def relate_obs_ids_to_chosen_alts(obs_id_array,
                                  alt_id_array,
                                  choice_array):
    chosen_alts_to_obs_ids = {}
    for alt_id in np.sort(np.unique(alt_id_array)):
        selection_condition =\
            np.where((alt_id_array == alt_id) & (choice_array == 1))
        chosen_alts_to_obs_ids[alt_id] =\
            np.sort(np.unique(obs_id_array[selection_condition]))
    return chosen_alts_to_obs_ids