https://blog.csdn.net/lanchunhui/article/details/49725065
>>> import numpy as np
>>> b = np.array(a)
>>> print(b)
[[1 2 3 4]
[1 2 3 4]
[1 2 3 4]]
>>> c=b[:,0]
>>> print(c)
[1 1 1]
>>> np.maximum(2,c)
array([2, 2, 2])
>>> np.maximum(2,c)[:,np.newaxis]
array([[2],
[2],
[2]])
>>> d=np.maximum(2,c)[:,np.newaxis]
>>> print(d)
[[2]
[2]
[2]]
>>> d[2]
array([2])
>>> e=np.maximum(2,c)
>>> print(e)
[2 2 2]
>>> e[2]
2