Question

I am trying to index neighbours of a particular numpy array item. For example if I have the array shown below and I check for items over a certain value how can I index the cell above, below, left, and right of true items in an efficient manner without resorting to loops etc.

In [34]: x
Out[34]: 
array([[ 10.,  10.,  10.,  10.,  10.],
   [ 10.,  10.,  10.,  10.,  10.],
   [ 10.,  20.,  10.,  10.,  10.],
   [ 10.,  10.,  10.,  20.,  10.],
   [ 10.,  10.,  10.,  10.,  10.]])

In [37]: ans = x > 10

In [38]: ans
Out[38]: 
array([[False, False, False, False, False],
   [False, False, False, False, False],
   [False,  True, False, False, False],
   [False, False, False,  True, False],
   [False, False, False, False, False]], dtype=bool)

Answer 1

This will give you the indices of the neighbors:

>>> def neighbors(x, y):
...    return np.array([(x-1, y), (x, y-1), (x+1, y), (x, y+1)])
...
>>> ind = zip(*np.where(x > 10))
>>> neighb = np.concatenate([neighbors(*i) for i in ind])
array([[1, 1],
       [2, 0],
       [3, 1],
       [2, 2],
       [2, 3],
       [3, 2],
       [4, 3],
       [3, 4]])