Thanks for your replies, finally i find if i set tableView.tableHeaderView, tableView.estimatedSectionHeaderHeight tableView.delegate = self and tableView.dataSource = self before setting tableView.frame, it will call numberOfRowsInSection first and get the Index out of range error. Then i place tableView.delegate = self and tableView.dataSource = self below the tableView.frame, it calls numberOfSections first and do not call numberOfRowsInSection. Commenting one of the tableView.tableHeaderView and tableView.estimatedSectionHeaderHeight also work well. this is the complete code. import UIKit class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {       let dataSource = [[String]]()       let tableView = UITableView(frame: .zero, style: .plain)   override func viewDidLoad() {     super.viewDidLoad()           let bounds = UIScreen.main.bounds           tableView.tableHeaderView = UIView() //    tableView.tableFooterView = UIView()     tableView.estimatedSectionHeaderHeight = 0 //    tableView.estimatedSectionFooterHeight = 0           // before frame     tableView.delegate = self     tableView.dataSource = self           tableView.frame = CGRect(x: 0, y: 0, width: bounds.width, height: bounds.height)           // after frame //      tableView.delegate = self //      tableView.dataSource = self           tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")     view.addSubview(tableView)   }       func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {     return dataSource[section].count   }       func numberOfSections(in tableView: UITableView) -> Int {     return dataSource.count   }       func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {     let cell = tableView.dequeueReusableCell(withIdentifier: "Cell")!     return cell   } }

i just define the dataSource in the Class