def get_G(self, k, add_noise=True):
        """ get G matrix from angles. """
        G = np.ones((self.N - 1, self.N - 1))
        if (add_noise):
            noise = pi * 0.1 * np.random.rand(
                (self.N - 1) * (self.N - 1)).reshape((self.N - 1, self.N - 1))
        other_indices = np.delete(range(self.N), k)
        for idx, i in enumerate(other_indices):
            for jdx, j in enumerate(other_indices):
                if (add_noise and
                        i != j):  # do not add noise on diagonal elements.
                    thetak_ij = self.get_inner_angle(k,
                                                     (i, j)) + noise[idx, jdx]
                else:
                    thetak_ij = self.get_inner_angle(k, (i, j))
                G[idx, jdx] = cos(thetak_ij)
                G[jdx, idx] = cos(thetak_ij)
        return G
get G matrix from angles.