int, frequency: Int) -> CellType {
        
        var cellType = CellType.contentCell
        
        let computeRow = row+1
        
        let contentRows = contentRowsWithAds(row, frequency: frequency)
        
        if frequency > baseRowCount && row >= baseRowCount {
            cellType = .advertisingCell
        } else if row != 0 && computeRow % (frequency+1) == 0 {
            cellType = .advertisingCell
        } else if contentRows >= baseRowCount {
            cellType = .advertisingCell
        }
        
        return cellType
        
    }
    
    func adCountForRowCount(rowCount: Int, frequency: Int) -> Int {
        var adCount = rowCount / (frequency)
        
        if adCount == 0 {
            adCount = 1
        }
        
        return adCount
    }
    
    func contentRowsWithAds(row: Int, frequency: Int) -> Int {
        let numberOfAds = row / (frequency+1)
        return row - numberOfAds
    }

}

Obj-c

 -(int) calculatedIndexForRow:(int)row adFrequency:(int)frequency  {
        int numberOfAds = row / (frequency+1)
        return row - numberOfAds
  }
  
  int calculatedIndex = [self calculatedIndexForRow:indexPath.row adFrequency:kAdCellFrequency];