Return vectors uniformly picked on the unit disk.
    The unit disk is the space enclosed by the unit circle.
    Vectors are in a polar representation.

    Parameters
    ----------
    n: integer
        Number of points to return.

    Returns
    -------
    r: array, shape (n, 2)
        Sample vectors.

def disk_pick_polar(n=1, rng=None):
    """Return vectors uniformly picked on the unit disk.
    The unit disk is the space enclosed by the unit circle.
    Vectors are in a polar representation.

    Parameters
    ----------
    n: integer
        Number of points to return.

    Returns
    -------
    r: array, shape (n, 2)
        Sample vectors.
    """
    if rng is None:
        rng = np.random
    a = np.zeros([n, 2], dtype=np.float)
    a[:, 0] = np.sqrt(rng.uniform(size=n))
    a[:, 1] = rng.uniform(0.0, 2.0 * np.pi, size=n)
    return a