Numpy array of multiple indices replace with a different matrix

Question

I have an array of 2d indices.

indices = [[2,4], [6,77], [102,554]]

Now, I have a different 4-dimensional array, arr, and I want to only extract an array (it is an array, since it is 4-dimensional) with corresponding index in the indices array. It is equivalent to the following code.

for i in range(len(indices)):
    output[i] = arr[indices[i][0], indices[i][1]]

However, I realized that using explicit for-loop yields a slow result. Is there any built-in numpy API that I can utilized? At this point, I tried using np.choose, np.put, np.take, but did not succeed to yield what I wanted. Thank you!

Divakar · Accepted Answer · 2017-03-26 21:21:58Z

1

We need to index into the first two axes with the two columns from indices (thinking of it as an array).

Thus, simply convert to array and index, like so -

indices_arr = np.array(indices)
out = arr[indices_arr[:,0], indices_arr[:,1]]

Or we could extract those directly without converting to array and then index -

d0,d1 = [i[0] for i in indices], [i[1] for i in indices]
out = arr[d0,d1]

Another way to extract the elements would be with conversion to tuple, like so -

out = arr[tuple(indices_arr.T)]

If indices is already an array, skip the conversion process and use indices in places where we had indices_arr.

edited Mar 26, 2017 at 21:21

answered Mar 26, 2017 at 21:13

Divakar

222k19 gold badges273 silver badges374 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Average Algorithm Guy Over a year ago

Is the first method faster than using the explicit for-loop?

Divakar Over a year ago

@AverageAlgorithmGuy If you already have indices as an array, I would go with the first approach, else go with the second one.

Pang · Accepted Answer · 2017-10-24 00:39:40Z

0

Try using the take function of numpy arrays. Your code should be something like:

outputarray= np.take(arr,indices)

edited Oct 24, 2017 at 0:39

Pang

10.2k146 gold badges87 silver badges126 bronze badges

answered Mar 26, 2017 at 21:19

Naveen Kumar reddy kambham

2741 silver badge4 bronze badges

1 Comment

KevinOrr Over a year ago

From the numpy docs: "If indices is not one dimensional, the output also has these dimensions." That means it will return [[arr[indices[0][0]], arr[indices[0][1]], ...], [arr[indices[1][0]], arr[indices[1][1]], ...], ...]

Collectives™ on Stack Overflow

Numpy array of multiple indices replace with a different matrix

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related