Question

Ok so I have two lists:

x = [1, 2, 3, 4]
y = [1, 1, 2, 5, 6]

I compare them in such a way so I get the following output:

x = [3, 4]
y = [1, 5, 6]

The basic is idea to go through each list and compare them. If they have an element in common remove that element. But only one of that element not all of them. If they don’t have an element in common leave it. Two identical lists would become x = [], y = []

Here is my rather hacked up and pretty lame solution. I hoping other can recommend a better and / or more pythonic way of doing this. 3 loops seems excessive…

    done = True

    while not done:
        done = False
        for x in xlist:
            for y in ylist:
                if x == y:
                    xlist.remove(x)
                    ylist.remove(y)
                    done = False
        print xlist, ylist

Thanks as always for taking the time to read this question. XOXO

Answer 1

It’s possible that the data structure you are looking for is the multiset (or “bag”), and if so, a good way to implement it in Python is to use collections.Counter:

>>> from collections import Counter
>>> x = Counter([1, 2, 3, 4])
>>> y = Counter([1, 1, 2, 5, 6])
>>> x - y
Counter({3: 1, 4: 1})
>>> y - x
Counter({1: 1, 5: 1, 6: 1})

If you want to convert the Counter objects back to lists with multiplicity, you can use the elements method:

>>> list((x - y).elements())
[3, 4]
>>> list((y - x).elements())
[1, 5, 6]