Numpy基本用法

文章目录

  • 一、Numpy基本用法
  • 二、Numpy创建数组
    • 1.使用np.array()由python list创建
    • 2.使用np的routines函数创建
      • 2.1 np.ones()
      • 2.2 zeros()
      • 2.3 np.full()
      • 2.4 np.eye()
      • 2.5 np.linspace()
      • 2.6 arange()
      • 2.7 randint()
      • 2.8 randn() 正太分布
      • 2.9 normal() 正太分布
      • 2.10 random_sample() 生成0到1的随机数
  • 三、Numpy查看数组属性
    • 1.数组元素个数
    • 2.数组形状
    • 3.数组维度
    • 4.数组元素类型
  • 四、数组的基本操作
    • 1.索引
    • 2.切片
    • 3.变形
    • 4.级联
      • np.hstack与np.vstack
    • 5.切分
    • 6.副本
  • 五、Numpy计算
    • 条件运算
    • 统计运算
    • 数组运算
    • 矩阵运算np.dot()
    • 矩阵的广播
  • 六、排序
    • 1.快速排序
    • 2.部分排序
    • numpy案例

一、Numpy基本用法

NumPy是Python语言的一个扩充程序库。支持高级大量的维度数组与矩阵运算,此外也针对数组运算提供大量的数学函数库。Numpy内部解除了Python的PIL(全局解释器锁),运算效率极好,是大量机器学习框架的基础库!

#导入numpy库,并查看numpy库版本
In [1]: import numpy as np

In [2]: np.__version__
Out[2]: '1.14.3'
#利用numpy+matplotlib处理图片
In [7]: import matplotlib.pyplot as plt

In [8]: cat = plt.imread('D:\myProject\Python\Jupyter\image\cat.jpg')

In [9]: type(cat)
Out[9]: numpy.ndarray

In [10]: cat1 = cat -50
#图片是一个三维数组,长、宽、颜色
In [11]: cat.shape
Out[11]: (333, 500, 3)
In [12]: plt.imshow(cat1)
Out[12]: <matplotlib.image.AxesImage at 0x222924b0320>
In [13]: plt.show()

Numpy基本用法_第1张图片

二、Numpy创建数组

1.使用np.array()由python list创建

In [14]: import numpy as np
#创建列表
In [15]: a = [1,2,3,4,5]
#将列表转换为数组
In [16]: b = np.array(a)

In [17]: b
Out[17]: array([1, 2, 3, 4, 5])

2.使用np的routines函数创建

2.1 np.ones()

# 1、np.ones(shape, dtype=None, order='C')
In [18]: np.ones(shape = (10,8),dtype= int)
Out[18]:
array([[1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 1, 1]])
In [19]: ones = np.ones(shape=(100,90,4),dtype=int)

In [20]: plt.imshow(ones)
    ...: plt.show()

Numpy基本用法_第2张图片

2.2 zeros()

# 2、zeros(shape, dtype=float, order='C')
In [21]: np.zeros((4,4))
Out[21]:
array([[0., 0., 0., 0.],
       [0., 0., 0., 0.],
       [0., 0., 0., 0.],
       [0., 0., 0., 0.]])

2.3 np.full()

# 3、 np.full(shape, fill_value, dtype=None, order='C')
In [22]: np.full((10,10),fill_value=20)
Out[22]:
array([[20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20],
       [20, 20, 20, 20, 20, 20, 20, 20, 20, 20]])

2.4 np.eye()

# 4、np.eye(N, M=None, k=0, dtype=, order='C')
#对角线位1,其他位置位0
In [23]: np.eye(10)
Out[23]:
array([[1., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
       [0., 1., 0., 0., 0., 0., 0., 0., 0., 0.],
       [0., 0., 1., 0., 0., 0., 0., 0., 0., 0.],
       [0., 0., 0., 1., 0., 0., 0., 0., 0., 0.],
       [0., 0., 0., 0., 1., 0., 0., 0., 0., 0.],
       [0., 0., 0., 0., 0., 1., 0., 0., 0., 0.],
       [0., 0., 0., 0., 0., 0., 1., 0., 0., 0.],
       [0., 0., 0., 0., 0., 0., 0., 1., 0., 0.],
       [0., 0., 0., 0., 0., 0., 0., 0., 1., 0.],
       [0., 0., 0., 0., 0., 0., 0., 0., 0., 1.]])

2.5 np.linspace()

# 5、np.linspace(start, stop, num=50, endpoint=True, retstep=False, dtype=None)
#lin = linear 线性
In [24]: np.linspace(1,100,20)
Out[24]:
array([  1.        ,   6.21052632,  11.42105263,  16.63157895,
        21.84210526,  27.05263158,  32.26315789,  37.47368421,
        42.68421053,  47.89473684,  53.10526316,  58.31578947,
        63.52631579,  68.73684211,  73.94736842,  79.15789474,
        84.36842105,  89.57894737,  94.78947368, 100.        ])

2.6 arange()

# 6、arange([start,] stop[, step,], dtype=None)
In [25]: np.arange(0,10,2)
Out[25]: array([0, 2, 4, 6, 8])

2.7 randint()

# 7、randint(low, high=None, size=None, dtype='l')
In [26]: np.random.randint(0,10,size = 10)
Out[26]: array([0, 9, 8, 9, 5, 5, 2, 6, 8, 7])

2.8 randn() 正太分布

# 8、randn(d0, d1, ..., dn)  正太分布
In [27]: np.random.randn(4,5)
Out[27]:
array([[ 1.05035386,  0.77231903, -0.33446991, -0.54562315, -0.84341866],
       [ 0.6616944 ,  0.89325969, -0.89042489,  1.61227272, -0.05136764],
       [-0.5346068 , -2.39236557, -0.54563468, -1.07850467,  0.56603284],
       [-0.33361988,  1.11445864, -0.11636709,  0.57361067,  0.80099792]])

2.9 normal() 正太分布

# 9、normal(loc=0.0, scale=1.0, size=None) 正太分布
In [28]: np.random.normal(loc=170,scale=1,size=20)
Out[28]:
array([168.56872023, 168.30641964, 169.98891802, 168.6781631 ,
       169.63611345, 167.81502056, 169.69833366, 169.05166886,
       170.96688262, 168.30677654, 170.15970816, 169.59783795,
       170.77979873, 170.20138558, 169.75464312, 172.16309013,
       169.37900208, 171.23951841, 169.7903996 , 168.62081606])

2.10 random_sample() 生成0到1的随机数

# 10、random_sample(size=None) 生成0到1的随机数
#np.random.random(size=20)
In [29]: r = np.random.random(size=(200,100,4)) #生成一张图片

In [30]: plt.imshow(r)
Out[30]: <matplotlib.image.AxesImage at 0x22293b60dd8>

In [31]: plt.show()

Numpy基本用法_第3张图片

三、Numpy查看数组属性

1.数组元素个数

In [33]: b
Out[33]: array([1, 2, 3, 4, 5])

In [34]: b.size
Out[34]: 5

2.数组形状

In [35]: b.shape
Out[35]: (5,)

3.数组维度

In [36]: b.ndim
Out[36]: 1

4.数组元素类型

In [37]: b.dtype
Out[37]: dtype('int32')

四、数组的基本操作

1.索引

In [39]: array1 = np.random.randint(0,100,(4,4))

In [40]: array1
Out[40]:
array([[85, 18, 96, 92],
       [42, 49, 76, 85],
       [42,  1, 80, 99],
       [33, 91, 74, 95]])

In [41]: array1[0,1]
Out[41]: 18

2.切片

#切片时,左闭右开
In [42]: array1[1:3,2:4]
Out[42]:
array([[76, 85],
       [80, 99]])

In [43]: array2 = np.arange(0,10)

In [44]: array2
Out[44]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
#将数组反转
In [45]: array2[::-1]
Out[45]: array([9, 8, 7, 6, 5, 4, 3, 2, 1, 0])
#通过两个::进行切片
In [46]: print(array2[::2])
[0 2 4 6 8]

In [47]: print(array2[::-2])
[9 7 5 3 1]

3.变形

  • 使用reshape函数,注意参数用tuple
In [48]: import numpy as np

In [49]: n = np.arange(0,10)

In [50]: n
Out[50]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

In [51]: n.shape
Out[51]: (10,)
#使用reshape函数进行变形
In [52]: n.reshape((5,2))
Out[52]:
array([[0, 1],
       [2, 3],
       [4, 5],
       [6, 7],
       [8, 9]])

In [53]: n1 = np.random.randint(0,100,(3,4,5))

In [54]: n1
Out[54]:
array([[[48,  8,  2, 35, 65],
        [81, 56, 20, 85, 76],
        [94, 65, 62, 59, 78],
        [48, 57,  7,  3, 63]],

       [[56, 27,  4, 21, 28],
        [92, 10, 73, 63, 82],
        [88, 28, 10, 76, 99],
        [49,  6, 39, 21, 50]],

       [[84, 87, 65, 15, 52],
        [94, 44, 40,  2, 94],
        [ 7, 90, 78, 18, 94],
        [94, 64, 83, 54,  6]]])

In [55]: n1.shape
Out[55]: (3, 4, 5)

In [56]: n1.reshape(3*4*5)
Out[56]:
array([48,  8,  2, 35, 65, 81, 56, 20, 85, 76, 94, 65, 62, 59, 78, 48, 57,
        7,  3, 63, 56, 27,  4, 21, 28, 92, 10, 73, 63, 82, 88, 28, 10, 76,
       99, 49,  6, 39, 21, 50, 84, 87, 65, 15, 52, 94, 44, 40,  2, 94,  7,
       90, 78, 18, 94, 94, 64, 83, 54,  6])
#使用负数直接转换成一维数组
In [57]: n1.reshape(-1)
Out[57]:
array([48,  8,  2, 35, 65, 81, 56, 20, 85, 76, 94, 65, 62, 59, 78, 48, 57,
        7,  3, 63, 56, 27,  4, 21, 28, 92, 10, 73, 63, 82, 88, 28, 10, 76,
       99, 49,  6, 39, 21, 50, 84, 87, 65, 15, 52, 94, 44, 40,  2, 94,  7,
       90, 78, 18, 94, 94, 64, 83, 54,  6])

4.级联

  • np.concatenate()级联需要注意的点:
    • 1.级联的参数是列表:一定要加中括号或小括号
    • 2.维度必须相同
    • 3.形状相符
    • 4.【重点】级联的方向默认是shape这个tuple的第一个值所代表的维度方向
    • 5.可通过axis参数改变级联的方向
In [58]: import numpy as np

In [59]: n1 = np.random.randint(0,10,(5,5))

In [60]: n1
Out[60]:
array([[1, 0, 1, 3, 2],
       [0, 1, 9, 2, 9],
       [2, 4, 1, 1, 1],
       [5, 2, 2, 6, 2],
       [8, 8, 8, 7, 2]])
#列级联
In [61]: np.concatenate((n1,n1),axis = 0)
Out[61]:
array([[1, 0, 1, 3, 2],
       [0, 1, 9, 2, 9],
       [2, 4, 1, 1, 1],
       [5, 2, 2, 6, 2],
       [8, 8, 8, 7, 2],
       [1, 0, 1, 3, 2],
       [0, 1, 9, 2, 9],
       [2, 4, 1, 1, 1],
       [5, 2, 2, 6, 2],
       [8, 8, 8, 7, 2]])
#行级联
In [62]: np.concatenate((n1,n1),axis = 1)
Out[62]:
array([[1, 0, 1, 3, 2, 1, 0, 1, 3, 2],
       [0, 1, 9, 2, 9, 0, 1, 9, 2, 9],
       [2, 4, 1, 1, 1, 2, 4, 1, 1, 1],
       [5, 2, 2, 6, 2, 5, 2, 2, 6, 2],
       [8, 8, 8, 7, 2, 8, 8, 8, 7, 2]])

np.hstack与np.vstack

  • 水平级联与垂直级联,处理自己。进行维度的变更
In [63]: n2 = np.random.randint(0,100,size=10)

In [64]: n2
Out[64]: array([73, 65, 92, 23, 32, 47, 78, 83, 97, 37])
#vertical 垂直
In [65]: n3 = np.vstack(n2) 
                            
In [66]: print(n3.shape)    
(10, 1)                     
                            
In [67]: n3                 
Out[67]:                    
array([[73],                
       [65],                
       [92],                
       [23],                
       [32],                
       [47],                
       [78],                
       [83],                
       [97],                
       [37]])
# 水平
In [68]: n4 = np.array([[1,2,3,4,5],[6,7,8,9]])

In [69]: n4
Out[69]: array([list([1, 2, 3, 4, 5]), list([6, 7, 8, 9])], dtype=object)

In [70]: np.hstack(n4)
Out[70]: array([1, 2, 3, 4, 5, 6, 7, 8, 9])
In [71]: np.hstack(n3)
Out[71]: array([73, 65, 92, 23, 32, 47, 78, 83, 97, 37])

5.切分

  • 与级联类似,常用函数:
    • np.split
    • np.vsplit
    • np.hsplit
In [72]: n5 = np.random.randint(0,150,size=(5,7))

In [73]: n5
Out[73]:
array([[122, 102,  37,  69,  28, 102, 131],
       [ 48,  84,  27, 119,   7,  65,  61],
       [ 17, 125, 142, 145, 132,  21,  57],
       [ 96,  23,  72,  45,  77,  54,  65],
       [120,  31, 104, 132,  64,  72, 145]])

In [74]: np.split(n5,(1,3))
Out[74]:
[array([[122, 102,  37,  69,  28, 102, 131]]),
 array([[ 48,  84,  27, 119,   7,  65,  61],
        [ 17, 125, 142, 145, 132,  21,  57]]),
 array([[ 96,  23,  72,  45,  77,  54,  65],
        [120,  31, 104, 132,  64,  72, 145]])]
#水平切分,即按行进行切分
In [75]: np.split(n5,(1,3),axis=1)
Out[75]:
[array([[122],
        [ 48],
        [ 17],
        [ 96],
        [120]]), array([[102,  37],
        [ 84,  27],
        [125, 142],
        [ 23,  72],
        [ 31, 104]]), array([[ 69,  28, 102, 131],
        [119,   7,  65,  61],
        [145, 132,  21,  57],
        [ 45,  77,  54,  65],
        [132,  64,  72, 145]])]
In [76]: np.split(n5,(1,3))[1]
Out[76]:
array([[ 48,  84,  27, 119,   7,  65,  61],
       [ 17, 125, 142, 145, 132,  21,  57]])
In [81]: cat.shape
Out[81]: (333, 500, 3)
    
In [77]: import matplotlib.pyplot as plt

In [78]: cat1 = np.split(cat,(100,200))[1]

In [79]: plt.imshow(cat1)
Out[79]: <matplotlib.image.AxesImage at 0x22293e9b978>

In [80]: plt.show()

Numpy基本用法_第4张图片

#垂直切分
In [82]: np.vsplit(n5,(1,3))
Out[82]:
[array([[122, 102,  37,  69,  28, 102, 131]]),
 array([[ 48,  84,  27, 119,   7,  65,  61],
        [ 17, 125, 142, 145, 132,  21,  57]]),
 array([[ 96,  23,  72,  45,  77,  54,  65],
        [120,  31, 104, 132,  64,  72, 145]])]
#水平切分
In [83]: np.hsplit(n5,(1,3))
Out[83]:
[array([[122],
        [ 48],
        [ 17],
        [ 96],
        [120]]), array([[102,  37],
        [ 84,  27],
        [125, 142],
        [ 23,  72],
        [ 31, 104]]), array([[ 69,  28, 102, 131],
        [119,   7,  65,  61],
        [145, 132,  21,  57],
        [ 45,  77,  54,  65],
        [132,  64,  72, 145]])]

6.副本

  • 所有赋值运算不会为ndarray的任何元素创建副本,对赋值后的对象的操作也对原来的对象生效。
In [84]: a = [x for x in range(5)]

In [85]: n = np.array(a)

In [86]: n
Out[86]: array([0, 1, 2, 3, 4])

In [87]: n[2] = 512

In [88]: n
Out[88]: array([  0,   1, 512,   3,   4])
#当数据是ndarray,用=赋值,内存没有改变
In [89]: n2 = n

In [90]: n2[2] = 1024

In [91]: display(n,n2)
array([   0,    1, 1024,    3,    4])
array([   0,    1, 1024,    3,    4])
  • 使用copy()函数创建副本
In [92]: n3 = n.copy()

In [93]: n3
Out[93]: array([   0,    1, 1024,    3,    4])

In [94]: n3[2] = 0

In [95]: display(n,n3)
array([   0,    1, 1024,    3,    4])
array([0, 1, 0, 3, 4])

五、Numpy计算

条件运算

Numpy基本用法_第5张图片

# 条件判断
In [97]: stus_score = np.array([[80,88],[82,81],[84,75],[86,83],[75,81]])

In [98]: stus_score > 80
Out[98]:
array([[False,  True],
       [ True,  True],
       [ True, False],
       [ True,  True],
       [False,  True]])
# 三目运算(如果数值小于80,替换为0,如果大于80,替换为90)
In [100]: stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])

In [101]: np.where(stus_score < 80, 0, 90)
Out[101]:
array([[90, 90],
       [90, 90],
       [90,  0],
       [90, 90],
       [ 0, 90]])

统计运算

# 指定轴最大值amax(参数1: 数组; 参数2: axis=0/1; 0表示列1表示行)
stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])
# 求每一列的最大值(0表示列)
In [106]: print("每一行的最大值为:")
每一行的最大值为:

In [107]: print("每一列的最大值为:")
     ...: result = np.amax(stus_score, axis=0)
     ...: print(result)
     ...: print("每一行的最大值为:")
     ...: result = np.amax(stus_score, axis=1)
     ...: print(result)
     ...:
     ...:
每一列的最大值为:
[86 88]
每一行的最大值为:
[88 82 84 86 81]

# 指定轴最大值amax(参数1: 数组; 参数2: axis=0/1/2;分别对应各个维度)
In [108]: import numpy as np

In [109]: n = np.random.randint(0,150,(4,4,4))

In [110]: n
Out[110]:
array([[[ 91,  45,  10,  51],
        [102,  88, 129, 100],
        [148,  72,  52, 114],
        [ 99,   5,  67,  26]],

       [[ 23, 102, 125, 116],
        [140,  61, 107,  15],
        [ 39, 125, 139,  38],
        [ 95, 148, 100, 109]],

       [[ 92,  68, 140,  75],
        [ 49, 113,  68,  70],
        [149, 107,  78,  69],
        [120, 109,  27, 138]],

       [[126,  79, 113,  89],
        [ 29, 126,   3,  90],
        [ 40,  23,  20,  14],
        [ 72,  38,  99,  55]]])
In [111]: n.max(axis=0)
Out[111]:
array([[126, 102, 140, 116],
       [140, 126, 129, 100],
       [149, 125, 139, 114],
       [120, 148, 100, 138]])

In [112]: n.max(axis=1)
Out[112]:
array([[148,  88, 129, 114],
       [140, 148, 139, 116],
       [149, 113, 140, 138],
       [126, 126, 113,  90]])

In [113]: n.max(axis=2)
Out[113]:
array([[ 91, 129, 148,  99],
       [125, 140, 139, 148],
       [140, 113, 149, 138],
       [126, 126,  40,  99]])
# 指定轴最小值amin
In [114]: stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])
# 求每一行的最小值(0表示列)
In [115]: print("每一列的最小值为:")
     ...: result = np.amin(stus_score, axis=0)
     ...: print(result)
     ...:
     ...: # 求每一行的最小值(1表示行)
     ...: print("每一行的最小值为:")
     ...: result = np.amin(stus_score, axis=1)
     ...: print(result)
     ...:
     ...:
每一列的最小值为:
[75 75]
每一行的最小值为:
[80 81 75 83 75]

# 指定轴平均值mean
In [116]: stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])
In [117]: # 求每一行的平均值(0表示列)
     ...: print("每一列的平均值:")
     ...: result = np.mean(stus_score, axis=0)
     ...: print(result)
     ...:
     ...: # 求每一行的平均值(1表示行)
     ...: print("每一行的平均值:")
     ...: result = np.mean(stus_score, axis=1)
     ...: print(result)
     ...:
     ...:
每一列的平均值:
[81.4 81.6]
每一行的平均值:
[84.  81.5 79.5 84.5 78.]
# 方差std
In [118]: stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])

In [119]: # 求每一行的方差(0表示列)
     ...: print("每一列的方差:")
     ...: result = np.std(stus_score, axis=0)
     ...: print(result)
     ...:
     ...: # 求每一行的方差(1表示行)
     ...: print("每一行的方差:")
     ...: result = np.std(stus_score, axis=1)
     ...: print(result)
     ...:
     ...:
每一列的方差:
[3.77359245 4.1761226 ]
每一行的方差:
[4.  0.5 4.5 1.5 3. ]

数组运算

# 数组与数的运算
In [120]: import numpy as np

In [121]: stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])

In [122]: print("加之前:")
     ...: print(stus_score)
     ...:
     ...: #为所有平时成绩都加5分
     ...: stus_score[:,0]=stus_score[:,0]+5
     ...: print("加之后:")
     ...: print(stus_score)
     ...:
     ...:
加之前:
[[80 88]
 [82 81]
 [84 75]
 [86 83]
 [75 81]]
加之后:
[[85 88]
 [87 81]
 [89 75]
 [91 83]
 [80 81]]

# 数组也支持加减乘除运算
In [123]: import numpy as np

In [124]: a = np.array([1,2,3,4])
     ...: b = np.array([10,20,30,40])
     ...: c = a + b
     ...: d = a - b
     ...: e = a * b
     ...: f = a / b
     ...: print("a+b=",c)
     ...: print("a-b=",d)
     ...: print("a*b=",e)
     ...: print("a/b=",f)
     ...:
     ...:
a+b= [11 22 33 44]
a-b= [ -9 -18 -27 -36]
a*b= [ 10  40  90 160]
a/b= [0.1 0.1 0.1 0.1]

矩阵运算np.dot()

Numpy基本用法_第6张图片

# 计算规则(计算学生总成绩)
#(M行,N列)*(N行,Z列)=(M行,Z列)
In [125]: import numpy as np

In [126]: stus_score = np.array([[80, 88], [82, 81], [84, 75], [86, 83], [75, 81]])
     ...: #平时成绩占40%,期末成绩占60%,计算结果
     ...: q = np.array([[0.4],[0.6]])
     ...: result = np.dot(stus_score,q)
     ...: print("最终结果为:")
     ...: print(result)
     ...:
     ...:
最终结果为:
[[84.8]
 [81.4]
 [78.6]
 [84.2]
 [78.6]]
# 矩阵拼接
## 矩阵垂直拼接
In [127]: print("v1为:")
     ...: v1 = [[0, 1, 2, 3, 4, 5],
     ...:       [6, 7, 8, 9, 10, 11]]
     ...: print(v1)
     ...: print("v2为:")
     ...: v2 = [[12, 13, 14, 15, 16, 17],
     ...:       [18, 19, 20, 21, 22, 23]]
     ...: print(v2)
     ...:
v1为:
[[0, 1, 2, 3, 4, 5], [6, 7, 8, 9, 10, 11]]
v2为:
[[12, 13, 14, 15, 16, 17], [18, 19, 20, 21, 22, 23]]

# 垂直拼接
In [128]: result = np.vstack((v1, v2))
     ...: print("v1和v2垂直拼接的结果为")
     ...: print(result)
     ...:
     ...:
v1和v2垂直拼接的结果为
[[ 0  1  2  3  4  5]
 [ 6  7  8  9 10 11]
 [12 13 14 15 16 17]
 [18 19 20 21 22 23]]
# 矩阵水平拼接
In [129]: print("v1为:")
     ...: v1 = [[0, 1, 2, 3, 4, 5],
     ...:       [6, 7, 8, 9, 10, 11]]
     ...: print(v1)
     ...: print("v2为:")
     ...: v2 = [[12, 13, 14, 15, 16, 17],
     ...:       [18, 19, 20, 21, 22, 23]]
     ...: print(v2)
     ...:
v1为:
[[0, 1, 2, 3, 4, 5], [6, 7, 8, 9, 10, 11]]
v2为:
[[12, 13, 14, 15, 16, 17], [18, 19, 20, 21, 22, 23]]

# 垂直拼接
In [130]: result = np.hstack((v1, v2))
     ...: print("v1和v2水平拼接的结果为")
     ...: print(result)
     ...:
     ...:
v1和v2水平拼接的结果为
[[ 0  1  2  3  4  5 12 13 14 15 16 17]
 [ 6  7  8  9 10 11 18 19 20 21 22 23]]

矩阵的广播

  • ndarray广播的两条规则:
    • 规则一:为缺失的维度补1
    • 规则二:假定缺失元素用已有值填充
#实例1:m = np.ones((2,3)) a = np.arange(3)求m+a
In [131]: import numpy as np

In [132]: m = np.ones((2,3))
     ...: a = np.arange(3)
     ...: display(m,a)
     ...:
     ...:
array([[1., 1., 1.],
       [1., 1., 1.]])
array([0, 1, 2])
#numpy广播,维度不同,自动补全
In [133]: m + a
Out[133]:
array([[1., 2., 3.],
       [1., 2., 3.]])
#实例2:a = np.arange(3).reshape((3,1)) b = np.arange(3) 求a+b
In [134]: a = np.arange(3).reshape((3,1))
     ...: b = np.arange(3)
     ...: display(a,b)
     ...:
     ...:
array([[0],
       [1],
       [2]])
array([0, 1, 2])
In [135]: a + b
Out[135]:
array([[0, 1, 2],
       [1, 2, 3],
       [2, 3, 4]])

六、排序

1.快速排序

  • np.sort()与ndarray.sort(),区别:
    • np.sort()不改变输入
    • ndarray.sort()本地处理,不占用空间,改变输入
In [136]: n1 = np.random.randint(0,150,size=15)

In [137]: n1
Out[137]:
array([ 80,  11,  31,  68,  83,  73,  42,   6,  40, 125, 147, 147,  88,
       117,  85])

In [138]: n2 = n1.sort()

In [139]: display(n1,n2)
array([  6,  11,  31,  40,  42,  68,  73,  80,  83,  85,  88, 117, 125,
       147, 147])
None

In [140]: n3 = np.sort(n1)
     ...: display(n1,n3)
     ...:
     ...:
array([  6,  11,  31,  40,  42,  68,  73,  80,  83,  85,  88, 117, 125,
       147, 147])
array([  6,  11,  31,  40,  42,  68,  73,  80,  83,  85,  88, 117, 125,
       147, 147])

2.部分排序

  • np.partition(a,k)
    • 当k为正时:我们想要得到最小的k个数
    • 当k为负时:我们想要得到最大的k个数
In [141]: n4 = np.random.randint(0,150,size=20)

In [142]: n4
Out[142]:
array([ 37,  74,  41,  53,  31,  11,  23, 108,  12, 128,  27,  88,  74,
       114,  97, 127,  60,  47, 130, 135])
In [143]: np.partition(n4,-5)
Out[143]:
array([ 37,  12,  41,  27,  31,  11,  23,  47,  74,  60,  53,  74,  88,
        97, 108, 114, 127, 128, 130, 135])
In [144]: np.partition(n4,5)
Out[144]:
array([ 12,  23,  11,  27,  31,  37,  41,  47, 108,  74,  53,  88,  74,
       114,  97, 127,  60, 128, 130, 135])
#取数组中最大的k个数
In [145]: np.partition(n4,-2)[-2:]
Out[145]: array([130, 135])

numpy案例

  • 使用numpy将图片进行翻转
import matplotlib.pyplot as plt
import numpy as np
%matplotlib inline
dianxin = plt.imread('./image/dianxin.jpg')
plt.imshow(dianxin)

Numpy基本用法_第7张图片

#将图片在竖直方向翻转
dianxin1 = dianxin[::-1]
plt.imshow(dianxin1)

Numpy基本用法_第8张图片

#在水平方向翻转
dianxin2 = dianxin[::,::-1]
plt.imshow(dianxin2)

Numpy基本用法_第9张图片

# 即在水平方向翻转又在垂直方向翻转
dianxin3 = dianxin[::-1,::-1]
plt.imshow(dianxin3)

Numpy基本用法_第10张图片

# 改变图片颜色
dianxin4 = dianxin[::,::,-2]
plt.imshow(dianxin4)

Numpy基本用法_第11张图片

#修改图片,是图片模糊
dianxin5 = dianxin[::3,::3]
plt.imshow(dianxin5)

Numpy基本用法_第12张图片

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