Compare lists within a python to list to find common elements

Question

consider a list of lists as given here.

many_lists = [[1,999,91,120,11], [909,800,7620,11], [1,101,800,7620]]

the output of this should be:

output_lst = [11, 1, 800, 7620]

I need to compare each list with the rest of the lists and if there is a common element then it should be in the output.

is there a simpler way to do this without two loops?

edit: (order within the output_list is not important)

Convert the lists to sets, intersect them, and convert the final result back to a list. — Barmar
– Barmar, Commented Jun 23, 2021 at 6:04
Does this answer your question? How to find common elements in list of lists? — user15801675
– user15801675, Commented Jun 23, 2021 at 6:06
@Barmar I don't think that answers this because the intersection of all lists is empty here and is not the expected output. — Mustafa Aydın
– Mustafa Aydın, Commented Jun 23, 2021 at 6:07
I see, it's the elements that are common to any pair of sublists, not all sublists. — Barmar
– Barmar, Commented Jun 23, 2021 at 6:09
@Barmar This is not a duplicate, OP wants to find items that appear in two lists at least, not all of them - and apparently in the order when the duplicates appear, but that should be made clear — Thierry Lathuille
– Thierry Lathuille, Commented Jun 23, 2021 at 6:09

Mustafa Aydın · Accepted Answer · 2021-06-23 06:15:36Z

2

One way is to run a counter over the flattened list and choose those that appeared more than once:

from collections import Counter
from itertools import chain

flattened = chain.from_iterable(many_lists)

result = [elem
          for elem, count in Counter(flattened).items()
          if count > 1]

to get

>>> result
[1, 11, 800, 7620]

edited Jun 23, 2021 at 6:15

answered Jun 23, 2021 at 6:13

Mustafa Aydın

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2 Comments

oskros Over a year ago

you can also flatten with sum(many_lists, [])

Mustafa Aydın Over a year ago

@oskros true, but that's an anti-pattern perhaps :)

Chandella07 · Accepted Answer · 2021-06-23 07:32:14Z

0

Without third loop, using set and intersaction:

many_lists = [[1,999,91,120,11], [909,800,7620,11], [1,101,800,7620]]

alist = []
for _ind, id in enumerate(many_lists):
    for j in many_lists[_ind+1:]:
        alist.append(list(set(id) & set(j)))
print(*alist)

answered Jun 23, 2021 at 7:32

Chandella07

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