How to find the difference between 3 lists that may have duplicate numbers

Question

I have 3 lists and I want to find the difference between the 1st/2nd and 2nd/3rd and print them.

Here is my code:

n1 = [1,1,1,2,3] 
n2 = [1,1,1,2] # Here the 3 is not found ("3" is not present in n1 at all)
n3 = [1,1,2]   # here 1 is not found (there are fewer "1"s in n3 than in n2)
for x in n1: 
   if x not in n2:
      print x  
for m in n2: 
   if m not in n3: 
      print m

but i get only 3 as output.

How to make it output 1 and 3? I tried using sets as well, but it only printed 3 again.

@bkaf Curious to which of those solutions applies here, as they all involve using sets, which quite obviously isn't applicable to this question. — MrAlexBailey, Oct 30 '15 at 12:03
please help me or tell me how to solve it or in what i search and thank you i search many times — user7179690, Oct 30 '15 at 12:05

score 6 · Accepted Answer · answered Oct 30 '15 at 12:04

Since you seem to care about the number of times an item is found in both lists, you need to either remove matched items from the list you're comparing with:

comp = n2[:]  # make a copy
for x in n1:
    if x not in comp:
        print x
    else:
        comp.remove(x)
# output: 3

or use a collections.Counter

from collections import Counter
print Counter(n1) - Counter(n2)
# output: Counter({3: 1})

which tells you which items in n1 are not in n2 or can be found more often in n1 than in n2.

So, for example:

>>> Counter([1,2,2,2,3]) - Counter([1,2])
Counter({2: 2, 3: 1})

I didn't know there was a `__sub__` for `Counter` objects. That's pretty neat. — TigerhawkT3, Oct 30 '15 at 12:13

score -3 · Answer 2 · answered Oct 30 '15 at 12:18

-3

you can use set to find the difference between list.

print set(n1).difference(set(n2))

answered Oct 30 '15 at 12:18

saikumarm

1,565
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That doesn't work. It doesn't get the second case where there is an extra `1` in `n2` – SirParselot Oct 30 '15 at 12:19
yeah I agree. it gets the difference as a whole and does not consider repetitions as unique – saikumarm Oct 30 '15 at 12:41

How to find the difference between 3 lists that may have duplicate numbers

2 Answers2