How to zip the elements of a single nested list

Question

Consider a nested list:

d = [[1,2,3],[4,5,6]]

I want to zip its elements for this result:

 [[1,4],[2,5],[3,6]]

How to do that? An incorrect approach is

list(zip(d))

But that gives:

[([1, 2, 3],), ([4, 5, 6],)]

What is the correct way to do the zip ?

Does this answer your question? [How to zip lists in a list](https://stackoverflow.com/questions/4112265/how-to-zip-lists-in-a-list) — Lakshya Srivastava, Feb 25 '20 at 19:29
this might be useful : https://stackoverflow.com/questions/4112265/how-to-zip-lists-in-a-list — Sajan, Feb 25 '20 at 19:29
What do you mean by "nuked"? Everything is alright, isnt it? :) — tim, Feb 25 '20 at 19:31
@tim I clicked on that time-piece icon under the check-mark of one answer and Momentarily _all_ answers disappeared! Freaked me out. I opened this page in a new tab and things were back. What's up with that? — WestCoastProjects, Feb 25 '20 at 19:35
Not so sure :) You should have rather clicked the "Accept Answer" on mine before this question got closed. But dont worry :) — tim, Feb 25 '20 at 19:37
i'm waiting another 10 seconds for min wait to accept your answer. That's why i had clicked on the time-piece: maybe it would tell me how much longer i needed to wait to accept Instead it nuked both answers from my page. — WestCoastProjects, Feb 25 '20 at 19:38
The same way as if you were calling *anything else* besides `zip`. You have a list of arguments; you want them to be used as separate arguments for the call; that is exactly what `*` does. — Karl Knechtel, Aug 29 '22 at 13:33

tim · Accepted Answer · 2020-02-25T19:29:52.660

4

You need to give the single sub-lists via unpacking (*) as single arguments to zip() like this:

d = [[1,2,3],[4,5,6]]          
zip(*d)  # You need this one
[(1, 4), (2, 5), (3, 6)]

This even works for longer lists, in case this is the behaviour you want:

zip(*[[1,2,3],[4,5,6],[7,8,9]])
[(1, 4, 7), (2, 5, 8), (3, 6, 9)]

If you want to have a list of lists instead of a list of tuples, just do this:

map(list, zip(*d))
[[1, 4], [2, 5], [3, 6]]

edited Feb 25 '20 at 19:29

answered Feb 25 '20 at 19:27

tim

1

i originally tested with `d = [a,b]` and tried `d=[*a,*b]` . that did not work - but I might have happened upon this answer if had originally built it the way in the question ;) – WestCoastProjects Feb 25 '20 at 19:29
1

`zip(*d)` returned `[(1, 4), (2, 5), (3, 6)]` in good old python2. With python3 you get `` – Friedrich -- Слава Україні Feb 25 '20 at 19:47
1

@Friedrich yea needing to drop `list(g)` on any generator `g` is annoying. I put a helper `def lmap(fn,x): return list(map(fn,x))` just to avoid seeing `list(map(...))` . too tiring to read – WestCoastProjects Feb 25 '20 at 20:22
1

@javadba A bit evil, I use `xzip = zip; zip = lambda *x: list(xzip(*x))`. This gives old python2 behaviour for python2 and 3 . – Friedrich -- Слава Україні Feb 25 '20 at 20:38

score 2 · Answer 2 · answered Feb 25 '20 at 19:28

2

This will do the trick:

list(zip(*d))

answered Feb 25 '20 at 19:28

score 1 · Answer 3 · answered Feb 25 '20 at 19:28

1

you should unpack d before zipping it:

list(zip(*d))

The output is a list of tuples, as follows:

[(1, 4), (2, 5), (3, 6)]

I hope this fits you well.

answered Feb 25 '20 at 19:28

bqbastos

3 Answers3