def combine_names(names, ids=None):
    """Combine the selected names into one new name.

    Parameters
    ----------
    names : list of str
        String names
    ids : numpy.ndarray, optional
        Selected index

    Returns
    -------
    str
    """
    if ids is None:
        return '+'.join(sorted(names))
    else:
        selected = sorted([names[i] for i in ids])
        return '+'.join(selected)