def twoballs_filename(data_dir, num_examples, num_feat, num_noise_feat, frac_flip):
    """Generate filename to save data and permutations"""
    data_filename = data_dir + '/twoballs_n=%d_%d:%d_rcn=%1.1f.csv'\
        % (num_examples, num_feat, num_noise_feat, frac_flip)
    perm_filename = data_dir + '/twoballs_n=%d_%d:%d_rcn=%1.1f_perm.txt'\
        % (num_examples, num_feat, num_noise_feat, frac_flip)
    return (data_filename, perm_filename)