How do you remove duplicates between two lists in python?

I am trying to remove duplicates from 2 lists. so I wrote this function:

a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]

b = ["ijk", "lmn", "opq", "rst", "123", "456", ]

for i in b:
    if i in a:
        print "found " + i
        b.remove[i]

print b

But I find that the matching items following a matched item does not get remove.

I get result like this:

found ijk
found opq
['lmn', 'rst', '123', '456']

but i expect result like this:

['123', '456']

How can I fix my function to do what I want?

Thank you.

Paolo

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asked Aug 12, 2013 at 19:18

0

Here is what's going on. Suppose you have this list:

['a', 'b', 'c', 'd']

and you are looping over every element in the list. Suppose you are currently at index position 1:

['a', 'b', 'c', 'd']
       ^
       |
   index = 1

...and you remove the element at index position 1, giving you this:

['a',      'c', 'd']
       ^
       |
    index 1

After removing the item, the other items slide to the left, giving you this:

['a', 'c', 'd']
       ^
       |
    index 1

Then when the loop runs again, the loop increments the index to 2, giving you this:

['a', 'c', 'd']
            ^ 
            |
         index = 2

See how you skipped over 'c'? The lesson is: never delete an element from a list that you are looping over.

answered Aug 12, 2013 at 19:28

7stud7stud

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0

Your problem seems to be that you're changing the list you're iterating over. Iterate over a copy of the list instead.

for i in b[:]:
    if i in a:
        b.remove[i]


>>> b
['123', '456']

But, How about using a list comprehension instead?

>>> a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]
>>> b = ["ijk", "lmn", "opq", "rst", "123", "456", ]
>>> [elem for elem in b if elem not in a ]
['123', '456']

answered Aug 12, 2013 at 19:20

Sukrit KalraSukrit Kalra

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1

What about

b= set[b] - set[a]

If you need possible repetitions in b to also appear repeated in the result and/or order to be preserved, then

b= [ x for x in b if not x in a ] 

would do.

answered Aug 12, 2013 at 19:24

Mario RossiMario Rossi

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3

You asked to remove both the lists duplicates, here's my solution:

from collections import OrderedDict
a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]
b = ["ijk", "lmn", "opq", "rst", "123", "456", ]

x = OrderedDict.fromkeys[a]
y = OrderedDict.fromkeys[b]

for k in x:
    if k in y:
        x.pop[k]
        y.pop[k]


print x.keys[]
print y.keys[]

Result:

['abc', 'def', 'xyz']
['123', '456']

The nice thing here is that you keep the order of both lists items

answered Aug 12, 2013 at 19:26

DevLoungeDevLounge

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or a set

set[b].difference[a]

be forewarned sets will not preserve order if that is important

answered Aug 12, 2013 at 19:22

Joran BeasleyJoran Beasley

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You can use lambda functions.

f = lambda list1, list2: list[filter[lambda element: element not in list2, list1]]

The duplicated elements in list2 are removed from list1.

>>> a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]
>>> b = ["ijk", "lmn", "opq", "rst", "123", "456"]
>>> f[a, b]
['abc', 'def', 'xyz']
>>> f[b, a]
['123', '456']

BcK

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answered Dec 14, 2020 at 12:31

GolvinGolvin

311 silver badge2 bronze badges

One way of avoiding the problem of editing a list while you iterate over it, is to use comprehensions:

a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]
b = ["ijk", "lmn", "opq", "rst", "123", "456", ]
b = [x for x in b if not x in a]

answered Aug 12, 2013 at 20:24

Mayur PatelMayur Patel

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2

There are already many answers on "how can you fix it?", so this is a "how can you improve it and be more pythonic?": since what you want to achieve is to get the difference between list b and list a, you should use difference operation on sets [operations on sets]:

>>> a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]
>>> b = ["ijk", "lmn", "opq", "rst", "123", "456", ]
>>> s1 = set[a]
>>> s2 = set[b]
>>> s2 - s1
set[['123', '456']]

answered Aug 12, 2013 at 20:29

Vincenzo PiiVincenzo Pii

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Along the lines of 7stud, if you go through the list in reversed order, you don't have the problem you encountered:

a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]

b = ["ijk", "lmn", "opq", "rst", "123", "456", ]

for i in reversed[b]:
    if i in a:
        print "found " + i
        b.remove[i]

print b

Output:
found rst
found opq
found lmn
found ijk
['123', '456']

answered Dec 14, 2020 at 13:09

MarkoMarko

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You can use the list comprehensive

a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]
b = ["ijk", "lmn", "opq", "rst", "123", "456", ]

duplicates value removed from a

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

duplicate value removed from b

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

answered Sep 8, 2021 at 11:44

a = ["abc", "def", "ijk", "lmn", "opq", "rst", "xyz"]

b = ["ijk", "lmn", "opq", "rst", "123", "456","abc"]

for i in a:
    if i in b:
        print["found", i]
        b.remove[i]
print[b]

output:
found abc
found ijk
found lmn
found opq
found rst
['123', '456']

answered May 1 at 13:36

1

A simple fix would be to instead iterate through a range, look at the element at the index, delete that element, then decrement the counter by 1.
Mock untested code

for i in range[0, len[b]]:
    if b[i] in a:
        del b[i]
        i -= 1

answered Aug 27, 2021 at 10:14

2

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