##### Why is one duplicate not removed?
 Why is one duplicate not removed? Emekadavid Silly Frenchman Posts: 42 Threads: 17 Joined: May 2020 Reputation: Jun-09-2020, 02:11 PM This code removes the duplicates in two lists. But when I run it, one of the duplicates is not removed. It beats me. Can someone explain to me why. ```def removeDups(L1, L2): """Assumes that L1 and L2 are lists. Removes any element from L1 that also occurs in L2""" for e in L1: if e in L2: L1.remove(e) L1 = [1,2,3,4,6] L2 = [1,2,5,6] removeDups(L1, L2) print('L1 =', L1)```It prints: [2,3,4] but item 2 is still a duplicate. Have run it several times and baffled. Reply Posts: 1,839 Threads: 33 Joined: Sep 2016 Reputation: Jun-09-2020, 02:30 PM (This post was last modified: Jun-09-2020, 02:34 PM by Yoriz.) You shouldn't alter a list while iterating over it, it loses track of the position, make a copy of the list to iterate and alter the original. ```def removeDups(L1, L2): """Assumes that L1 and L2 are lists. Removes any element from L1 that also occurs in L2""" for e in L1[:]: if e in L2: L1.remove(e) L1 = [1, 2, 3, 4, 6] L2 = [1, 2, 5, 6] removeDups(L1, L2) print('L1 =', L1)`````Output:L1 = [3, 4]`` Reply Emekadavid Silly Frenchman Posts: 42 Threads: 17 Joined: May 2020 Reputation: Jun-09-2020, 02:35 PM Ok. Iterate over the copy and then remove the duplicate in the original list. Got it. I changed the code to: `for i in L1[:]:`and it worked. Thanks. solved. Reply divyansh Programmer named Tim Posts: 8 Threads: 3 Joined: May 2020 Reputation: Jun-09-2020, 05:56 PM for more info you can watch MIT 60001 course 5th lecture on youtube you will find the same example in the last 5 min of that lecture Reply Posts: 1,780 Threads: 8 Joined: Jun 2018 Reputation: Jun-09-2020, 06:34 PM Another way is to use list comprehension: ```>>> lst_1 = [1, 2, 3, 4, 5, 6] >>> lst_2 = [1, 2, 5, 6] >>> lst_1 = [item for item in lst_1 if item not in lst_2] >>> lst_1 [3, 4]``` I'm not 'in'-sane. Indeed, I am so far 'out' of sane that you appear a tiny blip on the distant coast of sanity. Bucky Katt, Get Fuzzy Da Bishop: There's a dead bishop on the landing. I don't know who keeps bringing them in here. ....but society is to blame. Reply

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