Python list subtraction

Question

This is the practice example :

Write a function (list1, list2) that takes in two lists as arguments and return a list that is the result of removing elements from list1 that can be found in list2.

Why does this function return [1, 3, 4] and not [4] as I thought it would? I think it has something to do with list1.remove(). I guess it's something obvious, but I can't see it.

It works on these examples:

subtractList (['a', 'b', 'c', 'd'], ['x', 'y', 'z']) =
    ['a', 'b', 'c', 'd']

subtractList([1,2,3,4,5], [2, 4]) =
    [1, 3, 5]

but fails on this:

subtractList(range(5), range(4))

Also i noticed it removes only even numbers from the list.

Here is the code:

def subtractList(list1,list2):
    list1 = list(list1)
    list2 = list(list2)

    for i in list1:
        if i in list2:
            list1.remove(i)

    return list1

I have solved this practice with:

def subtractList(list1,list2):

    new_list = []

    list1 = list(list1)
    list2 = list(list2)

    for i in list1:
        if i not in list2:
            new_list.append(i)

    return new_list

Please fix your formatting, in particular your indentation. As it stands it is difficult to help you. — Ffisegydd, Jun 20 '14 at 13:40
Also, since you're already passing in lists (range(x) is also a list) turning them into lists again (`list1 = list(list1)`) is unnecessary — thumbtackthief, Jun 20 '14 at 13:45
Python3,yes range returns also a list by i wanted to be sure if someone enters something like tuple it would still work — Jasko, Jun 20 '14 at 13:51
Good thinking, but both my list comprehension solution and your function solution will work on tuples anyway. — thumbtackthief, Jun 20 '14 at 13:59
Is this 'pure' subtraction such that `[1, 1, 1, 1]-[1]=[1, 1, 1]` or is it set removal such that `[1, 1, 2, 2]-[1]=[2, 2]` — dawg, Jun 20 '14 at 14:00
You need to return elements that are not in both lists so its basicaly like sets difference. — Jasko, Jun 20 '14 at 14:07

score 4 · Accepted Answer · answered Jun 20 '14 at 13:42

4

Changing a list as you iterate over it will cause problems. Your second solution is decent, or you can check out "list comprehensions". In this case:

new_list = [num for num in list1 if num not in list2]

answered Jun 20 '14 at 13:42

thumbtackthief

6,093
10
41
87

For what it's worth, for a Python beginner I don't think the cause for your error is something obvious at all. :) – thumbtackthief Jun 20 '14 at 13:43

score 4 · Answer 2 · edited Apr 09 '20 at 13:34

4

Impress your professor by cheating:

def subtract_list(l1, l2):
    return list(sorted(set(l1) - set(l2)))

edited Apr 09 '20 at 13:34

Akash Chauhan

3
3

answered Jun 20 '14 at 13:47

jaime

2,234
1
19
22

yes i also thought about sets lol – Jasko Jun 20 '14 at 13:48
This may change the order of list1. – dawg Jun 20 '14 at 13:57

Python list subtraction

2 Answers2