def pack(self, topic_dims, t):
        """ Packs selected logs into a numpy array
            :param list topic_dims: list of (topic, dims) tuples, where topic is a string and dims a list dimensions to be packed for that topic
            :param int t: time indexes to be packed
        """

        data = []
        for topic, dims in topic_dims:
            for d in dims:
                data.append(self.logs[topic][t, d])
        return array(data).T