How to get all possible combinations of a list’s elements?


Question

I have a list with 15 numbers in, and I need to write some code that produces all 32,768 combinations of those numbers.

I've found some code (by Googling) that apparently does what I'm looking for, but I found the code fairly opaque and am wary of using it. Plus I have a feeling there must be a more elegant solution.

The only thing that occurs to me would be to just loop through the decimal integers 1–32768 and convert those to binary, and use the binary representation as a filter to pick out the appropriate numbers.

Does anyone know of a better way? Using map(), maybe?

1
345
10/25/2016 4:20:47 PM

Accepted Answer

Have a look at itertools.combinations:

itertools.combinations(iterable, r)

Return r length subsequences of elements from the input iterable.

Combinations are emitted in lexicographic sort order. So, if the input iterable is sorted, the combination tuples will be produced in sorted order.

Since 2.6, batteries are included!

381
1/21/2009 12:52:47 PM

This answer missed one aspect: the OP asked for ALL combinations... not just combinations of length "r".

So you'd either have to loop through all lengths "L":

import itertools

stuff = [1, 2, 3]
for L in range(0, len(stuff)+1):
    for subset in itertools.combinations(stuff, L):
        print(subset)

Or -- if you want to get snazzy (or bend the brain of whoever reads your code after you) -- you can generate the chain of "combinations()" generators, and iterate through that:

from itertools import chain, combinations
def all_subsets(ss):
    return chain(*map(lambda x: combinations(ss, x), range(0, len(ss)+1)))

for subset in all_subsets(stuff):
    print(subset)

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