def reindex(actual, base=1):
        """
        Re-index a numeric list with missing values removed (assuming both are unique and sorted ascendingly):
        Ex:
                input: original:[1,2,3,4,5], actual:[2,4];
                return: {2:1,4:2}
        """
        reindex = {v:(i+base) for i,v in enumerate(actual)}
        return reindex