Notice! list and array are different

　　 The list in python is a built-in data type of python, the data types in the list do not have to be the same , and the data types in the array must all be the same .

　　The array encapsulated in numpy has a very powerful function, and the same data type is stored in it.

　　The list code is as follows:

a=[[1,2],[1,2]] # two-dimensional list 
print (a)
b=a[1][1]
print(b)

　　operation result:

　　[[1, 2], [1, 2]]

　　2

　　Note: The method to read the elements of the two-dimensional list in the above code is a[1][1], not a[1,1]

　　The array code is as follows:

import numpy as np
a = np.array([[1,2],[1,2]]) #Convert   the list to array type 
print (a)
b=a[1][1]
print(b)
c=a[1,1]
print(c)

　　operation result:

　　[[1 2]
　　[1 2]]
　　2
　　2

　　Note: After converting the list to an array type, the method to read the elements in the two-dimensional array can be a[1][1] or a[1,1]. But the previous list can only be a[1][1]