https://www.zhihu.com/question/46988087
>>> import numpy as np
>>> a = np.random.randint(-5, 5, (5, 5))
>>> a
array([[-4, -4, -5, 2, 1],
[-1, -2, -1, 3, 3],
[-1, -2, 3, -5, 3],
[ 0, -3, -5, 1, -4],
[ 0, 3, 1, 3, -4]])
# 方式一
>>> np.maximum(a, 0)
array([[0, 0, 0, 2, 1],
[0, 0, 0, 3, 3],
[0, 0, 3, 0, 3],
[0, 0, 0, 1, 0],
[0, 3, 1, 3, 0]])
# 方式二
>>> (a + abs(a)) / 2
array([[0, 0, 0, 2, 1],
[0, 0, 0, 3, 3],
[0, 0, 3, 0, 3],
[0, 0, 0, 1, 0],
[0, 3, 1, 3, 0]])
# 方式三
>>> b = a.copy()
>>> b[b < 0] = 0
>>> b
array([[0, 0, 0, 2, 1],
[0, 0, 0, 3, 3],
[0, 0, 3, 0, 3],
[0, 0, 0, 1, 0],
[0, 3, 1, 3, 0]])
# 方式四
>>> np.where(a > 0, a, 0)
array([[0, 0, 0, 2, 1],
[0, 0, 0, 3, 3],
[0, 0, 3, 0, 3],
[0, 0, 0, 1, 0],
[0, 3, 1, 3, 0]])
