https://www.scirra.com/tutorials/202/to ... put-method
If you are new to touch controls such as the swipe, check out the tutorial there. It sounds like you have a pretty tough question for me to answer, though, but I think I understand what you're saying.
First of all, when your finger makes contact with the object being swiped, in that moment an invisible (visible when prototyping) line needs to be placed with its origin directly under your finger. The location of the line will remain there until you lift up your finger. As you move your finger down the line, the location of your finger on that line when you take your hands from the screen/finish swiping can represent which slot on the array to choose.
Admittedly there are other factors in play here: the speed of the swipe, for one, but I am not sure at the moment of the best way to handle that.