def get_ranking(self, alt):
        """
        Description:
            Returns the ranking of a given alternative in the
            computed aggregate ranking.  An error is thrown if
            the alternative does not exist.  The ranking is the
            index in the aggregate ranking, which is 0-indexed.
        Parameters:
            alt: the key that represents an alternative
        """
        if self.alts_to_ranks is None:
            raise ValueError("Aggregate ranking must be created first")
        try:
            rank = self.alts_to_ranks[alt]
            return rank
        except KeyError:
            raise KeyError("No alternative \"{}\" found in ".format(str(alt)) +
                           "the aggregate ranking")