def set_group(self, group):
        """Set group size of DMatrix (used for ranking).

        Parameters
        ----------
        group : array like
            Group size of each group
        """
        _check_call(_LIB.XGDMatrixSetGroup(self.handle,
                                           c_array(ctypes.c_uint, group),
                                           c_bst_ulong(len(group))))