def ids_to_str(ids):
    """Convert a list of article IDs to a comma separated string of IDs.

    Parameters
    ----------
    ids : bs4.element.ResultSet
        List of article IDs.

    Returns
    -------
    ids_str : str
        A string of all concatenated IDs.
    """
    n_ids = len(ids)
    ids_str = str(ids[0].text)
    for ind in range(1, n_ids):
        ids_str = ids_str + ',' + str(ids[ind].text)
    return ids_str
