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关于numpy:18-个-实用的Numpy-代码片段总结

jiezi

Numpy 长期以来始终是 Python 开发人员进行数组操作的通用抉择，它是基于 C 语言构建的这使得它成为执行数组操作的疾速和牢靠的抉择，并且它曾经成为机器学习和数据迷信必备的根底库。

在本文中，我整顿了一些 NumPy 代码的片段，这些代码片段都是在日常开发中常常用到的。

1、创立数组 

import numpy as np

new_array = np.array([1,2,3])

print(new_array)

# Output[1 2 3]

2、获取 Numpy 数组的形态、维度和大小 

# shape
print(new_array.shape)

# dimensions
print(new_array.ndim)

# size
print(new_array.size)

# Output(3,)
1
3

3、查看 Numpy 数组中元素的类型 

array = np.arange(0,10,1)
print(array.dtype)

# Output
int64

4、获取数组中每个元素的占用字节大小 

array = np.array([1,2])
print(array.itemsize)

# Output
8

5、创立时指定数组的类型 

array = np.array([[1,2], [3,4]], dtype=complex)
array

# Output
array([[1.+0.j, 2.+0.j],
       [3.+0.j, 4.+0.j]])

6、应用占位符创立数组 

# 全零数组
array = np.zeros((3,4))
print(array)
print("---")

# 全 1 数组
array = np.ones((1,2))
print(array)
print("---")

# shape (2,3) 的空数组，随机产生数据
array = np.empty((2,3))
print(array)
print("---")

# Output
[[0. 0. 0. 0.]
 [0. 0. 0. 0.]
 [0. 0. 0. 0.]]
---
[[1. 1.]]
---
[[4.67280967e-310 0.00000000e+000 0.00000000e+000]
 [0.00000000e+000 0.00000000e+000 0.00000000e+000]]
---

7、创立序列 

# 应用 np.arange 创立一个从 0 到 42 个元素的序列，步长 1
array = np.arange(0,42,1)
print(array)
print("---")

# 应用 np.linspace 在 0 到 100 之间插入 42 个元素
array = np.linspace(0,100,42)
print(array)
print("---")

# Output
[ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41]
---
[  0.           2.43902439   4.87804878   7.31707317   9.75609756
  12.19512195  14.63414634  17.07317073  19.51219512  21.95121951
  24.3902439   26.82926829  29.26829268  31.70731707  34.14634146
  36.58536585  39.02439024  41.46341463  43.90243902  46.34146341
  48.7804878   51.2195122   53.65853659  56.09756098  58.53658537
  60.97560976  63.41463415  65.85365854  68.29268293  70.73170732
  73.17073171  75.6097561   78.04878049  80.48780488  82.92682927
  85.36585366  87.80487805  90.24390244  92.68292683  95.12195122
  97.56097561 100.        ]
---

8、Numpy 中的数学函数 

import numpy as np
import matplotlib.pyplot as plt

# sine function
x = np.linspace(0,2*np.pi, 100)
f = np.sin(x)
plt.figure(figsize=(15,7))
plt.subplot(1,3,1)
plt.plot(f, color="green")
plt.title("np.sin(x)")

# cosine function
f = np.cos(x)
plt.subplot(1,3,2)
plt.plot(f, color="blue")
plt.title("np.cos(x)")

# tangent function
f = np.tan(x)
plt.subplot(1,3,3)
plt.plot(f, color="red")
plt.title("np.tan(x)")
plt.show()

9、通过在每个坐标上执行函数来创立数组 

some_function = lambda x: np.cos(x)+1
array = np.fromfunction(some_function, (100,))
plt.figure(figsize=(15,7))
plt.plot(array, color="green")
plt.title("np.cos(x) +1")
plt.show()

10、遍历 Numpy 数组的所有元素 

a = np.arange(0,23,1)
for i in a.flat:
    print(i)

    
# Output
0
1
2
...
22

11、获取浮点数的上限 

np.floor(10.5)

10.0

12、应用.ravel() 压扁数组 

array = np.full(shape=(5,5),fill_value=10)
print(array)
print("---")
print("Flattened array:")
print(array.ravel())

# Output

[[10 10 10 10 10]
 [10 10 10 10 10]
 [10 10 10 10 10]
 [10 10 10 10 10]
 [10 10 10 10 10]]
---
Flattened array:
[10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
 10]

13、获取一个数组的转置 

array = np.random.random((2,5))
print(array)
print(array.T)

[[0.18735704 0.22800582 0.02552177 0.93552346 0.20720663]
 [0.74303284 0.1897481  0.91389602 0.23099501 0.07565492]]
[[0.18735704 0.74303284]
 [0.22800582 0.1897481]
 [0.02552177 0.91389602]
 [0.93552346 0.23099501]
 [0.20720663 0.07565492]]

14、应用. shape() 和.resize() 进行重塑 

a = np.random.randint(100,size=(3,4))
print(a)
a_reshaped = np.reshape(a, (1,12))
print(a_reshaped)

# 应用.resize() 办法
a.resize((1,12))

# Output

[[29 18 39 24]
 [53 45 49  8]
 [90 75 61 61]]
[[29 18 39 24 53 45 49  8 90 75 61 61]]

15、将数组沿不同轴进行重叠 

a = np.random.random((2,2))
print(a)
b = np.random.random((2,2))
print(b)

# 沿垂直轴重叠 (取得更多行)
print(np.vstack((a,b)))
print(np.vstack((a,b)).shape)

# 沿程度轴重叠 (取得更多列)
print(np.hstack((a,b)))
print(np.hstack((a,b)).shape)

# column_stack 列叠加
print(np.column_stack((a,b)))

# Output
[[0.67028492 0.86322792]
 [0.38906266 0.36967583]]
[[0.51419553 0.21937852]
 [0.50375453 0.31634597]]
[[0.67028492 0.86322792]
 [0.38906266 0.36967583]
 [0.51419553 0.21937852]
 [0.50375453 0.31634597]]
(4, 2)
[[0.67028492 0.86322792 0.51419553 0.21937852]
 [0.38906266 0.36967583 0.50375453 0.31634597]]
(2, 4)
[[0.67028492 0.86322792 0.51419553 0.21937852]
 [0.38906266 0.36967583 0.50375453 0.31634597]]

16、将一个数组拆分为几个较小的数组

应用 hsplit，通过指定要返回的雷同 shape 的 array 的数量，或者通过指定宰割应该产生之后的列来沿着其横轴拆分原 array。vsplit 沿着垂直轴宰割，其宰割形式与 hsplit 用法雷同 

# 沿着程度轴将数组拆分为 5 个较小的数组
a = np.arange(0, 5, 1)
print("Horizontal split")
print(np.hsplit(a, 5))
print("---")

# 沿着垂直轴将数组拆分成 5 个更小的数组
a = np.random.random((5,5))
print("Vertical split")
print(np.vsplit(a, 5))

Horizontal split
[array([0]), array([1]), array([2]), array([3]), array([4])]
---
Vertical split
[array([[0.69059321, 0.55703093, 0.20019592, 0.19697317, 0.37278251]]), array([[0.24597633, 0.87216661, 0.634432  , 0.35326185, 0.03130537]]), array([[0.18063077, 0.45045441, 0.06882852, 0.91273837, 0.07332161]]), array([[0.61738939, 0.11291748, 0.73152623, 0.49177006, 0.95750985]]), array([[0.90212777, 0.53825846, 0.86733505, 0.76165564, 0.17337721]])]

17、数组的浅拷贝

.view() 办法创立了一个与原数组对象雷同的对象，它创立了该数组的浅拷贝，浅拷贝只复制指向某个对象的指针，而不复制对象数据，新旧对象还是共享同一块内存。所以如果其中一个对象扭转了内存的数值，就会影响到另一个对象，也就是说一个对象的数值扭转了，其余的也会扭转（应用雷同的内存）。

a = np.array([[0,1,2,3,4],
    [5,6,7,8,9],
    [10,11,12,13,14]
    ])

array_object = np.arange(0,10,1)

shallow_copy_object = array_object.view() # shallow copy

print("Array")
print(array_object)
print(f"Id = {id(array_object)}")
print("---")

print("Shallow Copy")
print(shallow_copy_object)
print(f"Id = {id(shallow_copy_object)}")
print("---")

shallow_copy_object[0] = 200

print("After assigment: shallow_copy_object[0] = 200")

print("Array")
print(array_object)

print("Shallow copy")
print(shallow_copy_object)

# Output

Array
[0 1 2 3 4 5 6 7 8 9]
Id = 139980496768528
---
Shallow Copy
[0 1 2 3 4 5 6 7 8 9]
Id = 139980496768720
---
After assigment: shallow_copy_object[0] = 200
Array
[200   1   2   3   4   5   6   7   8   9]
Shallow copy
[200   1   2   3   4   5   6   7   8   9]

18、数组的深拷贝

copy 办法复制对象及其数据的残缺正本。齐全拷贝了一个正本，外部元素地址都不一样，数值的扭转不会相互影响。

array_object = np.arange(0, 23, 1)
deep_copy_object = array_object.copy()

print(deep_copy_object is array_object)

print("Array")
print(array_object)
print(f"Array id = {id(array_object)}")
print("---")
      
print("Deep Copy")
print(deep_copy_object)
print(f"Deep copy id = {id(deep_copy_object)}")
print("---")

deep_copy_object[0] = 234

print("After assignment: deep_copy_object[0] = 234")
print("Array")
print(array_object)
print("Deep copy")

print(deep_copy_object)

False
Array
[0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22]
Array id = 139980498767472
---
Deep Copy
[0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22]
Deep copy id = 139980496039824
---
After assignment: deep_copy_object[0] = 234
Array
[0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22]
Deep copy
[234   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17
  18  19  20  21  22]

作者：Lucas Soares

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