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.rs 
fn linear_search(arr: &[i32], target: i32) -> i32 {
	for i in 0..arr.len() {
		if arr[i] == target {
			return i as i32;
		}
	}
	-1
}

fn main() {
	let arr = [4, 7, 1, 9, 3, 8];
	let target = 9;
	let result = linear_search(&arr, target);
	println!("{}", result);
}

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Rust: explicit for i in 0..arr.len() with an early return i as i32 the moment arr[i] == target. The standard arr.iter().position(...) would hide the walk the lesson is teaching.
  • Function signature fn linear_search(arr: &[i32], target: i32) -> i32 documents the slice contract; the -1 sentinel mirrors the language-neutral spec rather than returning Option<usize>.
  • 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`.