how do you do this

The problem is asking you to sum all the values in the array with a while loop. It gives you some initial values and sets you up wit the variables they want you to use. First you have to think about how you're going to solve the problem in your head. The way I did it was, I wanted to start adding everything in the array starting in the first spot, numbers[0] and add all the way up until we got to the end of the array [numbers.count]. One way you can do it in swift is,

while counter < numbers.count

Since they set counter = 0 for you already, then this should read that while 0 is less than the whole length of the array do this

 { 
sum = sum + numbers[counter]
counter += 1 
}

since sum = 0, then the body of the while loop on the first iteration says, 0 = 0 + whatever is in the first spot of the array

next step is to increase the counter so we move onto the next spot in the array

then the second iteration is, whatever was in the first spot of the array = whatever was in the first spot of the array + the next spot

this goes on until everything is added. Donezo

Here is a working example as of swift 4.1 on 14 APR 18

let numbers = [2,8,1,16,4,3,9]
var sum = 0
var counter = 0

// Enter your code below

while counter < numbers.count { 
sum = sum + numbers[counter]
counter += 1 
}

we use "counter" to track the number of iterations of the while loop. So we increase counter until it's less than "numbers.count"

while counter < numbers.count {
sum += numbers[counter]   //sum = sum + numbers[counter]
counter += 1
}