Question

I want to get the neighbors of the certain element in the numpy array. Lets consider following example

    a = numpy.array([0,1,2,3,4,5,6,7,8,9])

So I want to specify position 5 and want to get three neighbors from both sides. It can be done

   index = 5
   num_neighbor=3
   left = a[index-num_neighbor:index]
   right= a[num_neighbor+1:num_neighbor+index+1]

The above code does not take care of the boundaries… I want that i get the neighbours within the boundaries of the array. For this consider the following example if index is 1 then the left neighbor is only one element which is 0.

Thanks a lot

Answer 1

import numpy as np
a = np.array([0,1,2,3,4,5,6,7,8,9])
num_neighbor=3    

for index in range(len(a)):
    left = a[:index][-num_neighbor:]
    right= a[index+1:num_neighbor+index+1]
    print(index,left,right)

yields

(0, array([], dtype=int32), array([1, 2, 3]))
(1, array([0]), array([2, 3, 4]))
(2, array([0, 1]), array([3, 4, 5]))
(3, array([0, 1, 2]), array([4, 5, 6]))
(4, array([1, 2, 3]), array([5, 6, 7]))
(5, array([2, 3, 4]), array([6, 7, 8]))
(6, array([3, 4, 5]), array([7, 8, 9]))
(7, array([4, 5, 6]), array([8, 9]))
(8, array([5, 6, 7]), array([9]))
(9, array([6, 7, 8]), array([], dtype=int32))

---

The reason why a[index-num_neighbor:index] does not work when index<num_neighbor is because of slicing rules #3 and #4:

Given s[i:j]:

If i or j is negative, the index is relative to the end of the string:
len(s) + i or len(s) + j is substituted.

The slice of s from i to j is defined as the sequence of items with
index k such that i <= k < j. If i or j is greater than len(s), use
len(s). If i is omitted or None, use 0. If j is omitted or None, use
len(s). If i is greater than or equal to j, the slice is empty.

So when index=1, then a[index-num_neighbor:index] = a[-2:1] = a[10-2:1] = a[8:1] = [].