How to delete elements from a list using a list of indexes?

Question

If I have the following list of elements:

list = ['one', 'two', 'three', 'four']

and I have another list that contains the indexes I need to delete from the list above:

to_delete = [0,2]

How can I delete those elements from list using to_delete? Using a for loop doesn't work since, obviously, when you delete one element from a list the index of each remaining element changes.

What can I do?

if `to_delete` is sorted, you can just remove in reverse order with for loop as you suggested — Will Molter, Jul 28 '16 at 22:03
@WillMolter: That can require a quadratic amount of element shifting, though. — user2357112, Jul 28 '16 at 22:04
@user2357112 well of course but if the list is small who cares? — Will Molter, Jul 28 '16 at 22:06
@WillMolter: It's a bad habit to get into, and the inputs might not always be small. — user2357112, Jul 28 '16 at 22:08

score 4 · Accepted Answer · answered Jul 28 '16 at 22:03

4

You can use enumerate together with a conditional list comprehension to generate a new list containing every element of my_list that is not in the index location from to_delete.

my_list = ['one', 'two', 'three', 'four']
to_delete = [0,2]

new_list = [val for n, val in enumerate(my_list) if n not in to_delete]

>>> new_list
['two', 'four']

answered Jul 28 '16 at 22:03

Alexander

96,739
27
183
184

Not sure how this is not upvoted as it is the best approach, bar you should make to_delete a set for large lists – Padraic Cunningham Jul 28 '16 at 22:08

score 2 · Answer 2 · edited Sep 27 '17 at 05:45

2

Use list comprehension filtering :

 [ my_list[i] for i in range(len(my_list)) if i not in to_delete ]

edited Sep 27 '17 at 05:45

Graham

7,035
17
57
82

answered Jul 28 '16 at 22:04

Ohad Eytan

7,319
1
20
30

You don't want to use that name `list` – Moses Koledoye Jul 28 '16 at 22:06
@MosesKoledoye following OP naming. – Ohad Eytan Jul 28 '16 at 22:08
1

It's still a bad idea. If the OP is doing something wrong, fix it, don't just ignore it. – MattDMo Jul 28 '16 at 23:16

How to delete elements from a list using a list of indexes?

2 Answers2

Linked