My friend Bill had previously alerted me to the coolness of Python
sets. However I hadn't found opportunity to use them until now. Here are three functions using
sets to remove duplicate entries from a list, find the intersection of two lists, and find the union of two lists. Note,
sets were introduced in Python 2.4, so Python 2.4 or later is required. Also, the items in the list must be hashable and order of the lists is not preserved.
For more information on Python
sets, see the Library Reference.
""" NOTES: - requires Python 2.4 or greater - elements of the lists must be hashable - order of the original lists is not preserved """ def unique(a): """ return the list with duplicate elements removed """ return list(set(a)) def intersect(a, b): """ return the intersection of two lists """ return list(set(a) & set(b)) def union(a, b): """ return the union of two lists """ return list(set(a) | set(b)) if __name__ == "__main__": a = [0,1,2,0,1,2,3,4,5,6,7,8,9] b = [5,6,7,8,9,10,11,12,13,14] print unique(a) print intersect(a, b) print union(a, b)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [8, 9, 5, 6, 7] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
- An example using Python's groupby and defaultdict to do the same task — posted 2014-10-09
- python enum types — posted 2012-10-10
- Python data object motivated by a desire for a mutable namedtuple with default values — posted 2012-08-03
- How to sort a list of dicts in Python — posted 2010-04-02
- Python setdefault example — posted 2010-02-09
- How to conditionally replace items in a list — posted 2008-08-22
You may also want to check out this: [http://stackoverflow.com/questions/642763/python-intersection-of-two-lists]
Derek: Thanks. My method is simple, but it has its limitations.
thanks for the above - very helpful. do you also know a good solution for intersection & union without eliminating the duplicates beforehand? say I have
a = [1,1,2,4,4]
b = [1,4,4,4]
I would like to get
for intersection = [1,4,4]
and for union = [1,1,2,4,4,4].
Right now, I cannot think of anything better than two nested loops.
Thank you. It was exactly what I was looking for!
How about this:
def to_multiset(x): result = set() max_rep = len(x) for elt in x: for n in xrange(max_rep): n_elt = (elt,n) if n_elt not in result: result.add(n_elt) break return result def from_multiset(x): return sorted([elt for elt,n in x]) def multi_union(a, b): aa = to_multiset(a) bb = to_multiset(b) return from_multiset(aa | bb) def multi_intersect(a, b): aa = to_multiset(a) bb = to_multiset(b) return from_multiset(aa & bb) a = [1, 1, 2, 4, 4] b = [1, 4, 4, 4] expected_intersection = [1, 4, 4] expected_union = [1, 1, 2, 4, 4, 4] print multi_union(a, b), expected_union print multi_intersect(a, b), expected_intersection
nice, but set works well only for sorted list, or for list where you don't care about order of the elements.