combine lists to make one list that has each element a sum of the individual elements

Question

I have 3 lists:

['1','2']

['a','b','c']

['X','Y']

and the result I am looking to get:

['1aX','1bX','1cX','2aX','2bX','2cX','1aY','1bY','1cY','2aY','2bY','2cY']

is there a way to set this up quickly?

possible duplicate of [Get the cartesian product of a series of lists in Python](http://stackoverflow.com/questions/533905/get-the-cartesian-product-of-a-series-of-lists-in-python) — Felix Kling, Jul 26 '12 at 18:55
I believe this operation is called the cartesian product of sets. Felix is a ninja. — Wug, Jul 26 '12 at 18:55

score 10 · Answer 1 · answered Jul 26 '12 at 18:54

10

You can use itertools.product():

map("".join, itertools.product(list1, list2, list3))

answered Jul 26 '12 at 18:54

Sven Marnach

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The results will be in a different order than the OP specified though. (Which might not really matter of course.) – millimoose Jul 26 '12 at 18:55
@millimoose doubt order is important - if so, it would have been specified! – Jon Clements Jul 26 '12 at 19:05
Thanks for the awesome responses. I'm new to python and this forum. The order is actually important. But I already learned so much from what has been posted here, so I appreciate this answer. – user1555609 Jul 26 '12 at 19:40
@JonClements: *Should*, not *would*. :) – Sven Marnach Jul 26 '12 at 19:43

score 2 · Answer 2 · answered Jul 26 '12 at 19:19

2

>>> x,y,z=['1','2'],['a','b','c'],['x','y']
>>> s=[a+b+c for c in z for a in x for b in y]
>>> s
['1ax', '1bx', '1cx', '2ax', '2bx', '2cx', '1ay', '1by', '1cy', '2ay', '2by', '2cy']

This way you can choose the order you want

answered Jul 26 '12 at 19:19

elssar

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Andrew Clark · Answer 3 · 2012-07-26T19:10:09.163

To get the exact result that you specified, you can use the following:

import operator
import itertools

list1 = ['1','2']
list2 = ['a','b','c']
list3 = ['X','Y']
getter = operator.itemgetter(1, 2, 0)
result = [''.join(getter(seq)) for seq in itertools.product(list3, list1, list2)]

The final argument to itertools.product() will be the first to change, so based on your example output we want list2 to be last, then list1, and then list3, since first the middle element advances, then the first element, then the last. The operator.itemgetter() call is used to reorder the elements so that the element from list1 comes first, etc.

I find list comprehensions easier to read here, but here is a one-line alternative that uses map():

map(''.join, map(operator.itemgetter(1, 2, 0), itertools.product(list3, list1, list2)))

(You could do the list comprehension in one line as well, but you shouldn't because then the operator.itemgetter() call would be executed on each iteration)

combine lists to make one list that has each element a sum of the individual elements

3 Answers3