Append list value by turns python

Question

So I'm trying to append list value to another one by turns. This is my code:

lis1 = [1, 2, 3, 4, 5]
lis2 = [6, 7, 8, 9, 10]
for i, value in enumerate(lis2):
  lis1.append(i)
print(lis1)

My expected output is [1, 6, 2, 7, 3, 8, 4, 9, 5, 10]

But what I got is [1, 2, 3, 4, 5, 0, 1, 2, 3, 4]

Any help would be appreciated

With minimal changes you can reach desired result. In for loop body (.append row) just replace it with `lis1.insert(i*2 + 1, value)`. — Aivar Paalberg, Sep 18 '20 at 09:46

Konrad Rudolph · Accepted Answer · 2020-09-18T10:12:29.633

2

You want to use zip for this. That generates a list of tuples, and to flatten that you can use itertools.chain:

import itertools

list(zip(lis1, lis2))
# [(1, 6), (2, 7), (3, 8), (4, 9), (5, 10)]

list(itertools.chain(*zip(lis1, lis2)))
# [1, 6, 2, 7, 3, 8, 4, 9, 5, 10]

# or:
list(itertools.chain.from_iterable(zip(lis1, lis2)))

edited Sep 18 '20 at 10:12

answered Sep 18 '20 at 09:28

Konrad Rudolph

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1

Or `[x for pair in zip(lis1, lis2) for x in pair]` – tobias_k Sep 18 '20 at 09:29
2

`chain(*whatever)` -> `chain.from_iterable(whatever)` – juanpa.arrivillaga Sep 18 '20 at 09:39
@juanpa.arrivillaga That’s nice, but I don’t actually understand what the benefit is: it’s longer, it’s not more readable (is it?). — Is it substantially more efficient? – Konrad Rudolph Sep 18 '20 at 09:51
1

unpacking requires creating a tuple and passing that as an argument under the hood. – juanpa.arrivillaga Sep 18 '20 at 09:58
1

FYI, `%timeit` for list-comp, chain, and chain.from_iterable: 720, 680, and 510 ns per loop – tobias_k Sep 18 '20 at 10:11

score 1 · Answer 2 · answered Sep 18 '20 at 09:32

1

You could do it like this:

out = []
for x in range(len(lis1)):
    out.append(lis1[x])
    out.append(lis2[x])

Output

[1, 6, 2, 7, 3, 8, 4, 9, 5, 10]

answered Sep 18 '20 at 09:32

Sebastien D

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4
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score 1 · Answer 3 · answered Sep 18 '20 at 09:33

1

If you don't want to use zip from itertools, you can use list.extend() as followings:

lis1 = [1, 2, 3, 4, 5]
lis2 = [6, 7, 8, 9, 10]
new_list = []

for index, value in enumerate(lis1):
  new_list.extend((lis1[index],lis2[index]))

print(new_list)

answered Sep 18 '20 at 09:33

Harun Yilmaz

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Append list value by turns python

3 Answers3