How can I find all ways of pairing elements between two lists of equal length?

Question

Suppose I have two input lists like:

list1 = [1, 2, 3]
list2 = [4, 5, 6]

I want to get a list of lists of tuples, of all the potential combinations between the two – i.e., of ways to pair all the elements up. Thus, the result should be like:

[
    [(1, 4), (2, 5), (3, 6)],
    [(1, 4), (2, 6), (3, 5)],
    [(1, 5), (2, 4), (3, 6)],
    [(1, 5), (2, 6), (3, 4)],
    [(1, 6), (2, 4), (3, 5)],
    [(1, 6), (2, 5), (3, 4)]
]

That is: the first list shows the 1 being paired with the 4, the 2 with the 5, and the 3 with the 6. There are 6 lists, corresponding to all the ways that the elements from list1 could be associated with the elements of list2.

I think the itertools standard library should be helpful here, but I couldn’t figure out a solution. I got this far:

list1 = [1, 2, 3]
list2 = [5, 6, 7]
print(list(itertools.product(list1, list2)))

but the result is:

[(1, 5), (1, 6), (1, 7), (2, 5), (2, 6), (2, 7), (3, 5), (3, 6), (3, 7)]

It shows every possible pairing of an element from list1 with an element of list2, but not the possible resulting lists composed from these pairing.

How can I fix the code?

Asked By: GeoJunkie

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Source

Answer 1

Edited my code to give you your desired output.

list1 = [1,2]
list2 = [3,4]
combined = []

for a in list1:
    new_list = []
    for b in list2:
        new_list.append([a, b])
    combined.append(new_list)

print combined

Answered By: Joe Smart

Answer 2

You can create a list by constructing all the permutations of two list members with this, containing the list combinations.

lst1 = [1,2]
lst2 = [3,4]

#lst = [[j,k] for j in lst1 for k in lst2] # [[1,3],[1,4],[2,3],[2,4]]
lst = [[[j,k] for j in lst1] for k in lst2] # [[[1,3],[2,3]],[[1,4],[2,4]]]
print lst

Answered By: Semih Yagcioglu

Answer 3

Try this:

combos=[]
for i in list1:
      for j in list2:
          combos.append([i,j])
print combos

Answered By: Luke B

Answer 4

repeat the first list, permutate the second and zip it all together

>>> from itertools import permutations, repeat
>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> list(list(zip(r, p)) for (r, p) in zip(repeat(a), permutations(b)))
[[(1, 4), (2, 5), (3, 6)],
 [(1, 4), (2, 6), (3, 5)],
 [(1, 5), (2, 4), (3, 6)],
 [(1, 5), (2, 6), (3, 4)],
 [(1, 6), (2, 4), (3, 5)],
 [(1, 6), (2, 5), (3, 4)]]

EDIT: As Peter Otten noted, the inner zip and the repeat are superfluous.

[list(zip(a, p)) for p in permutations(b)]

Answered By: pacholik

Answer 5

Try to use list generator to create the nested lists:

>>> [[[x,y] for x in list1] for y in list2]
[[[1, 3], [2, 3]], [[1, 4], [2, 4]]]
>>>

Or, if you want one-line list, just delete brackets:

>>> [[x,y] for x in list1 for y in list2]
[[1, 3], [1, 4], [2, 3], [2, 4]]

Answered By: Sdwdaw

Answer 6

The accepted answer can be simplified to

a = [1, 2, 3]
b = [4, 5, 6]
[list(zip(a, p)) for p in permutations(b)]

(The list() call can be omitted in Python 2)

Answered By: Peter Otten

Answer 7

As @pacholik s answer does not cover lists of different length, here is my solution, using a list comprehension with two variables:

first_list = [1, 2, 3]
second_list = ['a', 'b']

combinations = [(a,b) for a in first_list for b in second_list]

The output looks like this:

[(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'b')]

Answered By: René Jahn

How can I find all ways of pairing elements between two lists of equal length?

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