How to get a sublist of all other elements while iterating?

Question

Using list comprehension I can easily do the following:

l = [0, 1, 2]
for i in l:
    subl = [j for j in l if j != i]

Is there a non-list comprehension way to do this by manipulating indices with the % operator?

Edit

To clarify: the indices match the list elements

Wouldn't this be quicker with a combination operation? Like `import itertools; list(itertools.combination(l, len(l)-1))`? — Abdou, Aug 26 '17 at 14:13
@Abdou it's not combination, it's all the sublists that are missing one element from the original list — Tal, Aug 26 '17 at 14:14
Your algorithm wouldn't work if you have repeating values in the list. You really should be doing this on indices (`enumerate()` may help). — AChampion, Aug 26 '17 at 14:18
You are right, I should clarify that the indices match the list elements — Sam Hammamy, Aug 26 '17 at 14:19

Tal · Answer 1 · 2017-08-26T14:15:50.297

1

Here's a solution that doesn't use list comprehension:

l = [0, 1, 2]
for i in range(len(l)):
    subl = l[:]
    subl.remove(i)

Output

[1, 2]
[0, 2]
[0, 1]

edited Aug 26 '17 at 14:15

answered Aug 26 '17 at 14:13

Tal

145
8

That's fine, but I am looking for a way to do it with `%` – Sam Hammamy Aug 26 '17 at 14:15
@SamHammamy that's the modulo operator, which doesn't work with lists – Tal Aug 26 '17 at 14:17
by the way, look at what champion wrote above (which you agreed with), isn't it exactly what this solution suggests ? – Tal Aug 26 '17 at 14:22

AChampion · Answer 2 · 2017-08-26T16:05:34.297

This is completely artificial and there is really no reason for doing it this way:

>>> l = [0, 1, 2]
>>> n = len(l)
>>> [[l[(j+i)%n] for j in range(n-1)] for i in range(n)]
[[0, 1], [1, 2], [2, 0]]

It rotates around the list which is what % will do, perhaps more obvious with a longer list:

>>> l = [0, 1, 2, 4, 5]
>>> n = len(l)
>>> [[l[(j+i)%n] for j in range(n-1)] for i in range(n)]
[[0, 1, 2, 4], [1, 2, 4, 5], [2, 4, 5, 0], [4, 5, 0, 1], [5, 0, 1, 2]]

If you really do need this rotational output then suggest looking into collections.deque().

How to get a sublist of all other elements while iterating?

2 Answers2