def sub_sample(pointcloud, n=100):
    """ Function to randomly subsample pointcloud to n
        number of points. If n is larger than pointcloud
        None is returned.

        args:
            pointcloud : pandas dataframe
            n          : int number of point to return
        returns:
            pandas dataframe
    """
    n_greater = False
    if pointcloud.values.shape[0] > n:
        pointcloud = pointcloud.sample(n=n)
    elif pointcloud.values.shape[0] < n:
        n_greater = True
    if n_greater == True:
        pointcloud = None
    return pointcloud
