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20
Binary Search Written by Kelvin Lau

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Binary search is one of the most efficient searching algorithms with a time complexity of O(log n). This is comparable with searching for an element inside a balanced binary search tree.

There are two conditions that need to be met before binary search may be used:

  • The collection must be able to perform index manipulation in constant time. This means that the collection must be a RandomAccessCollection.
  • The collection must be sorted.

Example

The benefits of binary search are best illustrated by comparing it with linear search. Swift’s Array type uses linear search to implement its firstIndex(of:) method. This means that it traverses through the whole collection or until it finds the element:

Linear search for the value 31.
Linear search for the value 31.

Binary search handles things differently by taking advantage of the fact that the collection is already sorted.

Here’s an example of applying binary search to find the value 31:

Binary search for the value 31.
Binary search for the value 31.

Instead of eight steps to find 31, it only takes three. Here’s how it works:

Step 1: Find middle index

The first step is to find the middle index of the collection. This is fairly straightforward:

Step 2: Check the element at the middle index

The next step is to check the element stored at the middle index. If it matches the value you’re looking for, you return the index. Otherwise, you’ll continue to Step 3.

Step 3: Recursively call binary Search

The final step is to recursively call binary search. However, this time, you’ll only consider the elements exclusively to the left or to the right of the middle index, depending on the value you’re searching for. If the value you’re searching for is less than the middle value, you search the left subsequence. If it is greater than the middle value, you search the right subsequence.

Implementation

Open the starter playground for this chapter. Create a new file in the Sources folder named BinarySearch.swift. Add the following to the file:

// 1
public extension RandomAccessCollection where Element: Comparable {
  // 2
  func binarySearch(for value: Element, in range: Range<Index>? = nil)
      -> Index? {
    // more to come
  }
}

// 1
let range = range ?? startIndex..<endIndex
// 2
guard range.lowerBound < range.upperBound else {
  return nil
}
// 3
let size = distance(from: range.lowerBound, to: range.upperBound)
let middle = index(range.lowerBound, offsetBy: size / 2)
// 4
if self[middle] == value {
  return middle
// 5
} else if self[middle] > value {
  return binarySearch(for: value, in: range.lowerBound..<middle)
} else {
  return binarySearch(for: value, in: index(after: middle)..<range.upperBound)
}
let array = [1, 5, 15, 17, 19, 22, 24, 31, 105, 150]

let search31 = array.firstIndex(of: 31)
let binarySearch31 = array.binarySearch(for: 31)

print("firstIndex(of:): \(String(describing: search31))")
print("binarySearch(for:): \(String(describing: binarySearch31))")
index(of:): Optional(7)
binarySearch(for:): Optional(7)

Key points

  • Binary search is only a valid algorithm on sorted collections.
  • Sometimes, it may be beneficial to sort a collection just to leverage the binary search capability for looking up elements.
  • The firstIndex(of:) method on sequences uses linear search, which has a O(n) time complexity. Binary search has a O(log n) time complexity, which scales much better for large data sets if you are doing repeated lookups.

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