What is the meaning of the last line of code, x1[ x1[:,1]>3 ]

Question

import numpy as np
x1 = np.array([[1,2,3],[4,5,6],[7,8,9]])
x1[ x1[:,1]>3 ]

For the code shown in upon, I don't understand why the output is array([[4, 5, 6],[7, 8, 9]]).

Answer 1

Break it down:

In [10]: x1                                                                                                                                                                                                 
Out[10]: 
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

In [11]: x1[:,1] # select all rows, second column                                                                                                                                                           
Out[11]: array([2, 5, 8])

In [12]: x1[:,1]>3 # for each one of these, return whether it's > 3                                                                                                                                         
Out[12]: array([False,  True,  True])

In [13]: x1[ x1[:,1]>3 ] # This is "Boolean array indexing"                                                                                                                                                 
Out[13]: 
array([[4, 5, 6],
       [7, 8, 9]])

The "Boolean array indexing" part filters the rows of x1 depending on the booleans contained in the boolean array x1[:,1]>3.

See Boolean array indexing in numpy doc.

Answer 2

It will retrieve all rows whose value is greater than 3. : is used to slice row and columns from array