CODEWITHSUNDEEP
pythonnumpynumpy-slicing
Laplacian
Laplacian

Reputation: 117

Numpy multi-dimensional array slicing from end to first

From a numpy array

a=np.arange(100).reshape(10,10)

I want to obtain an array

[[99, 90, 91],
 [9, 0, 1],
 [19, 10, 11]]

I tried

a[[-1,0,1],[-1,0,1]]

but this instead gives array([99, 0, 11]). How can I solve this problem?

Upvotes: 1

Views: 56

Answers (3)

Tls Chris
Tls Chris

Reputation: 3824

Split the slicing into two seperate operations

arr[ [ -1,0,1] ][ :, [ -1,0,1]]
# array([[99., 90., 91.],
#        [ 9.,  0.,  1.],
#        [19., 10., 11.]])

Equivalent to:

temp = arr[ [ -1,0,1] ]  # Extract the rows
temp[ :, [ -1,0,1]]      # Extract the columns from those rows
# array([[99., 90., 91.],
#        [ 9.,  0.,  1.],
#        [19., 10., 11.]])

Upvotes: 1

Corralien
Corralien

Reputation: 120391

Roll your array over two axis and slice 3x3:

>>> np.roll(a, shift=1, axis=[0,1])[:3, :3]
array([[99, 90, 91],
       [ 9,  0,  1],
       [19, 10, 11]])

Upvotes: 2

Tejas Navalkhe
Tejas Navalkhe

Reputation: 21

a[[-1,0,1],[-1,0,1]] This is wrong, this means you want an elements from row -1, column -1 i.e (99) and row 0, column 0 i.e (0) and row 1, column 1 i.e (11) this is the reason you are getting array([99, 0, 11])

Your Answer:

a[ [[-1],[0],[1]], [-1,0,1] ]: This means, we want every element from column -1, 0, 1 from row [-1], [0], [1].

Upvotes: 1

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