Chichi Chichi - 1 month ago 16
Python Question

Python: how to remove every n-th element from list?

I had already looked through this post:
Python: building new list from existing by dropping every n-th element, but for some reason it does not work for me:

I tried this way:

def drop(mylist, n):
del mylist[0::n]
print(mylist)


This function takes a list and
n
. Then it removes every n-th element by using n-step from list and prints result.

Here is my function call:

drop([1,2,3,4],2)


Wrong output:

[2, 4]
instead of
[1, 3]





Then I tried a variant from the link above:

def drop(mylist, n):
new_list = [item for index, item in enumerate(mylist) if index % n != 0]
print(new_list)


Again, function call:

drop([1,2,3,4],2)


Gives me the same wrong result:
[2, 4]
instead of
[1, 3]





How to correctly remove n-th element from a list?

Answer

In your first function mylist[0::n] is [1, 3] because 0::n means first element is 0 and other elements are every nth element after first. As Daniel suggested you could use mylist[::n] which means every nth element.

In your second function index is starting with 0 and 0 % 0 is 0, so it doesn't copy first element. It's same for third element (2 % 2 is 0). So all you need to do is new_list = [item for index, item in enumerate(mylist) if (index + 1) % n != 0]

Tip: you may want to use return instead of print() in functions like these.

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