Manipulation of large lists in python

Question

Let's say that I have this list with 3 items and every item contains 3 more items:

list = [ [1,2,3], [4,5,6], [7,8,9] ]

I want to create a 3**2 = 9 list which contains all the combinations of the items, which means

combination of (x,y,z) with x in [1,4,7], y in [2,5,8] and z in [3,6,9], so I will use list comprehension like this:

new_list =[(x,y,z) for x in [1,2,3] for y in [4,5,6] for z in [7,8,9]]

This is a "manual" approach. But if I want to use in my code large lists which every time vary in length, let's say a 20D list (20 items and every item contains 20 more items), how can I create a generic type of code?

Kasravnd · Accepted Answer · 2016-02-13 20:14:06Z

4

You can use itertools.product but the result would be 3³ = 27 item :

>>> from itertools import product
>>> lst = [ [1,2,3], [4,5,6], [7,8,9] ]
>>> 
>>> list(product(*lst))
[(1, 4, 7), (1, 4, 8), (1, 4, 9), (1, 5, 7), (1, 5, 8), (1, 5, 9), (1, 6, 7), (1, 6, 8), (1, 6, 9), (2, 4, 7), (2, 4, 8), (2, 4, 9), (2, 5, 7), (2, 5, 8), (2, 5, 9), (2, 6, 7), (2, 6, 8), (2, 6, 9), (3, 4, 7), (3, 4, 8), (3, 4, 9), (3, 5, 7), (3, 5, 8), (3, 5, 9), (3, 6, 7), (3, 6, 8), (3, 6, 9)]

Note that when you are dealing with large datasets you don't need to convert the result to list. Since the product returns an iterator you can simple loop overt the products and access to items.

answered Feb 13, 2016 at 20:14

Kasravnd

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1 Comment

user3060854 Over a year ago

That was exactly what I was looking for, also you are right about the result, that would be 3**3=27.

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