How to add all items at index to new list

Question

I am currently trying to re-sort a list I got from parsing a website.

I have tried everything but I don't think I found the best solution to my problem.

Let's say we have the following list:

my_list = [['a', 'b', 'c'], ['a', 'b', 'c']]

What I am trying to convert it to:

new_list = [['a', 'a'], ['b', 'b'], ['c', 'c']]

I came up with the following loop:

result = [[], [], []]

for sublist in my_list:
    for i in range(0, len(sublist)):
        result[i].append(sublist[i])
print(result)
# output: [['a', 'a'], ['b', 'b'], ['c', 'c']]

My method is not the best I assume and I am searching for the most pythonic way to do it if you know what I'm saying.

use zip https://docs.python.org/3/library/functions.html#zip — Boris Verkhovskiy, Aug 15 '19 at 17:42
I recommend reading the Python tutorial https://docs.python.org/3/tutorial/ — Boris Verkhovskiy, Aug 15 '19 at 17:44
Do you want `[[1,2,3],[3,2,1]]` as `[[1,1],[2,2],[3,3]]` or `[[1,3],[2,2],[3,1]]` ? — Artog, Aug 15 '19 at 17:47
@Boris you even simpler: `list(zip(*my_list))` (though the output will be `tuple`s instead of `list`s) — pault, Aug 15 '19 at 17:48
I would go with @pault's recommendation so you don't run into index errors and it works if there are more than two sublists. — Anna Nevison, Aug 15 '19 at 17:57

score 0 · Accepted Answer · edited Jan 19 '21 at 21:05

0

The Python builtin function zip() is your friend here.

From the official documentation, it

returns an iterator of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables.

What that means is that given two lists, zip(list1, list2) will pair list1[0] with list2[0], list1[1] with list2[1], and so on.

In your case, the lists you want to zip together are inside another list, my_list, so you can unpack it with *my_list. Since zip() returns an iterator, you want to create a list out of the return value of zip(). Final solution:

new_list = list(zip(*my_list))

edited Jan 19 '21 at 21:05

Boris Verkhovskiy

14,854
11
100
103

answered Aug 15 '19 at 17:51

jeremye

1,368
9
19

1

Output will be: [('a', 'a'), ('b', 'b'), ('c', 'c')]. The questioner want to get lists instead of tuples. – milanbalazs Aug 15 '19 at 18:07

score 0 · Answer 2 · answered Aug 15 '19 at 17:51

0

I have written a code which exactly does what you want. You should use map after zip to convert tuples to lists.

Code:

my_list = [['a', 'b', 'c'], ['a', 'b', 'c']]
my_new_list = list(map(list, zip(my_list[0], my_list[1])))
print(my_new_list)

Output:

>>> python3 test.py 
[['a', 'a'], ['b', 'b'], ['c', 'c']]

answered Aug 15 '19 at 17:51

milanbalazs

4,811
4
23
45

equivalent list comp: `my_new_list = [list(t) for t in zip(my_list[0], my_list[1])]` – wjandrea Aug 15 '19 at 18:06
Yes, you are right. I don't see any advantage of list comp. I think both solutions are proper. :) – milanbalazs Aug 15 '19 at 18:09
1

Agreed, just wanted to mention it for OP's sake :) – wjandrea Aug 15 '19 at 18:10

How to add all items at index to new list

2 Answers2