Binary search

/*
 * NOTE: Binary search does not find an index if the key you search for is not present in the array
 */
func binarySearch<T: Comparable>(_ a: [T], key: T, range: Range<Int>) -> Int? {
    if range.lowerBound >= range.upperBound {
        return nil// If we get here, then the search key is not present in the array.
    }else{
        let midIndex = range.lowerBound + (range.upperBound - range.lowerBound) / 2// Calculate where to split the array.
        if(a[midIndex] > key){// Is the search key in the left half?
            return binarySearch(a, key: key, range: range.lowerBound ..< midIndex)
        }else if(a[midIndex] < key ){// Is the search key in the right half?
            return binarySearch(a, key: key, range: midIndex + 1 ..< range.upperBound)
        }else {// If we get here, then we've found the search key!
            return midIndex
        }
    }
}
let numbers = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67]
binarySearch(numbers, key: 43, range: 0 ..< numbers.count)//output: 13 index of where the key is