CODEWITHSUNDEEP
pythonlistmerge
user47467
user47467

Reputation: 1093

Merging lists of lists

I have two lists of lists that have equivalent numbers of items. The two lists look like this:

L1 = [[1, 2], [3, 4], [5, 6, 7]]

L2 =[[a, b], [c, d], [e, f, g]]

I am looking to create one list that looks like this:

Lmerge = [[[a, 1], [b,2]], [[c,3], [d,4]], [[e,5], [f,6], [g,7]]]

I was attempting to use map() :

map(list.__add__, L1, L2) but the output produces a flat list.

What is the best way to combine two lists of lists? Thanks in advance.

Upvotes: 4

Views: 166

Answers (2)

arekolek
arekolek

Reputation: 9610

You were on the right track with the map.

Here's a shorter alternative to the first version by tobias_k, zip together corresponding elements from both lists:

>>> zipped = map(zip, L2, L1)
>>> list(map(list, zipped)) # evaluate to a list of lists
[[('a', 1), ('b', 2)], [('c', 3), ('d', 4)], [('e', 5), ('f', 6), ('g', 7)]]

As noted in the comments, in Python 2, the brief map(zip, L2, L1) is enough.

map(zip, L2, L1) will work for you in Python 3 too, given that you iterate over it just once and that you don't need sequence access by index. And if you need to iterate many times, you may be interested in itertools.tee.

A shorter alternative to the second version:

>>> [list(map(list, x)) for x in map(zip, L2, L1)]
[[['a', 1], ['b', 2]], [['c', 3], ['d', 4]], [['e', 5], ['f', 6], ['g', 7]]]

Lastly, there's also this:

>>> from functools import partial
>>> map(partial(map, list), map(zip, L2, L1))
[[['a', 1], ['b', 2]], [['c', 3], ['d', 4]], [['e', 5], ['f', 6], ['g', 7]]]

Upvotes: 3

tobias_k
tobias_k

Reputation: 82889

You can zip the lists and then zip the resulting tuples again...

>>> L1 = [[1, 2], [3, 4], [5, 6, 7]]
>>> L2 =[['a', 'b'], ['c', 'd'], ['e', 'f', 'g']]
>>> [list(zip(a,b)) for a,b in zip(L2, L1)]
[[('a', 1), ('b', 2)], [('c', 3), ('d', 4)], [('e', 5), ('f', 6), ('g', 7)]]

If you need lists all the way down, combine with `map:

>>> [list(map(list, zip(a,b))) for a,b in zip(L2, L1)]
[[['a', 1], ['b', 2]], [['c', 3], ['d', 4]], [['e', 5], ['f', 6], ['g', 7]]]

Upvotes: 7

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