binary search

func binarySearch(for number: Int, at list: [Int]) -> String {
    var left = list.startIndex
    var right = list.endIndex - 1

    while true {
        let errorMessage = "The number \(number) does not exist in this list"
        let numberFound = "The number \(number) is in the list at index: "
        let middle = (left + right) / 2

        if number < list[left] || number > list[right] {
            return errorMessage
        }
        else if number == list[left] {
            return numberFound + "\(left)"
        }
        else if number == list[right] {
            return numberFound + "\(right)"
        }
        else if number == list[middle] {
            return numberFound + "\(middle)"
        }
        else if number < list[middle] {
            right = middle
        }
        else if number > list[middle] {
            left = middle
        } else {
            return errorMessage
        }
    }
}