numpy：矩阵的元素选取

>>> a = np.random.random((3,4))
>>> a
array([[0.890582  , 0.43671675, 0.19531267, 0.13555252],
       [0.17254557, 0.3088476 , 0.36916878, 0.81551741],
       [0.31869139, 0.16168486, 0.58004271, 0.4279148 ]])
>>> a.nonzero()
(array([0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2]), array([0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]))
>>> a[a.nonzero()]   ##方法一，使用矩阵去选择
array([0.890582  , 0.43671675, 0.19531267, 0.13555252, 0.17254557,
       0.3088476 , 0.36916878, 0.81551741, 0.31869139, 0.16168486,
       0.58004271, 0.4279148 ])
>>> a
array([[0.890582  , 0.43671675, 0.19531267, 0.13555252],
       [0.17254557, 0.3088476 , 0.36916878, 0.81551741],
       [0.31869139, 0.16168486, 0.58004271, 0.4279148 ]])
>>> a[1,1]         ##方法二
0.3088475978138986
>>> a[1][1]         ##方法三
0.3088475978138986
>>> a[np.array([[False,False,False,False],[True,True,True,True,],
    [False,False,False,False]])]
array([0.72159605, 0.90488492, 0.60745105, 0.20538722])  ##方法四