code: public function fn(array &$arr_x)
      {
          $arr = array();
  
          foreach ($arr_x as $x) {
              $item = new \stdClass();
              $item->x = $x;
              $item->fx = $this->f($x);
              $arr[] = $item;
          }
  
          return $arr;
      }
docs: Do same things like NormalDistribution::f() but for several values at once.
  
  @see NormalDistribution::precision()
  @param  array $arr_x Several x values
  @access public
  @return array Each items is small object having x and fx attributes.