Taking the union of 2 lists of lists in Python

May 16, 2022

I have 2 list of lists.

One is say list1, which is :

[['1', '2', '*', '2', '1', '0.8'],
 ['1', '2', '3', '1', '1', '0.7'],
 ['*', '*', '3', '4', '1', '0.5'],
 ['1', '2', '*', '1', '1', '0.3'],
 ['2', '2', '*', '2', '1', '0.1']]

And list2 is :

[['3', '*', '1', '4', '1', '0.9'],
 ['1', '2', '2', '2', '1', '0.4'],
 ['2', '2', '*', '2', '1', '0.1']]

Now I want to get the union of these 2 list of lists and create a 3rd list of list and as ['2', '2', '*', '2', '1', '0.1'] this list is there in both of them so I want this to be in the final list of list for only 1 time.

I am doing:

final_list=list1 + list2

Final list is producing :

[['3', '*', '1', '4', '1', '0.9'],
 ['1', '2', '*', '2', '1', '0.8'],
 ['1', '2', '3', '1', '1', '0.7'],
 ['*', '*', '3', '4', '1', '0.5'],
 ['1', '2', '2', '2', '1', '0.4'],
 ['1', '2', '*', '1', '1', '0.3'],
 ['2', '2', '*', '2', '1', '0.1'],
 ['2', '2', '*', '2', '1', '0.1']]

My desired outcome is :

[['3', '*', '1', '4', '1', '0.9'],
 ['1', '2', '*', '2', '1', '0.8'],
 ['1', '2', '3', '1', '1', '0.7'],
 ['*', '*', '3', '4', '1', '0.5'],
 ['1', '2', '2', '2', '1', '0.4'],
 ['1', '2', '*', '1', '1', '0.3'],
 ['2', '2', '*', '2', '1', '0.1']]

>Solution :

You should be able to create a set, add each element of each list to that set, then create a list from that set, something like:

new_set = set()
for item in list1:
    set.add(item)
for item in list2:
    set.add(item)
final_list = list(set)

It won’t be guaranteed to be in order but it will have only unique elements. Of course, if you don’t care about order, you may as well be using sets exclusively so you don’t have to concern yourself with duplicates.

If you want to maintain order, you just maintain both a final list and the set, and only add to the final list if it’s not already in the set:

dupe_set = set()
final_list = []
for item in list1:
    if item not in dupe_set:
        set.add(item)
        final_list.append(item)
for item in list1:
    if item not in dupe_set:
        set.add(item)
        final_list.append(item)