how to find number of rows in codeigniter code example

Example 1: codeigniter count rows

$this->db
  ->where(['field'=>'foo'])
  ->from("tablename")
  ->count_all_results();

//OR 
$this->db
  ->where(['field'=>'foo'])
  ->count_all_results("tablename");

Example 2: codeigniter get num_rows

// num rows example
$this->db->select('*');
$this->db->where('whatever');
$query = $this->db->get('table');
$num = $query->num_rows();