How to sort each row of a 3D numpy array by another 2D array?

Question

I have a 2D numpy array of 2D points:

np.random.seed(0)   
a = np.random.rand(3, 4, 2) # each value is a 2D point

I would like to sort each row by the norm of every point

norms = np.linalg.norm(a, axis=2) # shape(3, 4)

indices = np.argsort(norms, axis=0) # indices of each sorted row

Now I would like to create an array with the same shape and values as a. that will have each row of 2D points sorted by their norm.

How can I achieve that?

I tried variations of np.take & np.take_along_axis but with no success.

for example:

np.take(a, indices, axis=1) # shape (3,3,4,2)

This samples a 3 times, once for each row in indices. I would like to sample a just once. each row in indices has the columns that should be sampled from the corresponding row.

I'll be happy to nominate for reopening and post an answer when you fix the question — Mad Physicist
– Mad Physicist, Commented Oct 7, 2020 at 1:46
@MadPhysicist I edited the question with an example and a clarification. hopefully it is clear now. — Elad Maimoni
– Elad Maimoni, Commented Oct 7, 2020 at 1:49
@NinaKaprez Is the posted answer not what you are looking for? if not, could you please post a sample input and desired output? — Ehsan
– Ehsan, Commented Oct 7, 2020 at 1:49
What about take along axis? Also, shouldn't your argsort have axis=1? Or do you intend to sort columns rather than rows? — Mad Physicist
– Mad Physicist, Commented Oct 7, 2020 at 1:50

Ehsan · Accepted Answer · 2020-10-07 01:46:20Z

5

If I understand you correctly, you want this:

norms = np.linalg.norm(a,axis=2) # shape(3,4)
indices = np.argsort(norms , axis=1)
np.take_along_axis(a, indices[:,:,None], axis=1)

output for your example:

[[[0.4236548  0.64589411]
  [0.60276338 0.54488318]
  [0.5488135  0.71518937]
  [0.43758721 0.891773  ]]

 [[0.07103606 0.0871293 ]
  [0.79172504 0.52889492]
  [0.96366276 0.38344152]
  [0.56804456 0.92559664]]

 [[0.0202184  0.83261985]
  [0.46147936 0.78052918]
  [0.77815675 0.87001215]
  [0.97861834 0.79915856]]]

answered Oct 7, 2020 at 1:46

Ehsan

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5 Comments

Elad Maimoni Over a year ago

no, the output should have the same shape as the variable 'a'.

Ehsan Over a year ago

@NinaKaprez It is the same shape. Is a not of shape (3,4,2)? If not, you would need to clarify better.

Elad Maimoni Over a year ago

@Eshan, my bad. I think this is what I need. thanks! can you elaborate what indices[::None] does?

Mad Physicist Over a year ago

@NinaKaprez. It adds a new dimension at the None index. None is np.newaxis.

Ehsan Over a year ago

@NinaKaprez You are welcome. Could not have said better than Mad Physicist's comment.

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