work4

import numpy as np
x = np.array([[1,2,3],[4,5,6],[1,2,3]])
x.flatten()

array([1, 2, 3, 4, 5, 6, 1, 2, 3])

import numpy as np
x = np.array([[1,2,3],[4,5,6],[1,2,3]])
x.ravel()

array([1, 2, 3, 4, 5, 6, 1, 2, 3])

x = np.array([[1,2,3],[2,3,4]])
print(np.prod(x))

print(np.prod(x,axis=1))

np.prod(x,axis=0)

144
[ 6 24]





array([ 2,  6, 12])

import numpy as np
x = np.array([[1,2,3],[-3,2,4],[5,-2,9]])
print(x)


y1 = np.maximum(0,x) # 把小于0的元素置0，比改变x的值
print(y1)


y2 = np.minimum(0,x) # 把大于0的元素置0，不改变x的值
print(y2)

x1 = x.copy()
print(x1)

x1[x1 < 0] = 0 # 把小于0的元素置0，改变x1的值
print(x1)


x2 = x.copy()
x2[x2 > 0] = 0 # 把大于0的元素置0，改变x2的值
print(x2)

[[ 1  2  3]
 [-3  2  4]
 [ 5 -2  9]]
[[1 2 3]
 [0 2 4]
 [5 0 9]]
[[ 0  0  0]
 [-3  0  0]
 [ 0 -2  0]]
[[ 1  2  3]
 [-3  2  4]
 [ 5 -2  9]]
[[1 2 3]
 [0 2 4]
 [5 0 9]]
[[ 0  0  0]
 [-3  0  0]
 [ 0 -2  0]]

import numpy as np
x = np.array([[1,2,3],[-3,2,4],[5,-2,9]])
x


x1 = x.copy() # copy()，开辟新地址
x1[x1 > 0] = 0
x1


x # x不变


x2 = x # 直接等于，未开辟新地址，x2与x相关联
x2


x2[x2>0] = 0
x2


x # x也改变


x = np.array([[1,2,3],[-3,2,4],[5,-2,9]])
x3 = x[2] # 取x的第3行
x3


x3[2] = 100 # 将x3第3个元素置100
x # x中对应的元素置也被置成100了

array([[  1,   2,   3],
       [ -3,   2,   4],
       [  5,  -2, 100]])

import numpy as np
x = np.array([[1,2,3],[4,5,6]])
np.zeros_like(x) # 生成一个和x大小相同的全零矩阵

array([[0, 0, 0],
       [0, 0, 0]])