如下所示：
>>> 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],
