CODEWITHSUNDEEP
pythonlistcombinatorics
Utsav Vakil
Utsav Vakil

Reputation: 319

Get all pairwise combinations from a list

For example, if the input list is

[1, 2, 3, 4]

I want the output to be

[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

If possible, I would like a solution which is better than the brute force method of using two for loops. How do I implement this?

Upvotes: 31

Views: 30566

Answers (2)

MooingRawr
MooingRawr

Reputation: 4991

import itertools

x = [1,2,3,4]

for each in itertools.permutations(x,2):
    print(each)

Note that itertools is a generator object, meaning you need to iterate through it to get all you want. The '2' is optional, but it tells the function what's the number per combination you want.

You can read more here

Edited:

As ForceBru said in the comment, you can unpack the generator to print, skipping the for loop together But I would still iterate through it as you might not know how big the generated object will be:

print(*itertools.permutations(x, 2))

Upvotes: 6

brianpck
brianpck

Reputation: 8254

Though the previous answer will give you all pairwise orderings, the example expected result seems to imply that you want all unordered pairs.

This can be done with itertools.combinations:

>>> import itertools
>>> x = [1,2,3,4]
>>> list(itertools.combinations(x, 2))
[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

Compare to the other result:

>>> list(itertools.permutations(x, 2))
[(1, 2), (1, 3), (1, 4), (2, 1), (2, 3), (2, 4), (3, 1), (3, 2), (3, 4), (4, 1), (4, 2), (4, 3)]

Upvotes: 52

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