Python combinations with list and items in other lists

Question

I try the code below, is there a efficent way to do this?

c = []
l = [['A1','A2'], ['B1','B2'],  ['C1','C2'] ]

for i in range(0, len(l) - 1):
    for j in range(i+1, len(l)): 
        c.append(sorted([l[i][0],l[i][1],l[j][0]]))
        c.append(sorted([l[i][0],l[i][1],l[j][1]]))
        c.append(sorted([l[i][0],l[j][0],l[j][1]]))
        c.append(sorted([l[i][1],l[j][0],l[j][1]]))

print(c)

Out put:

[['A1', 'A2', 'B1'], ['A1', 'A2', 'B2'], ['A1', 'B1', 'B2'],
['A2', 'B1', 'B2'], ['A1', 'A2', 'C1'], ['A1', 'A2', 'C2'], 
['A1', 'C1', 'C2'], ['A2', 'C1', 'C2'], ['B1', 'B2', 'C1'], 
['B1', 'B2', 'C2'], ['B1', 'C1', 'C2'], ['B2', 'C1', 'C2']

Try `list(itertools.combinations(['A1', 'A2', 'B1', 'B2', 'C1', 'C2'], 3))` — poke, Oct 18 '16 at 07:05
@vaultah You’re right. And now I’m confused. Nevermind me then. — poke, Oct 18 '16 at 07:12
It seems that way combinate 'A2', 'B2', 'C2', we need 2 items of the 3 items in one list — Tom, Oct 18 '16 at 07:12

score 2 · Answer 1 · answered Oct 18 '16 at 07:29

2

Try this:

# group every 2 lists in list l
ll = list(itertools.combinations(l, 2))

# generate all combinations of 3 elements out from each 2 lists
c = [list(itertools.combinations(a + b, 3)) for (a, b) in ll]

# concate all elements
c = sum(c, [])

answered Oct 18 '16 at 07:29

Pan Long

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Also you need to `list(map(list, c))` to have `list` of `list` objects as OP's output. – vishes_shell Oct 18 '16 at 07:36
Since the OP asks about efficiency, `c = sum(c, [])` is probably not ideal (`O(n^2)`). A nested comprehension is more performant: `c = [x for l in c for x in l]`. (http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python) – user2390182 Oct 18 '16 at 07:42

Sardorbek Imomaliev · Accepted Answer · 2016-10-18T08:03:43.703

1

Or in one line

from itertools import product

c = [[k] + i for i, j in product(l, l) if j!=i for k in j]

edited Oct 18 '16 at 08:03

answered Oct 18 '16 at 07:40

Sardorbek Imomaliev

14,861
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Python combinations with list and items in other lists

2 Answers2