Patrick Patrick - 6 months ago 11
Python Question

what's a good way to combinate through a set?

Given a set

{a,b,c,d}
, what's a good way to produce
{a,b,c,d,ab,ac,ad,bc,bd,cd,abc,abd,abcd}
?

A recursive algorithm, I think, but maybe some weird lambda thing would work better. I'm using python.

Answer

The Python itertools page has exactly a powerset recipe for this:

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

Output:

>>> list(powerset("abcd"))
[(), ('a',), ('b',), ('c',), ('d',), ('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd'), ('a', 'b', 'c'), ('a', 'b', 'd'), ('a', 'c', 'd'), ('b', 'c', 'd'), ('a', 'b', 'c', 'd')]

If you don't like that empty tuple at the beginning, you can just change the range statement to range(1, len(s)+1) to avoid a 0-length combination.