Joining Lists of Lists in Python in a specific manner

Question

Given two very large lists (about 50K records) list1 = [['a','b','c'],['d','e','f']] and list2 = [['a','r','t'],['d','e','n']] . How to obtain list3 = [['a','b','c','r','t'],['d','e','f','e','n']]

Here I'm joining by comapring the first character of the second list's sublists and taking only one of them in final list

I am new to python I tried

i=0
final=[]
    while (i<len(list1)) :
        for row in list2 :
            if(list1[i][0]==list2[0]) :
                final= row + list[i][1:]

    i+=1

But this does not work

Hello and welcome! It looks like you want us to write some code for you. While many users are willing to produce code for a coder in distress, they usually only help when the poster has already tried to solve the problem on their own. A good way to demonstrate this effort is to include the code you've written so far, example input (if there is any), the expected output, and the output you actually get (console output, tracebacks, etc.). The more detail you provide, the more answers you are likely to receive. Check the FAQ and How to Ask. — MooingRawr
– MooingRawr, Commented May 16, 2017 at 19:43
Post your attempt and we'll tell you how to fix it or do it more efficiently. — timgeb
– timgeb, Commented May 16, 2017 at 19:44
Please also explain why the two lists in the result only have five elements each. — timgeb
– timgeb, Commented May 16, 2017 at 19:48

MooingRawr · Accepted Answer · 2017-05-16 20:30:02Z

1

list1 = [['a','b','c'],['d','e','f']]
list2 = [['a','r','t'],['d','e','n']]

print([x+y[1:] for x,y in zip(list1,list2)])

Using list comprehensions, list slicing, and zip() function this can be done rather quickly.

Edited based on comments, use an if case to only allow certain things through:

list1 = [['a','b','c'],['d','e','f']]
list2 = [['a','r','t'],['d','e','n']]

print([x+y[1:] for x,y in zip(list1,list2) if x[0] == y[0]])

edited May 16, 2017 at 20:30

answered May 16, 2017 at 19:48

MooingRawr

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4 Comments

Harshit Dhawan Over a year ago

Thank you, I am new here. And sorry for not posting my attempt before

Harshit Dhawan Over a year ago

What if I want to join using the condition that the first element of the sublists are same

MooingRawr Over a year ago

What happens if they aren't the same ?

Harshit Dhawan Over a year ago

It does not print them, loop continues

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