How to merge lists with alternating elements?

Question

We have two lists list1 = [10,30,50], list2 = [2,4,6], and we want the output [10,2,30,4,50,6] (as opposed to [10,30,50,2,4,6], which is easier to generate). The lists could be very long so the manual solution list1[0] + list2[0] + list1[1] + ... is not feasible for us.

thanks but I would like to avoid implementing my own merge algorithm, i assume there's an easier solution in python — Canovice, Aug 24 '21 at 20:53
concatenate and sort, or use `zip()` those are basically your options — Gillespie, Aug 24 '21 at 20:55
Do you know that the lists can be sorted in this specific way, or could they be e.g. `[1,4,5]` and `[2,3,6]` and what should be the result in this case? — tobias_k, Aug 24 '21 at 20:57
no the lists cannot be sorted - this was a bad example using integers.. — Canovice, Aug 24 '21 at 20:59

John Kugelman · Accepted Answer · 2021-08-24T21:01:08.410

5

zip() grabs one item at a time from each list and pairs them up:

>>> list(zip(list1, list2))
[(1, 2), (3, 4), (5, 6)]

You can then use a second loop to flatten the pairs:

>>> [item for pair in zip(list1, list2) for item in pair]
[1, 2, 3, 4, 5, 6]

Note that if the lists are different lengths zip() will ignore the extra items in the longer one.

edited Aug 24 '21 at 21:01

answered Aug 24 '21 at 20:54

John Kugelman

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Prophet · Answer 2 · 2021-08-24T20:58:46.083

0

In case the lists are of integers, as appearing in the question you can simply combine the two lists, and then sort them

list1.extend(list2)
sorted(l1)

edited Aug 24 '21 at 20:58

answered Aug 24 '21 at 20:54

Prophet

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perhaps this is a bad example using integers because our actual list is of strings, the point is that the order of the lists matter. we cannot simply sort – Canovice Aug 24 '21 at 20:55
3

Well, you didn't mention that in the question. – Prophet Aug 24 '21 at 20:56

score 0 · Answer 3 · answered Aug 24 '21 at 20:57

0

def sortLists(list1, list2):
    return sorted(list1+list2)
print(sortLists([1,2,5], [3,4]))

The output would be:

[1,2,3,4,5]

answered Aug 24 '21 at 20:57

Arka Mukherjee

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How to merge lists with alternating elements?

3 Answers3