[FIXED] Python Numpy 2d array slicing minus index to plus index

Issue

I would like to slice the array so that the last value is followed by the first value.

Code :

import numpy as np

arr = [[1, 2, 3, 4], 
       [5, 6, 7, 8], 
       [9, 10, 11, 12], 
       [13, 14, 15, 16]]

m = np.array(arr)

print(m[-1:1, -1:1])

expected output :

[[16 13]
 [4 1]]

actual output :

[]

Solution

You have to use advanced indexing:

In [64]: arr=np.arange(1,17).reshape(4,4)   
In [65]: arr[[[3],[0]],[3,0]]       # or -1 as in mozway's answer
Out[65]: 
array([[16, 13],
       [ 4,  1]])

On further thought, you can use a -3 step:

In [67]: arr[-1::-3, -1::-3]
Out[67]: 
array([[16, 13],
       [ 4,  1]])

The numbers and shape are the same, but strides is different (one is a copy, the other a view (with somewhat funny strides):

In [69]: arr[[[-1],[0]],[-1,0]].strides
Out[69]: (8, 4)
In [70]: arr[-1::-3, -1::-3].strides
Out[70]: (-48, -12)

Answered By - hpaulj