Perform action selectively on a set without losing the rest of items

Hello to all,

I already solved this, but I think I took the very long road to do it. I am going to illustrate it with a more specific example which is where the problem arose:

I have a set of lines which are parallel two by two. However I would also like to align them in the same direction. Hence I perform a dot product which gives me the ones that have the same direction (result equals 1.0) and the ones that are in opposite direction (result equals -1.0). Then basically what I did was select the lines that I wanted to flip with a dispatch, and then merge them with the ones that weren't flip with the "Item index". 

Any easier way of doing it? Thank you all again!