numpy知識點補充

import numpy as np

a = np.zeros((2,2)) #建立2x2的全零矩陣
print(a)
b = np.ones((1,2)) #建立1x2的全一矩陣
print(b) 
c = np.full((2,2), 7) #建立2x2的全為7的矩陣
print(c) 
d = np.eye(2) #建立全1的2x2對角矩陣
print(d) 
e = np.random.random((2,2)) #建立2x2的隨機數矩陣
print(e)
[[0. 0.]
[0. 0.]]
[[1. 1.]]
[[7 7]
[7 7]]
[[1. 0.]
[0. 1.]]
[[0.57761083 0.61231605]
[0.10407747 0.88935241]]

a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])

b = a[:2, 1:3] #獲取前兩行，第二列第三列的資料
[[2 3]
[6 7]]
a = np.array([[1,2], [3, 4], [5, 6]])
print(a[[0, 1, 2], [0, 1, 0]]) 
[1 4 5]

import numpy as np

a = np.array([[1,2], [3, 4], [5, 6]])
bool_idx = (a > 2) 
print(bool_idx) 
print(a[bool_idx]) # prints "[3 4 5 6]"
print(a[a > 2]) # prints "[3 4 5 6]"

import numpy as np

x = np.array([1, 2]) 
print(x.dtype) # prints "int64"
x = np.array([1.0, 2.0]) 
print(x.dtype) # prints "float64"
x = np.array([1, 2], dtype=np.int64) 
print(x.dtype) # prints "int64"

import numpy as np

x = np.array([[1,2],[3,4]])
print(np.sum(x)) # prints "10"
print(np.sum(x, axis=0)) # prints "[4 6]"
print(np.sum(x, axis=1)) # prints "[3 7]"
print(x.t) # prints "[[1 3]
# [2 4]]"
print(x.dot(v))
print(np.dot(x, v)) #向量內積

import numpy as np

x = np.array([[1,2,3], [4,5,6], [7,8,9], [10, 11, 12]])
v = np.array([1, 0, 1])
y = np.empty_like(x) # 建立乙個維度與x一樣的空矩陣
# x的每一行都加上v向量
for i in range(4):
y[i, :] = x[i, :] + v
print(y)
# [[ 2 2 4]
# [ 5 5 7]
# [ 8 8 10]
# [11 11 13]]

import numpy as np

x = np.array([[1,2,3], [4,5,6], [7,8,9], [10, 11, 12]])
v = np.array([1, 0, 1])
vv = np.tile(v, (4, 1)) # 堆疊行4次
print(vv) # prints "[[1 0 1]
# [1 0 1]
# [1 0 1]
# [1 0 1]]"
y = x + vv 
print(y) # prints "[[ 2 2 4
# [ 5 5 7]
# [ 8 8 10]
# [11 11 13]]"
#當前版本可直接加
x = np.array([[1,2,3], [4,5,6], [7,8,9], [10, 11, 12]])
v = np.array([1, 0, 1])
y = x + v 
print(y) # prints "[[ 2 2 4]
# [ 5 5 7]
# [ 8 8 10]
# [11 11 13]]"

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