Searching

Binary Search (Recursive)

Use the same binary-search window as the iterative lesson, but pass lo and hi through recursive calls.

Algorithm

Basic Implementation

basic.rb 
arr = [1, 3, 5, 7, 9, 11, 13]
target = 11

def search(arr, target, lo, hi)
	return -1 if lo > hi
	mid = lo + (hi - lo) / 2
	return mid if arr[mid] == target
	return search(arr, target, mid + 1, hi) if arr[mid] < target
	search(arr, target, lo, mid - 1)
end

puts search(arr, target, 0, arr.length - 1)

Complexity

  • Time: O(log n)
  • Space: O(log n) call stack

Implementation notes

  • Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
  • The final output is intentionally small and deterministic for cross-language comparison.
execution replay The checked-in replay follows the language-neutral state table for `search-binary-recursive`.
cross-language comparison This Ruby DSA version keeps the same data and final output as every other DSA book in this wave.