How to concatenate two lists so that elements are in alternative position?

Question

for example:

 a=[1,2,3,4,5,6]
 b=[7,8,9,10,11,12]

then result:

c=[1,7,2,8,3,9,4,10,5,11,6,12]

How do you concatenate two lists,so that the elements are in alternative positions??

i have tried to link them in to a new list and rearrange but its not coming. it would be nice if you could tell me the long way(without using built in functions too much).I m new to python and not much is taught in my school. Thank you.

a lot of answers have already been given here but I found a much simpler way to do this. `c = [None]*(len(a)+len(b)); c[::2]=a;c[1::2]=b` — Joe Ferndz, Nov 10 '20 at 03:41

score 6 · Accepted Answer · answered Feb 08 '15 at 15:50

6

Just append them with a for loop, assuming they're the same length:

c = []

for i in range(len(a)):
    c.append(a[i])
    c.append(b[i])

answered Feb 08 '15 at 15:50

Malik Brahimi

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1

enumerate would be a much better idea – Padraic Cunningham Feb 08 '15 at 16:06
1

He clearly said he was a beginner, otherwise I would use a better approach. – Malik Brahimi Feb 08 '15 at 16:14
3

beginner or not enumerate is the way to do it. – Padraic Cunningham Feb 08 '15 at 16:16

Padraic Cunningham · Answer 2 · 2015-02-08T16:14:26.533

4

for uneven size lists in python 3 you can use filter and zip_longest filtering None:

a = [1,2,3,4,5,6]
b = [7,8,9,10,11,12,13]

from itertools import chain, zip_longest

print(list(filter(None.__ne__ ,chain.from_iterable(zip_longest(a,b)))))
[1, 7, 2, 8, 3, 9, 4, 10, 5, 11, 6, 12, 13]

Using python2 use a list comp and filter None's with ele is not None:

print([ ele for ele in chain.from_iterable(zip_longest(a, b)) if ele is not None])

If you may have None as a value in your lists use a custom sentinel value, using that as the fillvalue in zip_longest:

my_sent = object()
print([ ele for ele in chain.from_iterable(zip_longest(a, b,fillvalue=my_sent)) if ele is not my_sent])

edited Feb 08 '15 at 16:14

answered Feb 08 '15 at 16:00

Padraic Cunningham

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I would recommend using a different sentinel value than `None`, for ex: `object()`. – Ashwini Chaudhary Feb 08 '15 at 16:11
@AshwiniChaudhary, yes if there were None's potentially in the list, I will add though. – Padraic Cunningham Feb 08 '15 at 16:12
`(None.__ne__ )` causes 'DeprecationWarning:'. Removing the `__ne__` works. – Tortenrandband Jul 09 '22 at 08:13

score 2 · Answer 3 · answered Feb 08 '15 at 15:50

2

Using zip and list comprehension:

>>> a = [1, 2, 3, 4, 5, 6]
>>> b = [7, 8, 9, 10, 11, 12]
>>> [x for xs in zip(a, b) for x in xs]
[1, 7, 2, 8, 3, 9, 4, 10, 5, 11, 6, 12]

answered Feb 08 '15 at 15:50

falsetru

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score 2 · Answer 4 · answered Feb 08 '15 at 15:51

2

>>> import itertools
>>> a = [1, 2, 3, 4, 5, 6]
>>> b = [7, 8, 9, 10, 11, 12]
>>> print list(itertools.chain.from_iterable(zip(a, b)))

answered Feb 08 '15 at 15:51

Ozgur Vatansever

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How to concatenate two lists so that elements are in alternative position?

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