def _word_to_score(self, ids, scores):
        """Return a map from each word to its score.

        :param (np.array) ids: a vector of word ids
        :param (np.array) scores: a vector of scores

        :return (dict[unicode, float]): a map from each word (unicode) to its score (float)
        """
        # should be 1-D vectors
        assert len(ids.shape) == 1
        assert ids.shape == scores.shape

        w2s = {}
        for i in range(len(ids)):
            w2s[self.vocab.index2word(ids[i])] = scores[i]
        return w2s