Is there a way to get the difference and intersect

Question

This question already has answers here:

Find intersection of two nested lists? (19 answers)

Closed 5 years ago.

If I have lists:

a = [1, 2, 3, 4, 5]
b = [4, 5, 6, 7, 8]

c = a * b

should give me:

c = [4, 5]

and

c = a - b

should give me:

c = [1, 2, 3]

Is this available for Python or do I have to write it myself?

Would the same work for tuples? I will likely use lists as I will be adding them, but just wondering.

Answer 1

If the order doesn't matter, you can use set for this. It has intersection and difference implemented.

>>> a = set([1, 2, 3, 4, 5])
>>> b = set([4, 5, 6, 7, 8])
>>> a.intersection(b)
set([4, 5])
>>> a.difference(b)
set([1, 2, 3])

Here is the info of time complexities of these operations: https://wiki.python.org/moin/TimeComplexity#set. Notice, that the order of subtrahends changes operation complexity.

If element can occur several times (formally it is called multiset), you can use Counter:

>>> from collections import Counter
>>> a = Counter([1, 2, 3, 4, 4, 5, 5])
>>> b = Counter([4, 4, 5, 6, 7, 8])
>>> a - b
Counter({1: 1, 2: 1, 3: 1, 5: 1})
>>> a & b
Counter({4: 2, 5: 1})