Arrays and Iteration

Linear Search

Walk an array once looking for a target value. Return the index of the first match, or -1 if none. The simplest possible search loop.

Algorithm

Canonical input arr = [4, 7, 1, 9, 3, 8] with target = 9 finishes after four compares; the matching index is 3.

Basic Implementation

basic.rb 
def linear_search(arr, target)
	i = 0
	while i < arr.length
		if arr[i] == target
			return i
		end
		i = i + 1
	end
	-1
end

arr = [4, 7, 1, 9, 3, 8]
target = 9
result = linear_search(arr, target)
puts result

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Ruby: explicit while i < arr.length with an early return i the moment arr[i] == target. The stdlib arr.index(target) or arr.find_index { |x| x == target } would hide the walk the lesson is teaching.
  • Method signature def linear_search(arr, target) documents the array contract; the -1 sentinel mirrors the language-neutral spec rather than returning nil.
  • The replay shows the running index, the element being checked, and a match indicator on each frame.
early exit Return the index the moment `arr[i]` equals the target. Walking past it would defeat the point.
sentinel return A no-match walk falls off the loop and returns `-1`.