def _sort_features(self, vocabulary):
        """Sort features by name

        Returns a reordered matrix and modifies the vocabulary in place
        """
        sorted_features = sorted(six.iteritems(vocabulary))
        map_index = np.empty(len(sorted_features), dtype=np.int32)
        for new_val, (term, old_val) in enumerate(sorted_features):
            map_index[new_val] = old_val
            vocabulary[term] = new_val

        return map_index