Contains of HashSet<Integer> in Python

python contains hashset
Borys Stepov picture Borys Stepov · Nov 3, 2014 · Viewed 84.2k times · Source

In Java we have HashSet<Integer>, I need similar structure in Python to use contains like below:

A = [1, 2, 3]
S = set()
S.add(2)
for x in A:
    if S.contains(x):
        print "Example"

Could you please help?

Answer

tttthomasssss picture tttthomasssss · Nov 3, 2014

Just use a set:

>>> l = set()
>>> l.add(1)
>>> l.add(2)
>>> 1 in l
True
>>> 34 in l
False

The same works for lists:

>>> ll = [1,2,3]
>>> 2 in ll
True
>>> 23 in ll
False

Edit: Note @bholagabbar's comment below that the time complexity for in checks in lists and tuples is O(n) on average (see the python docs here), whereas for sets it is on average O(1) (worst case also O(n), but is very uncommon and might only happen if __hash__ is implemented poorly).

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