add left and right portions of the array symmetrically - python

Question

a = np.array([[[ 1, 11],
               [ 3, 13],
               [ 5, 15],
               [ 7, 17],
               [ 9, 19]],

              [[ 2, 12],
               [ 4, 14],
               [ 6, 16],
               [ 8, 18],
               [10, 20]]])

I'm trying to add the left portion to the right portion of the array in a symmetrical way along the second dimension (so 1+9 , 11+19 , 3+7 , 13+17 for the first sub array above).

I tried this

>>> middle = int(np.ceil(a.shape[1]/2))
>>> a[:,:middle-1,:] + a[:,middle:,:]
array([[[ 8, 28],
        [12, 32]],

       [[10, 30],
        [14, 34]]], dtype=uint8)

which adds the left to the right but not symmetrically. This is what I'm hoping to get

array([[[10, 30],
        [10, 30]],

       [[12, 32],
        [12, 32]]], dtype=uint8)

Answer 1

Looks like you may invert the array, add and cut to the a.shape[1]//2 middle

(a + a[:,::-1,:])[:, :a.shape[1]//2, :]

---

array([[[10, 30],
        [10, 30]],

       [[12, 32],
        [12, 32]]])

Answer 2

A small modification to your code will work:

middle = int(np.ceil(a.shape[1]/2))
print(a[:,:middle-1,:] + a[:,:middle-1:-1,:])

The second addend is sliced differently here to reverse it. (Original was a[:,middle:,:] )

Result:

[[[10 30]
  [10 30]]

 [[12 32]
  [12 32]]]

Answer 3

You can either use backward slicing " [b:a:-1] "

i,j,k = a.shape
a[:,:j//2] + a[:,:(j-1)//2:-1]
# array([[[10, 30],
#         [10, 30]],
#
#        [[12, 32],
#         [12, 32]]])

Or to avoid the slightly error prone computation of the backward limits you can use np.fliplr

half = np.s_[:,:a.shape[1]//2]
a[half] + np.fliplr(a)[half]
# array([[[10, 30],
#         [10, 30]],
#
#        [[12, 32],
#         [12, 32]]])