All list combinations in Python

Question

For example I have an array, that looks like this [[1, 2, 3], [4, 5], [7, 8, 9]]

My mission is too show all combinations in the array, like this:

1-4-7, 1-4-8, 1-4-9

1-5-7, 1-5-8, 1-5-9

2-4-7, 2-4-8, 2-4-9

2-5-7, 2-5-8, 2-5-9

3-4-7, 3-4-8, 3-4-9

3-5-7, 3-5-8, 3-5-9

I have seen many different solutions, like this - How to get all possible combinations of a list’s elements?, but it isn't for me

Add-on: Array can't be mutated

why doesn't itertools.product work - it is exactly what you need surely. — Tony Suffolk 66, Jun 28 '21 at 09:35
Does this answer your question? [pick combinations from multiple lists](https://stackoverflow.com/questions/15305719/pick-combinations-from-multiple-lists) — MartijnVanAttekum, Jun 28 '21 at 09:38

user2390182 · Accepted Answer · 2021-06-28T09:38:17.660

4

Since you are looking for the cartesian product of the lists, use itertools.product:

from itertools import product

data = [[1, 2, 3], [4, 5], [7, 8, 9]]

for p in product(*data):
    print("-".join(map(str, p)))

1-4-7
1-4-8
1-4-9
1-5-7
# ...
3-4-9
3-5-7
3-5-8
3-5-9

edited Jun 28 '21 at 09:38

answered Jun 28 '21 at 09:35

user2390182

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this is clear duplicate of https://stackoverflow.com/q/533905/4046632 – buran Jun 28 '21 at 09:37
Yeah, it is, thanks – 9gress0r Jun 28 '21 at 09:38
@buran So mark it as such. You might throw in the output structure with dashes which could be this q's usp :) – user2390182 Jun 28 '21 at 09:40
1

I did, but with 56К reputation you should do so too – buran Jun 28 '21 at 09:43
True. Sometimes a quick answer is easier than a search for the best dupe. It will still be duped and the answer doesn't harm the quality of the site content. – user2390182 Jun 28 '21 at 09:46

score -1 · Answer 2 · answered Jun 28 '21 at 09:43

Something like the following should work, if you don't want to use itertools like the linked answer did:

First, maintain a list of indices, let's call it positions, that represent what elements you're currently looking at in each of the inner lists, and initialise all of these indices to 0. In your example given this would look like [0,0,0]

Print out / store the combination that results from these indices. Here that would be 1, 4, 7.

Increment the last index in the list. If this index is now the size of the list it corresponds to (in the example, for the last index, that would be 3, for the second last, 2, etc.) then reset it to 0 and increment the next index along instead. Do this check for all the values in positions

Repeat this process, printing out / storing the resulting combination each time, until the very first index in positions reaches the size of the first list in your list of lists, at which point you're done

All list combinations in Python

2 Answers2