数据为titanic数据    为什么转化不了float型呢
http://biostat.mc.vanderbilt.edu/wiki/pub/Main/DataSets/titanic.txt
ValueError                                Traceback (most recent call last)<ipython-input-67-bf634c37a5a0> in <module>()      1 mean_age = np.mean(titanic_X[ages != 'NA',----> 2 1].astype(np.float))ValueError: could not convert string to float: