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In python: difference between two lists

I have two lists:

found = ['CG', 'E6', 'E1', 'E2', 'E4', 'L2', 'E7', 'E5', 'L1', 'E2BS', 'E2BS', 'E2BS', 'E2', 'E1^E4', 'E5']
expected = ['E1', 'E2', 'E4', 'E1^E4', 'E6', 'E7', 'L1', 'L2', 'CG', 'E2BS', 'E3']

I want to find the differences between both lists. I did

list(set(expected)-set(found))

and

list(set(found)-set(expected))

Returns ['E3']and ['E5']accordingly.

However, I need the following answers:

'E3' is missing from found.
'E5' is missing from expected.
There are 2 copies of 'E5' in found.
There are 3 copies of 'E2BS' in found.
There are 2 copies of 'E2' in found.

Any help / suggestions are welcome!

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3 answers

The collections.Counter class will excel when enumerating the differences between multisets:

>>> from collections import Counter
>>> found = Counter(['CG', 'E6', 'E1', 'E2', 'E4', 'L2', 'E7', 'E5', 'L1', 'E2BS', 'E2BS', 'E2BS', 'E2', 'E1^E4', 'E5'])
>>> expected = Counter(['E1', 'E2', 'E4', 'E1^E4', 'E6', 'E7', 'L1', 'L2', 'CG', 'E2BS', 'E3'])
>>> list((found - expected).elements())
['E2', 'E2BS', 'E2BS', 'E5', 'E5']
>>> list((expected - found).elements())

You may also be interested in difflib.Differ :

>>> from difflib import Differ
>>> found = ['CG', 'E6', 'E1', 'E2', 'E4', 'L2', 'E7', 'E5', 'L1', 'E2BS', 'E2BS', 'E2BS', 'E2', 'E1^E4', 'E5']
>>> expected = ['E1', 'E2', 'E4', 'E1^E4', 'E6', 'E7', 'L1', 'L2', 'CG', 'E2BS', 'E3']
>>> for d in Differ().compare(expected, found):
...     print(d)

+ CG
+ E6
  E1
  E2
  E4
+ L2
+ E7
+ E5
+ L1
+ E2BS
+ E2BS
+ E2BS
+ E2
  E1^E4
+ E5
- E6
- E7
- L1
- L2
- CG
- E2BS
- E3
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Use Python setclass and Counterclass instead of overturning your own solution:

  • symmetric_difference: , , .
  • intersection: .
  • difference: , , , ,

  • found.difference(expected) # set(['E5'])

  • expected.difference(found) # set(['E3'])

  • found.symmetric_difference(expected) # set(['E5', 'E3'])

  • : . , Counter, , . :

    collections.Counter(found)['E5'] # 2
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You have already answered the first two:

print('{0} missing from found'.format(list(set(expected) - set(found)))
print('{0} missing from expected'.format(list(set(found) - set(expected)))

The second two require you to look at counting duplicates in lists, for which there are many solutions that can be found on the Internet (including: Find and list duplicates in a list? ).

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