grasp/catch/cluth at straws
depend on sth．that is useless or unable to help in a time of trouble or danger；try sth．with little hope of succeeding捞救命稻草（将要溺死之人连一根草也要去抓）
To depend on your memory without studying for a test is to grasp at straws．应付考试只凭记忆而不学习那是枉费心机。