How to merge elements of two lists at the same index in Python?

Question

im new to python i've just started. so im trying to merge two lists into each other in one big list. input:

A = [[a,1],[b,2],[c,3]]
B = [10,20,30]

desired output:

c = [[a,1,10],[b,2,20],[c,3,30]]

i tried insert method but it does not add B elements inside each individual list inside the A list and i tried c.extend([a,b]) but it gave me this:

[10,[a,1],20,[b,2],30,[c,3]]

what seems to be the problem because im really confused. thanks

zip them together and concat: `[l+[n] for l, n in zip(a, b)]` — Mark, May 20 '21 at 23:27
Welcome to Stack Overflow! Please take the [tour] and read [ask]. What did you try exactly? I'm really not sure how you got that output. Also, are the variables `a, b, c` meant to be strings? If so they should be in quotes. — wjandrea, May 20 '21 at 23:32
Actual duplicate: [Append list elements to list of lists in python](https://stackoverflow.com/q/45082010/4518341) @prune please switch it — wjandrea, May 20 '21 at 23:46

score 0 · Answer 1 · answered May 20 '21 at 23:30

If you want c to be independent of a, you'll need to make a deep copy, either with the standard library copy.deepcopy or by hand-coding:

c = [ai[:] for ai in a]

and then extend each element list in c with the corresponding value from b:

for ci, bi in zip(c,b):
    ci.append(bi)

which does rely on the structure being exactly as you illustrate: each element of a is a list, and each element of b is a value.

If you just say c=a or c=a.copy() to start with, the lists inside a will also be affected by the append actions.

score 0 · Accepted Answer · answered May 20 '21 at 23:34

0

One way to do this is to zip the input lists then add them:

a = [['a', 1], ['b', 2], ['c', 3]]
b = [10, 20, 30]

c = [x+[y] for x, y in zip(a, b)]
print(c)  # -> [['a', 1, 10], ['b', 2, 20], ['c', 3, 30]]

answered May 20 '21 at 23:34

wjandrea

23,210
7
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oubaydos · Answer 3 · 2021-05-20T23:42:58.810

-2

if you always have a list of lists in the first variable, and a list of numbers, chars... in the second you could use this code :

a = [["a",1],["b",2],["c",3]]
b = [10,20,30]

def mergeLists(e,f):
    counter = 0
    for i in e:
        i.append(f[counter])
#add this line : 
mergeLists(a,b)

now if you print a you will get : a=[['a', 1, 10], ['b', 2, 10], ['c', 3, 10]] what this code do is iterating through the list e and adding every item of f, note that the lists must have the same length

note that you could do the same thing if you want to work just with a and b directly by using for i, j in zip(a, b): i.append(j)

edited May 20 '21 at 23:42

answered May 20 '21 at 23:29

oubaydos

163
1
12

`counter` is never iterated. But actually, it'd be easier to use `zip` instead: `for i, j in zip(e, f): i.append(j)` – wjandrea May 20 '21 at 23:38

How to merge elements of two lists at the same index in Python?

3 Answers3