Swift array set value of index not working

Question

I have a method (contents below) where queue2 is just an [Int]. I printed a lot of things to see if everything was working up to a point.

public func cool(item:         


        

Answer 1

If queue2 is empty, this line is illegal no matter what back is:

queue2[back] = item

You cannot refer to an index that doesn't exist, and an empty array has no indexes (indices).