def concat_t_vars(self):
        """
        Concatenate ``self.t`` with ``self.vars`` and output a single matrix
        for data dump

        :return matrix: concatenated matrix with ``self.t`` as the 0-th column
        """
        logger.warning('This function is deprecated and replaced by `concat_t_vars_np`.')
        out = np.array([])

        if len(self.t) == 0:
            return out

        out = np.ndarray(shape=(0, self.vars[0].size[0] + 1))

        for t, var in zip(self.t, self.vars):
            line = [[t]]
            line[0].extend(list(var))
            out = np.append(out, line, axis=0)

        return out