CODEWITHSUNDEEP
python
Junior hpc
Junior hpc

Reputation: 169

Extract the n-th element from a list of list of list

I have a list of list of list and I would like to extract the n-th element from each sub-sub list. For example, given the following list:

my_list = [[[0, 0], [1, 1]], [[2, 0], [3, 2]], [[2, 0], [3, 3]], [[4, 0], [5, 4]], [[6, 0], [7, 5]]]

I want to extract all the first elements keeping the structure of the original list (list of list of list), like:

all_first_elements = [[[0],[1]], [[2],[3]], [[2],[3]], [[4],[5]], [[6],[7]]]

The problem is similar to this one but with one additional nested list.

I have tried all_first_elements = [item[0] for item in my_list] but it returns the first elements of the list of list (and not the first elements of list of list of list).

Upvotes: 2

Views: 227

Answers (5)

bharatk
bharatk

Reputation: 4315

The enumerate() function adds a counter to an iterable.

So for each element in a cursor, a tuple is produced with (counter, element).

my_list = [[[0, 0], [1, 1]], [[2, 0], [3, 2]], [[2, 0], [3, 3]], [[4, 0], [5, 4]], [[6, 0], [7, 5]]]

for sub_list in my_list:
    for index,elm in enumerate(sub_list):
        sub_list[index] = [elm[0]]

print(my_list)

O/P:

[[[0], [1]], [[2], [3]], [[2], [3]], [[4], [5]], [[6], [7]]]

Upvotes: 1

random74823
random74823

Reputation: 33

Here is a generic solution that you will not have to update if the nesting level changes:

def extract(data, level, index):
    result = []

    if level == 0:
        return data[index]

    for elem in data:
        result.append(extract(elem, level-1, index))

    return result

You will call it like this for the example provided in the original question:

print extract(my_list, level=2, index=0)

Please be aware that I have excluded data validation checks for the sake of clarity and because I was not sure how your data may vary. For example, the lists at the n-th level of depth may or may not be empty.

Upvotes: 0

Chris Adams
Chris Adams

Reputation: 18647

If you are open to using numpy, you can slice and index using:

import numpy as np

np.array(my_list)[:, :, [0]]

[out]

array([[[0],
        [1]],

       [[2],
        [3]],

       [[2],
        [3]],

       [[4],
        [5]],

       [[6],
        [7]]])

If you need the result as a list, just chain on the .tolist method:

np.array(my_list)[:, :, [0]].tolist()

[out]

[[[0], [1]], [[2], [3]], [[2], [3]], [[4], [5]], [[6], [7]]]

Upvotes: 1

Devesh Kumar Singh
Devesh Kumar Singh

Reputation: 20490

You can use a double for loop within a list-comprehension

all_first_elements = [[[item_1[0]] for item_1 in item_0] for item_0 in my_list]

The output will be

[[[0], [1]], [[2], [3]], [[2], [3]], [[4], [5]], [[6], [7]]]

Upvotes: 3

Arkistarvh Kltzuonstev
Arkistarvh Kltzuonstev

Reputation: 6920

Try this :

all_first_elements = [[[i[0][0]],[i[1][0]]] for i in my_list]

Output :

[[[0], [1]], [[2], [3]], [[2], [3]], [[4], [5]], [[6], [7]]]

Upvotes: 0

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