data export in excel using CI

I am trying to export data in excel sheet. i have successfully exported the data but problem is that my data comes in form of array from database and i dont want all of the array to be displayed i want only selective data to be sent to excel sheet help please.

{       
  //load our new PHPExcel library
  $this->load->library('Excel');
  //activate worksheet number 1
  $this->excel->setActiveSheetIndex(0);
  //name the worksheet
  $this->excel->getActiveSheet()->setTitle('Attendance Report');

  $this->excel->getActiveSheet()->getColumnDimension('A')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('B')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('C')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('D')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('E')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('F')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('G')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('H')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('I')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('J')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('K')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('L')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('M')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('N')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('O')->setAutoSize(true);
  $this->excel->getActiveSheet()->getColumnDimension('P')->setAutoSize(true);


  $this->excel->getActiveSheet()->getStyle("A1:P1")->applyFromArray(array("font" => array("bold" => true)));

  $this->excel->setActiveSheetIndex(0)->setCellValue('A1', 'Date (Day)');
  $this->excel->setActiveSheetIndex(0)->setCellValue('B1', 'Worked Hour');
  $this->excel->setActiveSheetIndex(0)->setCellValue('C1', 'In Time');
  $this->excel->setActiveSheetIndex(0)->setCellValue('D1', 'In Remarks');
  $this->excel->setActiveSheetIndex(0)->setCellValue('E1', 'Out Date');
  $this->excel->setActiveSheetIndex(0)->setCellValue('F1', 'Out Time');
  $this->excel->setActiveSheetIndex(0)->setCellValue('G1', 'Remarks');
  $this->excel->setActiveSheetIndex(0)->setCellValue('H1', 'Difference');
  $this->excel->setActiveSheetIndex(0)->setCellValue('I1', 'In Time');
  $this->excel->setActiveSheetIndex(0)->setCellValue('J1', 'In Remarks');
  $this->excel->setActiveSheetIndex(0)->setCellValue('K1', 'Out Date');
  $this->excel->setActiveSheetIndex(0)->setCellValue('L1', 'Out Time');
  $this->excel->setActiveSheetIndex(0)->setCellValue('M1', 'Remarks');
  $this->excel->setActiveSheetIndex(0)->setCellValue('N1', 'Difference');
  $this->excel->setActiveSheetIndex(0)->setCellValue('O1', 'Total OT');
  $this->excel->setActiveSheetIndex(0)->setCellValue('P1', 'Remark');

  $data=$this->mainModel->exportAttendanceReport();

  // get all users in array formate
  $this->excel->getActiveSheet()->fromArray($data, null, 'A2');

      $filename='Attendance Report.xls'; //save our workbook as this file name

      header('Content-Type: application/vnd.ms-excel'); //mime type

      header('Content-Disposition: attachment;filename="'.$filename.'"'); //tell browser what's the file name

      header('Cache-Control: max-age=0'); //no cache

      //save it to Excel5 format (excel 2003 .XLS file), change this to 'Excel2007' (and adjust the filename extension, also the header mime type)
      //if you want to save it as .XLSX Excel 2007 format

      $objWriter = PHPExcel_IOFactory::createWriter($this->excel, 'Excel5');

      //force user to download the Excel file without writing it to server's HD
    $objWriter->save('php://output');
}

here is what i have done upto now.

Try Following code. I am done some changes in your code ,
// Pass name of your tables field name in $row[''] variable

{       
      //load our new PHPExcel library
      $this->load->library('Excel');
      //activate worksheet number 1
      $this->excel->setActiveSheetIndex(0);
      //name the worksheet
      $this->excel->getActiveSheet()->setTitle('Attendance Report');

      $this->excel->getActiveSheet()->getColumnDimension('A')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('B')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('C')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('D')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('E')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('F')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('G')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('H')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('I')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('J')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('K')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('L')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('M')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('N')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('O')->setAutoSize(true);
      $this->excel->getActiveSheet()->getColumnDimension('P')->setAutoSize(true);


      $this->excel->getActiveSheet()->getStyle("A1:P1")->applyFromArray(array("font" => array("bold" => true)));

      $this->excel->setActiveSheetIndex(0)->setCellValue('A1', 'Date (Day)');
      $this->excel->setActiveSheetIndex(0)->setCellValue('B1', 'Worked Hour');
      $this->excel->setActiveSheetIndex(0)->setCellValue('C1', 'In Time');
      $this->excel->setActiveSheetIndex(0)->setCellValue('D1', 'In Remarks');
      $this->excel->setActiveSheetIndex(0)->setCellValue('E1', 'Out Date');
      $this->excel->setActiveSheetIndex(0)->setCellValue('F1', 'Out Time');
      $this->excel->setActiveSheetIndex(0)->setCellValue('G1', 'Remarks');
      $this->excel->setActiveSheetIndex(0)->setCellValue('H1', 'Difference');
      $this->excel->setActiveSheetIndex(0)->setCellValue('I1', 'In Time');
      $this->excel->setActiveSheetIndex(0)->setCellValue('J1', 'In Remarks');
      $this->excel->setActiveSheetIndex(0)->setCellValue('K1', 'Out Date');
      $this->excel->setActiveSheetIndex(0)->setCellValue('L1', 'Out Time');
      $this->excel->setActiveSheetIndex(0)->setCellValue('M1', 'Remarks');
      $this->excel->setActiveSheetIndex(0)->setCellValue('N1', 'Difference');
      $this->excel->setActiveSheetIndex(0)->setCellValue('O1', 'Total OT');
      $this->excel->setActiveSheetIndex(0)->setCellValue('P1', 'Remark');

      $data=$this->mainModel->exportAttendanceReport();

      // get all users in array formate

      $n = 2;
      if(!empty($data))
        {
            foreach ($data as $row)
            { 
               // Pass name of your tables field name in following $row[''] variable 

              $this->excel->getActiveSheet()->setCellValue('A'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('B'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('C'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('D'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('E'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('F'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('G'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('H'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('I'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('J'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('K'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('L'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('M'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('N'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('O'.$n, $row['field_name']);
              $this->excel->getActiveSheet()->setCellValue('P'.$n, $row['field_name']);
              $n++;
            }
          }


          $filename='Attendance Report.xls'; //save our workbook as this file name

          header('Content-Type: application/vnd.ms-excel'); //mime type

          header('Content-Disposition: attachment;filename="'.$filename.'"'); //tell browser what's the file name

          header('Cache-Control: max-age=0'); //no cache

          //save it to Excel5 format (excel 2003 .XLS file), change this to 'Excel2007' (and adjust the filename extension, also the header mime type)
          //if you want to save it as .XLSX Excel 2007 format

          $objWriter = PHPExcel_IOFactory::createWriter($this->excel, 'Excel5');

          //force user to download the Excel file without writing it to server's HD
          $objWriter->save('php://output');
    }