Python: Find list overlap with duplicates in lists

Question

Lists being compared

list1 = ['Q, 'U', 'Z', 'Z']

List2 = ['Q, 'U', 'I', 'Z']

My query

common = []
for letter in list1: # QUZZ
    if letter in list2: # QUIZ --- this is the problematic condition (it is add 2 ZZ's instead of 1)
        common.append(letter)
print(common)

Expected Output

Common = ['Q, 'U', 'Z']

Actual Output

Common = ['Q, 'U', 'Z', 'Z']

To add, the query should also work for the reverse condition. I have tried using sets, but that doesn’t work for the second condition

list1 = ['Z','Z','Z','Z','E','E']
list2= ['Z', 'Z', 'Z']


print(list(set([x for x in list1 if x in list2])))

Expected Output:
”’
[‘Z’,’Z’,’Z’]

”’

Actual Output:

['Z']

Asked By: Ziba Joon

||

Source

Answer 1

If you use a set you can use the & operator, which for sets does an intersection. This will eliminate duplicates and find common elements.

Edit: For your problem you will actually need a multiset. I recommend using the multiset package which you can install with pip install multiset or pip3 install multiset if you’re using python3.

from multiset import Multiset

list1 = ['Z','Z','Z','Z','E','E']

list2= ['Z', 'Z', 'Z']

print(Multiset(list1) & Multiset(list2)) # Output: {'Z', 'Z', 'Z'}

Answered By: JRose

Answer 2

As @It_is_Chris said in the comment you can use sets to remove duplicates. Sets also have a method to find common items:

list1 = ['Q, 'U', 'Z', 'Z']
list2 = ['Q, 'U', 'I', 'Z']
common = set(list1).intersection(set(list2))

The output is now a set, but can be converted to a list if needed.

Answered By: Carlos Melus

Python: Find list overlap with duplicates in lists

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