CODEWITHSUNDEEP
pythonlistcombinations
Student NL
Student NL

Reputation: 429

Combinations of a list of list - Elementwise

I am looking for a method to generate all possible combinations of a list of lists, under the condition that there should only be "elementwise" combinations. Thus, if we have the lists [1,2] and [3,4], then the outcome [3,2] is allowed, but [4,2] is not allowed. With itertools.product(*lists) the last outcome is included.

Thus, I want the following output: [1,2], [3,2], [1,4], [3,4], and the following options should be 'skipped': [4,2], [1,3]. Note that the order is important! Thus I do not allow [2,3], just [3,2].

I know that I can check for this afterwards, but since I am generating many many lists in my code, I rather avoid this.

Upvotes: 1

Views: 435

Answers (1)

user5407570
user5407570

Reputation:

You could store the two lists in one list of lists and then after transposing the container list, use itertools.product() on it.

import itertools
original_list =  [[1,2], [3,4]]
transposed_list = list(map(list, zip(*original_list)))
print(list(itertools.product(*transposed_list)))

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

EDIT | Explaining how the list was transposed:

By definition, transposing is the process of exchanging places.

*original_list means [1,2] and [3,4]. The asterisk refers to the elements of the list, rather than the list as a whole

zip basically pairs values together 'element-wise': e.g. with our original_list we have [1,2] and [3,4]. Calling zip on our elements will result in (1,3) and (2,4). The values were paired element-wise. Note that the resulting pairs are not in list form.

map applies a function to every element in an input list. In our example, we want to apply the (built-in) list function to our element-wise pairs - i.e. convert each tuple of pairs into a list. This is what turns (1,3) and (2,4) into [1,3] and [2,4]

Finally, convert our result from the mapping into a containing list, holding our element-wise pairs, producing [ [1,3], [2,4] ]

Upvotes: 3

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