Python periodically remove items from a list

Question

Say I have a long list with 1000 elements and I want to periodically delete groups of elements based off of two variables.

So for my_list=[1,2,3,4...1000] , and a=5 , b=7 , I would keep the first 5 elements, delete the next 7, and repeat until the end of the list.

The list will then look like:

my_list = [1,2,3,4,5,12,13,14,15,16...]

I don't know a or b prior to using them, nor the length of the list, so I'm looking for a general solution.

Thanks!

Answer 1

Here's one way to do it using enumerate and taking the mod of the index on the sum of a and b . Filter out values whose mod is less than a :

l = range(1, 30)
a, b = 5, 7
r = [x for i, x in enumerate(l) if i%(a+b) < a]
print(r)
# [1, 2, 3, 4, 5, 13, 14, 15, 16, 17, 25, 26, 27, 28, 29]

_{PS if you're deleting the next 7, 12 should not be included.}

Answer 2

Here's a different approach using iterators:

import itertools as it

L = list(range(1000))
a, b = 5, 7

mask = it.chain(it.repeat(1, a), it.repeat(0, b))

Result = list(it.compress(L, it.cycle(mask)))

Documentation of the itertools standard module .

The cool thing here is that no intermediate data is stored in memory, and all the results are generated on-the-fly.