1

For example if i have:

list1 = [1, 2, 3]
list2 = [4, 5, 6, 7, 8, 9, 10, 11, 12]

How to add elements from list1 to list2 in a loop until list2 is over, where output becomes:

list3 = [1, 4, 2, 5, 3, 6, 1, 7, 2, 8, 3, 9, 1, 10, 2, 11, 3, 12]
wjandrea
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Adryan Ziegler
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  • If you are using numpy, there exists much cleaner solutions by interleaving the two arrays: https://stackoverflow.com/questions/5347065/interweaving-two-numpy-arrays – Kevin May 08 '21 at 15:03

3 Answers3

2

You can use itertools.cycle to cycle through the first list and zip the 2 lists together:

>>> from itertools import cycle
>>> [i for pair in zip(cycle(list1), list2) for i in pair]
[1, 4, 2, 5, 3, 6, 1, 7, 2, 8, 3, 9, 1, 10, 2, 11, 3, 12]

If you want to know what's going on, you can check the intermediate result:

>>> list(zip(cycle(list1), list2))
[(1, 4), (2, 5), (3, 6), (1, 7), (2, 8), (3, 9), (1, 10), (2, 11), (3, 12)]

You can remove the list comprehension and do the same thing with just builtin functions:

>>> from itertools import cycle, chain
>>> list(chain(*zip(cycle(list1), list2)))
[1, 4, 2, 5, 3, 6, 1, 7, 2, 8, 3, 9, 1, 10, 2, 11, 3, 12]

If you really want to do it in the long way:

>>> list3 = []
>>> for pair in zip(cycle(list1), list2):
...     print(f'{pair=}') # just for understanding what's going on
...     for i in pair:
...         list3.append(i)
...
pair=(1, 4)
pair=(2, 5)
pair=(3, 6)
pair=(1, 7)
pair=(2, 8)
pair=(3, 9)
pair=(1, 10)
pair=(2, 11)
pair=(3, 12)
>>> list3
[1, 4, 2, 5, 3, 6, 1, 7, 2, 8, 3, 9, 1, 10, 2, 11, 3, 12]
Asocia
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    i feel this is what i am looking for but i dont quite understand the loop format used, could it be simplified/extended? – Adryan Ziegler May 08 '21 at 15:09
  • The loop in here is called "list comprehension". It's so common in python and not that different from regular loops. `my_list = [i for i in range(10)]` is same as `my_list = []; for i in range(10): my_list.append(i)` – Asocia May 08 '21 at 15:13
  • @AdryanZiegler Edited my post. You may find it easier to understand now. – Asocia May 08 '21 at 15:27
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    FWIW, you can do `list3.extend(pair)` instead of `for i in pair: list3.append(i)` – wjandrea May 08 '21 at 15:32
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    @wjandrea Yeah, I also noticed it while writing. But decided to keep it like this because it's expanded version of list comprehension. – Asocia May 08 '21 at 15:34
0

You can interleave the two arrays by doing the following:

import numpy as np

list1_tiled = np.tile([1, 2, 3], 3)
list2 = np.array([4, 5, 6, 7, 8, 9, 10, 11, 12])

np.ravel(np.column_stack((list1_tiled, list2)))

Output:

array([ 1,  4,  2,  5,  3,  6,  1,  7,  2,  8,  3,  9,  1, 10,  2, 11,  3, 12])
Kevin
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0

If you want a really simple, no import solution, you can just continuously rotate out the first element in your first list.

list1 = [1, 2, 3]
list2 = [4 ,5 ,6, 7, 8, 9, 10, 11, 12]

list3 = []
for i in list2:
    list3.extend([list1[0], i])
    # this pops the first element from your list 
    # and appends it to the end.
    list1.append(list1.pop(0))

print(list3)

This essentially makes list1 an infinitely rotating list, sort of like collections.deque.

Axe319
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