Question

I have a list with N elements, and I slice it using certain step, let’s say 3:

slice0 = text[0::3]
slice1 = text[1::3]
slice2 = text[2::3]

After doing some processing separatedly, now I’d need to merge them back in the same positions they were in the original list. Is there a similar (easy) way to do this?

Example:

L = [1,2,3,4,5,6] -> L0 = [1,4], L1 = [2,5], L2 = [3,6]

Then some processing (say multiply each list by 1, 2 and 3 respectively:

L0 = [1,4], L1 = [4,10], L2 = [9,18] 

Merge them back to their original positions

L = [1,4,9,4,10,18]

Thank you.

Answer 1

You can use the zip() function to join them back together:

>>> l0 = [1,4]; l1 = [4,10]; l2 = [9,18]
>>> zip(l0, l1, l2)
[(1, 4, 9), (4, 10, 18)]
>>> [x for t in zip(l0, l1, l2) for x in t]
[1, 4, 9, 4, 10, 18]

Or use itertools.chain:

>>> from itertools import chain
>>> list(chain(*zip(l0, l1, l2)))
[1, 4, 9, 4, 10, 18]

In Python 3, where zip is a generator function, itertools.chain.from_iterable might be preferrable, as others have pointed out already.