CODEWITHSUNDEEP
pythonpython-2.7
Adrocks__
Adrocks__

Reputation: 29

Gather unique values from 2 lists

I currently have 2 "set" lists, both contain mostly the same values i.e:

a = [1,2,3,4,5,6]

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

What I'm trying to do is compare the two and only return 7 & 8 into another list however I'm having no luck, I've tried a few methods I've found on here such as:

c = [item for item in a if item not in b]

However I've had no luck does anyone know a quick and fairly easy method of doing this? Both lists have been "set" previously to remove any duplicates within their own lists also (just felt I should add that if that makes a difference)

Just to be clear, as I think I wasn't sorry, the example values are already in set format, in my head for some reason when you used set on a list it acted like the array_unique PHP function.

Thanks.

Upvotes: -1

Views: 160

Answers (3)

user2285236
user2285236

Reputation:

You can use sets:

set(b) - set(a)
Out[65]: {7, 8}

Or more explicitly:

set(b).difference(a)
Out[67]: {7, 8}

For symmetric difference, you can use ^. It will return elements that are in a or b, but not in both. Assume a has an additional element, 9:

a = [1, 2, 3, 4, 5, 6, 9]
b = [1, 2, 3, 4, 5, 6, 7, 8]
set(b) ^ set(a)
Out[70]: {7, 8, 9}

Or,

set(b).symmetric_difference(a)
Out[71]: {7, 8, 9}

If they are already sets, you can just do b - a or b ^ a.

Your attempt does not work because there is no item in a which is not in b. For that difference (b-a), you need: [item for item in b if item not in a].

Upvotes: 2

RoadRunner
RoadRunner

Reputation: 26335

Another approach is using a defaultdict to count the number of occurrences of each element from both lists:

from collections import defaultdict
from itertools import chain

a = [1,2,3,4,5,6,9]

b = [1,2,3,4,5,6,7,8,9]

d = defaultdict(int)

for num in chain(a, b):
    d[num] += 1


print([key for key, value in d.items() if value == 1])

Output:

[7, 8]

Upvotes: 0

Valera Grishin
Valera Grishin

Reputation: 441

list(set(a)-set(b))+list(set(b)-set(a))

Upvotes: -1

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