def _agg_avg(iur, item, sims, use):
    """
    Average aggregate.

    Args:
        iur(matrix._CSR): the item-user ratings matrix
        item(int): the item index in ``iur``
        use(numpy.ndarray): positions in sims and the rating row to actually use
    """

    rates = iur.row_vs(item)
    num = 0.0
    den = len(use) 
    for j in use:
        num += rates[j]
    return num /den