How to join lists within lists in Python based on a common element

Question

I have two lists of lists:

List1 = [['Jack','27','cat','house','fireman'], ['Tom','43','cat','Flat','Doctor'], ['Ben','30','Dog','house','Postman']]

List2 = [['Orange','5','Tom'], ['Blue','11','Ben'], ['Red','21','Jack']]

I want to join the lists together where the name matches:

Result_list = [['Jack','27','cat','house','fireman','Red','21','Jack'], ['Tom','43','cat','Flat','Doctor','Orange','5','Tom'], ['Ben','30','Dog','house','Postman','Blue','11','Ben']]

The names will always be at position [0] within the lists in list1 and [2] within the lists in list2

I'm am pretty new to python and cannot work out how to do this. Is anyone able to offer a solution?

Thanks

Rakesh · Accepted Answer · 2020-04-02 13:55:30Z

1

This is one approach by converting one of the list to a dict.

List1 = [['Jack','27','cat','house','fireman'], ['Tom','43','cat','Flat','Doctor'], ['Ben','30','Dog','house','Postman']]
List2 = [['Orange','5','Tom'], ['Blue','11','Ben'], ['Red','21','Jack']]

List2_d = {i[-1]: i for i in List2}
for j in List1:
    if j[0] in List2_d:
        j.extend(List2_d[j[0]])

print(List1)

Output:

[['Jack', '27', 'cat', 'house', 'fireman', 'Red', '21', 'Jack'],
 ['Tom', '43', 'cat', 'Flat', 'Doctor', 'Orange', '5', 'Tom'],
 ['Ben', '30', 'Dog', 'house', 'Postman', 'Blue', '11', 'Ben']]

answered Apr 2, 2020 at 13:55

Rakesh

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AyyAppAn Ajith · Accepted Answer · 2020-04-02 13:58:40Z

1

list1 = [['Jack','27','cat','house','fireman'], 
['Tom','43','cat','Flat','Doctor'], 
['Ben','30','Dog','house','Postman']]

list2 = [['Orange','5','Tom'], ['Blue','11','Ben'], ['Red','21','Jack']]

for i in list1:
    for j in list2:
        if i[0] == j[2]:
            i.extend(j)
print(list1)

output:

[['Jack', '27', 'cat', 'house', 'fireman', 'Red', '21', 'Jack'],
 ['Tom', '43', 'cat', 'Flat', 'Doctor', 'Orange', '5', 'Tom'],
 ['Ben', '30', 'Dog', 'house', 'Postman', 'Blue', '11', 'Ben']]

answered Apr 2, 2020 at 13:58

AyyAppAn Ajith

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Comments

Alain T. · Accepted Answer · 2020-04-02 17:36:30Z

1

You can do this using a dictionary as in index to List2 and build the merged list with a comprehension on List1 that concatenates the matching sublist in the dictionary:

List1 = [['Jack','27','cat','house','fireman'], ['Tom','43','cat','Flat','Doctor'], ['Ben','30','Dog','house','Postman']]
List2 = [['Orange','5','Tom'], ['Blue','11','Ben'], ['Red','21','Jack']]

match  = { sl[2]:sl for sl in List2 }
merged = [ sl+match[sl[0]] for sl in List1 ]

output:

print(merged)

[['Jack', '27', 'cat', 'house', 'fireman', 'Red', '21', 'Jack'],
 ['Tom', '43', 'cat', 'Flat', 'Doctor', 'Orange', '5', 'Tom'],
 ['Ben', '30', 'Dog', 'house', 'Postman', 'Blue', '11', 'Ben']]

answered Apr 2, 2020 at 17:36

Alain T.

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Comments

RoadRunner · Accepted Answer · 2020-04-02 14:02:18Z

You could use a defaultdict to group by the names:

from collections import defaultdict

List1 = [['Jack','27','cat','house','fireman'], ['Tom','43','cat','Flat','Doctor'], ['Ben','30','Dog','house','Postman']]

List2 = [['Orange','5','Tom'], ['Blue','11','Ben'], ['Red','21','Jack']]

d = defaultdict(list)
for lst in List1:
    d[lst[0]].extend(lst)

for lst in List2:
    d[lst[-1]].extend(lst)

print([v for v in d.values()])

Since both lists are of the same length, we can also get away with zip here:

d = defaultdict(list)

for x, y in zip(List1, List2):
    d[x[0]].extend(x)
    d[y[-1]].extend(y)

print([v for v in d.values()])

Output:

[['Jack', '27', 'cat', 'house', 'fireman', 'Red', '21', 'Jack'], ['Tom', '43', 'cat', 'Flat', 'Doctor', 'Orange', '5', 'Tom'], ['Ben', '30', 'Dog', 'house', 'Postman', 'Blue', '11', 'Ben']]

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