Question

I am trying to remove items from a nested list in Python. I have a nested list as follows:

families = [[0, 1, 2],[0, 1, 2, 3],[0, 1, 2, 3, 4],[1, 2, 3, 4, 5],[2, 3, 4, 5, 6]]

I want to remove the entries in each sublist that coorespond to the indexed position of the sublist in the master list. So, for example, I need to remove 0 from the first sublist, 1 from second sublist, etc. I am trying to use a list comrehension do do this. This is what I have tried:

familiesNew = [ [ families[i][j] for j in families[i] if i !=j ] for i in range(len(families)) ] 

This works for range(len(families)) up to 3, however beyond that I get IndexError: list index out of range. I am not sure why. Can somebody give me an idea of how to do this. Preferably a one-liner (list comprehension).

Thanks.

Answer 1

You almost got it right. Just replace families[i][j] with j and it works:

>>> [ [ j for j in families[i] if i !=j ] for i in range(len(families)) ]
[[1, 2], [0, 2, 3], [0, 1, 3, 4], [1, 2, 4, 5], [2, 3, 5, 6]]

It can be written a bit cleaner using the enumerate function:

>>> [[f for f in family if f != i] for i, family in enumerate(families)]
[[1, 2], [0, 2, 3], [0, 1, 3, 4], [1, 2, 4, 5], [2, 3, 5, 6]]

Or even using remove if you don’t mind changing the original list:

>>> for i, family in enumerate(families): family.remove(i)