Insert list values into another list (python)

Question

I'm still pretty new at programming. I have a two Lists:

List1=[[1,2,3,4],[5,6,7,8],[9,10,11,12]]
List2=['a','b','c']

and I want to put the values of one list into the other one, so my output looks like this:

List1=[[1,2,3,4,'a'],[5,6,7,8,'b'],[9,10,11,12,'c']]

So far I managed to write this:

for i in range(0,len(List1)):
    for row in List1:
        row.insert(5, List2[i])

But i get this:

List1=[[1,2,3,4,'a','b','c'],[5,6,7,8,'a','b','c'],[9,10,11,12,'a','b','c']]

I'm sure it's probably a simple mistake, but I can't find out what it is.

for row in list:, what is list? also, you're missing a closing ). — PiCTo
– PiCTo, Commented Dec 6, 2019 at 18:55

Prune · Accepted Answer · 2019-12-06 18:58:44Z

2

Your posted code very explicitly adds every letter to each list. Instead, you want to add only the corresponding letter. You need one loop index for both lists, not nested loops.

for i in range(len(List1)):
    List1[i].append(List2[i])

answered Dec 6, 2019 at 18:58

Prune

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5 Comments

Santhosh Kumar Over a year ago

what a coincidence :o

Prune Over a year ago

I typed faster! :-)

Santhosh Kumar Over a year ago

Well I checked in python interpreter before comment :)

Lau Over a year ago

Thank you both for your answer! What if I want to choose the index where to insert List2?

Prune Over a year ago

Then you revert to insert, as in your original code.

LeoE · Accepted Answer · 2019-12-06 19:03:25Z

2

You can use zip for that:

List1=[[1,2,3,4],[5,6,7,8],[9,10,11,12]]
List2=['a','b','c']

List1 = [ a + list(b) for a, b in zip(List1, List2)]
print(List1)

Out:

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

Please be aware, that this creates a new list and does not change the old one. If lists mutability is used this code will not work!

edited Dec 6, 2019 at 19:03

answered Dec 6, 2019 at 18:58

LeoE

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3 Comments

PiCTo Over a year ago

This creates a third list, though (List3) instead of modifying List1, as asked by OP.

LeoE Over a year ago

Depending, if OP is working with the list as a mutable object it makes a difference, you are right, changed code, so that List1 is now there instead of List3, but might still cause problems, if list is supposed to be changed in function. Added info to answer, thanks

PiCTo Over a year ago

It might also be a problem in terms of performance as all the items of List1 are copied to a temporary value (expressed by your list comprehension) before the "old" list that List1 used to refer to is freed, in order to allow the assignment. This means that the values 1, 2, ... are unnecessarily copied.

Santhosh Kumar · Accepted Answer · 2019-12-06 18:59:23Z

1

You can use append to just add the ith indexed element to each element of List1

for i in range(len(List1)):
    List1[i].append(List2[i])

answered Dec 6, 2019 at 18:59

Santhosh Kumar

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Comments

PiCTo · Accepted Answer · 2019-12-06 18:58:31Z

0

What you're looking for is zip:

for row, char in zip(List1, List2):
    row.append(char)

which gives:

>>> List1
[[1, 2, 3, 4, 'a'], [5, 6, 7, 8, 'b'], [9, 10, 11, 12, 'c']]

answered Dec 6, 2019 at 18:58

PiCTo

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Comments

Mykola Zotko · Accepted Answer · 2019-12-06 19:45:36Z

0

You can use an iterator:

it = iter(List2)
for lst in List1:
    lst.append(next(it))

answered Dec 6, 2019 at 19:45

Mykola Zotko

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