Create and access (numpy) array

Creating an array of three ways

1. Create a list python by

(1) create a one-dimensional

import numpy as np
list_1 = [1, 2, 3, 4]
array_1 = np.array(list)
array_1
>>>array([1, 2, 3, 4])

(2) create two-dimensional

list_2 = [5, 6, 7, 8]
array_2 = np.array([list_1, list_2])
array_2
>>>array([[1, 2, 3, 4],
		  [5, 6, 7, 8]])

2. arange function
arange can only create one-dimensional arrays

array_4 = np.arange(1, 10)
array_4
>>>array([1, 2, 3, 4, 5, 6, 7, 8, 9])   #左闭右开型
array_5 = np.arange(1, 10, 2)
array_5
>>>array([1, 3, 5, 7, 9])   #设置步长为2

3. Through a special function

np.zeros(5)
>>>array([0., 0., 0., 0., 0.])
np.zeros([2, 3])
>>>array([[0., 0., 0.],
          [0., 0., 0.]])
np.eye(5)
>>>array([[1., 0., 0., 0., 0.],
          [0., 1., 0., 0., 0.],
       	  [0., 0., 1., 0., 0.],
       	  [0., 0., 0., 1., 0.],
          [0., 0., 0., 0., 1.]])

Access to an array of ways

1. one-dimensional

a = np.arange(1,10)
a
>>>array([1, 2, 3, 4, 5, 6, 7, 8, 9])
a[1]
>>>2
a[1:5]
>>>array([2, 3, 4, 5]) #从0算起，第1到第4个数

2. The two-dimensional

b = np.array([[1, 2, 3], [4, 5, 6]])
b
>>>array([[1, 2, 3],
       	  [4, 5, 6]])
b[1][0]
>>>4
b[1,0]
>>>4
c = np.array([[1,2,3],[4,5,6],[7,8,9]]) 
c
>>>array([[1, 2, 3],
     	  [4, 5, 6],
       	  [7, 8, 9]])
c[:2, 1:]
>>>array([[2, 3],
          [5, 6]])