How to join two lists based on the list element occurrence of one of the two using Python

Question

I have the following lists

list1 = [[x1,1,b1],[x2,1,b1],[x3,1,b1],[x4,1,b1]]

and the following

list2 = [[x1,0,b1],[x5,0,b1],[x2,0,b1],[x7,0,b1]]

I don't know how to create a final list like the

list3 = [[x1,1,b1],[x2,1,b1],[x3,1,b1],[x4,1,b1],[x5,0,b1],[x7,0,b1]]

To keep the first list1 and add to list1 elements from list2 only if the list2[0][0] element does not exist in list1

I tried something like the following with several combinations

for i in list1:
    for i2 in list2:
        if i[0][0] != i2[0][0]
            list3.append(i2)

But list3 displays elements which are common

`list3 = list1 +list2` should do it, after filtering `list2`. — SuperKogito, Apr 11 '19 at 20:09
This post may be of help for you https://stackoverflow.com/questions/2864842/common-elements-comparison-between-2-lists — GILO, Apr 11 '19 at 20:09

Jean-François Fabre · Accepted Answer · 2019-04-11T20:21:43.600

The logic of your attempt is wrong. Your double loop fails because when looping on all elements of both combined lists, the difference test has to be true at some point.

Let me propose a faster & working alternative:

Extract the first elements of each list1 sublists in a set for quick match.
Then create new list by adding list1 and the filtered elements of list2

Like this:

list1 = [['x1',1,'b1'],['x2',1,'b1'],['x3',1,'b1'],['x4',1,'b1']]
list2 = [['x1',0,'b1'],['x5',0,'b1'],['x2',0,'b1'],['x7',0,'b1']]

list_items = {l[0] for l in list1}

list3 = list1 + [l for l in list2 if l[0] not in list_items]

result:

>>> list3
[['x1', 1, 'b1'],
 ['x2', 1, 'b1'],
 ['x3', 1, 'b1'],
 ['x4', 1, 'b1'],
 ['x5', 0, 'b1'],
 ['x7', 0, 'b1']]

Thank you very much Jean bon nuit :) – Evridiki Apr 11 '19 at 20:21 — Evridiki, Apr 11 '19 at 20:21

andreihondrari · Answer 2 · 2019-04-11T20:15:23.170

0

You have to use sets and make a union to eliminate duplicates:

list1 = [[1,1,1], [1,1,2]]
list2 = [[1,1,3], [1,1,2]]

list1 = set([tuple(x) for x in list1])
list2 = set([tuple(x) for x in list2])

res = list1.union(list2)

this will yield {(1, 1, 3), (1, 1, 1), (1, 1, 2)}

and to have it as a list of lists you can do: [list(x) for x in res]

edited Apr 11 '19 at 20:15

answered Apr 11 '19 at 20:09

andreihondrari

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I like the idea and I was about to suggest a similar approach but the end result type for the OP is a list. So you still have to convert your final result to a list. – perennial_noob Apr 11 '19 at 20:13
@perennial_noob can be done with `[list(x) for x in res]` – andreihondrari Apr 11 '19 at 20:15

Mykola Zotko · Answer 3 · 2019-04-11T20:50:45.667

You can use a dictionary to merge two lists:

from itertools import chain

x1, x2, x3, x4, x5, x7 = 'x1', 'x2', 'x3', 'x4', 'x5', 'x7'
b1, b2, b3 = 'b1', 'b2', 'b3'

list1 = [[x1, 1, b1], [x2, 1, b1], [x3, 1, b1], [x4, 1, b1]]
list2 = [[x1, 0, b1], [x5, 0, b1], [x2, 0, b1], [x7, 0, b1]]

d = {i[0]: i for i in chain(list2, list1)}
sorted(d.values(), key=lambda x: x[0])
# [['x1', 1, 'b1'], ['x2', 1, 'b1'], ['x3', 1, 'b1'], ['x4', 1, 'b1'], ['x5', 0, 'b1'], ['x7', 0, 'b1']]

score -3 · Answer 4 · edited Apr 11 '19 at 23:52

-3

Append the lists together, so

list3.append(list2)
list3.append(list1)

edited Apr 11 '19 at 23:52

Ru Chern Chong

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answered Apr 11 '19 at 20:08

Taylor Liang

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How to join two lists based on the list element occurrence of one of the two using Python

4 Answers4