def index_url(year):
    """ Generate url of the index file for future downloading.
    year - > the year to download
    Returns:
        url link of the index file
    """
    quarters = ['QTR1', 'QTR2', 'QTR3', 'QTR4']
    return [f'https://www.sec.gov/Archives/edgar/full-index/{year}/{q}/master.idx' for q in quarters]