[云炬python3玩转机器学习笔记] 3-5Numpy数组和矩阵的基本操作

62 阅读 0 评论 41 点赞

我是靠谱客的博主心灵美绿茶，最近开发中收集的这篇文章主要介绍[云炬python3玩转机器学习笔记] 3-5Numpy数组和矩阵的基本操作，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

import numpy as np
x=np.arange(10)
x
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
X= np.arange(15).reshape(3,5)
X
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
基本属性
x.ndim  # 查看数组维数
1
X.ndim
2
x.shape
(10,)
X.shape
(3, 5)
x.size
10
X.size
15
numpy.array的数据访问
x
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
x[0]
0
x[-1]
9
X
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
X[0][0]
0
X[(0,0)]
0
X[2,2]
12
x
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
x[0:5]
array([0, 1, 2, 3, 4])
x[:5]
array([0, 1, 2, 3, 4])
x[5:]
array([5, 6, 7, 8, 9])
x[::2]    #间隔
array([0, 2, 4, 6, 8])
x[::-1]  #倒序
array([9, 8, 7, 6, 5, 4, 3, 2, 1, 0])
X
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
X[:2,:3]
array([[0, 1, 2],
       [5, 6, 7]])
X[:2][:3]  #前两行的前三列
array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])
X[:2][:3] #X[:2]的前3行
array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])
X[:2,::2] 
array([[0, 2, 4],
       [5, 7, 9]])
X[::-1,::-1]
array([[14, 13, 12, 11, 10],
       [ 9,  8,  7,  6,  5],
       [ 4,  3,  2,  1,  0]])
X[0]
array([0, 1, 2, 3, 4])
X[0,:]
array([0, 1, 2, 3, 4])
X[0,:].ndim
1
X[:,0]
array([ 0,  5, 10])
X[:,0].ndim
1
subX=X[:2,:3]
subX
array([[0, 1, 2],
       [5, 6, 7]])
subX[0,0]=100
subX
array([[100,   1,   2],
       [  5,   6,   7]])
X  #子矩阵对原矩阵右影响，子矩阵引用的原矩阵的元素
array([[100,   1,   2,   3,   4],
       [  5,   6,   7,   8,   9],
       [ 10,  11,  12,  13,  14]])
X[0,0]=0
X
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
subX
array([[0, 1, 2],
       [5, 6, 7]])
subX =X[:2,:3].copy() #副本
subX
array([[0, 1, 2],
       [5, 6, 7]])
subX[0,0]=100
subX
array([[100,   1,   2],
       [  5,   6,   7]])
X
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
Reshape
x.shape
(10,)
x.ndim
1
x.reshape(2,5)
array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])
x
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
A= x.reshape(2,5)
A
array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])
x
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
B= x.reshape(1,10)
B #一维转二维
array([[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]])
B.shape
(1, 10)
x.reshape(10,-1) #指定行数，智能计算列数
array([[0],
       [1],
       [2],
       [3],
       [4],
       [5],
       [6],
       [7],
       [8],
       [9]])
x.reshape(-1,10)
array([[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]])
x.reshape(2,-1)
array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])
x.reshape(3,-1)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-65-27fb2acd3ab6> in <module>
----> 1 x.reshape(3,-1)

ValueError: cannot reshape array of size 10 into shape (3,newaxis)