Is there any simple way to return single value from the list in python?

Question

Let's say we have a list: x = [5, 5, 2, 5, 5, 3, 3, 4, 4]. How can I return the number which happens only once in this list, i.e. 2?

This question was asked while trying to solve the "Find the odd int" problem on Codewars. Initially I did not understand correctly the task, but could have solved it much easier, so read it carefully before solving :)

Finally following code was used to solve it (it's not entire solution, but the part of it, which is used after creating the dictionary out of the input):

return [int(k) for k,v in d.items() if v%2==1][0]

@the.salman.a: Not really - that's for when you already know which item you want to count. As the top answer states, performance becomes terrible if you count all the items in the list. — Tim Pietzcker, Mar 27 '18 at 07:27
what have you tried so far to solve your problem.Stack overflow is not a free coding service. You have to show your effort too. — Vikas Periyadath, Mar 27 '18 at 07:31
@TimPietzcker: counting all the items in a list can trivially be done in O(N) time, using the right approach. — Martijn Pieters, Mar 27 '18 at 07:34
It was just a part of another task. I tried, but could not find the same question. — user2467011, Mar 27 '18 at 09:53
@VikasDamodar, I did not know about the Counter collection and tried to solve this with standard for loop and if... else procedure — user2467011, Mar 27 '18 at 10:20

Tim Pietzcker · Accepted Answer · 2018-03-27T07:32:00.157

3

>>> x = [5, 5, 2, 5, 5, 3, 3, 4, 4, 8]
>>> from collections import Counter
>>> c = Counter(x)
>>> [item[0] for item in c.items() if item[1]==1]
[2, 8]

edited Mar 27 '18 at 07:32

answered Mar 27 '18 at 07:28

Tim Pietzcker

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It's not safe to assume they know there will be only one item that has one occurrence. – Alex Mar 27 '18 at 07:30
@Alex: Yes, better make the solution more general. – Tim Pietzcker Mar 27 '18 at 07:36

score 2 · Answer 2 · answered Mar 27 '18 at 07:31

You may use collections.Counter to achieve this. Below list comprehension will return list of all elements which occurred once in your list:

>>> from collections import Counter
>>> x = [5, 5, 2, 5, 5, 3, 3, 4, 4]

>>> [k for k, v in Counter(x).items() if v==1]
[2]

Is there any simple way to return single value from the list in python?

2 Answers2