Python Question: union of sets in a list

battra · 2009-10-15 16:25:41

Suppose you have a list of sets in Python:

>> lst = [ set([0, 1]), set([1, 2]) , set([2, 3]) ]

I want the union of the sets. Since, the list will be large, I need it to be very efficient. Normally I'd use reduce() for this:

>> reduce(lambda x, y : x |y, lst)

set([0, 1, 2, 3])

But, how would I do this WITHOUT using reduce()?

I know I could write a simple for loop, but, I wanted it to be as fast as possible (reduce() is pretty fast even with lambda call).

Is it possible to do this with a generator or list comprehension (which are both fast)?

Thanks.

Last edited by battra (2009-10-15 16:41:55)

keenerd · 2009-10-15 17:05:17

The union operation is also a method of set, set.union(). Unlike the infix union "|", the method can take any number of arguments.

big_union = set.union(set([0, 1]), set([1, 2]) , set([2, 3]))

Use * to break a list/tuple apart, so each element becomes a new argument.

big_union = set.union(*lst)

Last edited by keenerd (2009-10-15 17:10:01)

battra · 2009-10-15 18:40:30

Thanks a lot. Much appreciated.

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