Python: converting multi-dimensional numpy arrays into list of arrays

Question

Suppose I have 2D array, for simplicity, a=np.array([[1,2],[3,4]]). I want to convert it to list of arrays, so that the result would be:

b=[np.array([1,2]), np.array([3,4])]

I found out that there is np.ndarray.tolist() function, but it converts N-D array into nested list. I could have done in a for loop (using append method), but it is not efficient/elegant.

In my real example I am working with 2D arrays of approximately 10000 x 50 elements and I want list that contains 50 one dimensional arrays, each of the shape (10000,).

To iterate on the 2nd dimension, you may want to transpose the array first. — hpaulj, Aug 20 '18 at 15:38

score 5 · Accepted Answer · answered Aug 20 '18 at 14:18

5

How about using list:

a=np.array([[1,2],[3,4]])
b = list(a)

answered Aug 20 '18 at 14:18

jh314

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Wow. This is the shortest possible solution I guess :) – Sheldore Aug 20 '18 at 14:20

score 3 · Answer 2 · answered Aug 20 '18 at 14:17

3

Why don't you use list comprehension as follows without using any append:

a=np.array([[1,2],[3,4]])
b = [i for i in a]
print (b)

Output

[array([1, 2]), array([3, 4])]

answered Aug 20 '18 at 14:17

Sheldore

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`[i for i in a]` is a verbose way of sayin `list(a)` – juanpa.arrivillaga Aug 20 '18 at 14:19
Hmm you are right. But I guess in terms of timings, they shouldn't be different. Under the hood runs the same things IMO – Sheldore Aug 20 '18 at 14:20
about the same, but no they do not. `list` will be faster. – juanpa.arrivillaga Aug 20 '18 at 14:21
That's the beauty of SO. You get to know different and more efficient approaches. Thanks for the comment :) – Sheldore Aug 20 '18 at 14:22
check out [this question](https://stackoverflow.com/questions/40018398/list-uses-more-memory-than-list-comprehension) for some gory details – juanpa.arrivillaga Aug 20 '18 at 14:25
Thanks a lot for the interesting link. It seems the time difference becomes noticeable once the size crosses the order of 10**5. – Sheldore Aug 20 '18 at 14:28

Python: converting multi-dimensional numpy arrays into list of arrays

2 Answers2