Question

I have two sorted lists of positive integers which can have repeated elements and I must remove matching pairs of numbers, one from each list:

a=[1,2,2,2,3]
b=[2,3,4,5,5]

should become:

a=[1,2,2]
b=[4,5,5]

That is, the 2’s and the 3’s have been removed because they appear in both lists.

Set intersection can’t be used here because of the repeated elements.

How do I go about this?

Answer 1

To remove elements appearing in both lists, use the following:

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

To create a function which does it for you, simply do:

def removeCommonElements(a, b):
    for e in a[:]:
        if e in b:
            a.remove(e)
            b.remove(e)

Or to return new lists and not to edit the old ones:

def getWithoutCommonElements(a, b): # Name subject to change
    a2 = a.copy()
    b2 = b.copy()
    for e in a:
        if e not in b:
            a2.remove(e)
            b2.remove(e)
    return a2, b2

However the former could be replaced with removeCommonElements like so:

a2, b2 = a.copy(), b.copy()
removeCommonElements(a2, b2)

Which would keep a and b, but create a duplicates without common elements.