The Binary Search Algorithm

In this lesson you will learn how to implement binary search in Swift, and understand the thinking behind searching efficiently. This is for anyone searching for "Swift binary search implementation".

Binary search finds a value efficiently in already-sorted data. Picture opening a dictionary in the middle and halving the range you still have to look through each time.

The sample code computes the midpoint mid from the two boundaries low and high, compares it with the target, and updates low or high to narrow the range. Compared with checking each item from the start, it finds the value dramatically faster as the data grows.

A common stumbling block for beginners is that binary search requires the array to be sorted first. Sorted input is a precondition, so do not skip that step.

In real-world development the idea is widely applied whenever you need to find a value quickly among a large volume of data.

📖 Reference code

func binarySearch(_ arr: [Int], _ target: Int) -> Int {
    var low = 0
    var high = arr.count - 1
    while low <= high {
        let mid = (low + high) / 2
        if arr[mid] == target {
            return mid
        } else if arr[mid] < target {
            low = mid + 1
        } else {
            high = mid - 1
        }
    }
    return -1
}

let nums = [1, 3, 5, 7, 9, 11]
print(binarySearch(nums, 7))
print(binarySearch(nums, 4))

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Swift directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).