def get_drop_indices(num_chunks, keep_indices):
    """obtain the drop_indices given the keep_indices.

    Args:
        num_chunks (int): The total number of chunks
        keep_indices (List): The keep indices.

    Returns: List. The dropped indices (frozen chunks).

    """
    drop_indices = []
    for i in range(num_chunks):
        if i not in keep_indices:
            drop_indices.append(i)
    return drop_indices