Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.rs

fn main() {
	let arr = [1, 2, 4, 4, 4, 7, 9];
	let target = 4;
	let mut lo: i32 = 0;
	let mut hi: i32 = (arr.len() as i32) - 1;
	let mut result: i32 = -1;
	while lo <= hi {
		let mid = lo + (hi - lo) / 2;
		if arr[mid as usize] == target {
			result = mid;
			hi = mid - 1;
		} else if arr[mid as usize] < target {
			lo = mid + 1;
		} else {
			hi = mid - 1;
		}
	}
	println!("{}", result);
}

Complexity

Time: O(log n)
Space: O(1)

Implementation notes

The checked source uses a fixed sorted array, let arr = [1, 2, 4, 4, 4, 7, 9], not a Vec or slice parameter.
This version does not return Option; it keeps let mut result: i32 = -1 as a not-found sentinel and prints that scalar at the end.
lo, hi, and mid are i32 values. hi starts as (arr.len() as i32) - 1, and array reads cast with arr[mid as usize].
The loop guard while lo <= hi keeps mid nonnegative for each read in this trace. Using signed bounds avoids usize underflow when hi = mid - 1.
The branch order checks equality first. On a match it records result = mid and narrows left with hi = mid - 1, so duplicates at indices 2, 3, and 4 return the first index.
The trace records mid=3 as a match (result=3, hi=2), mid=1 as value 2 (lo=2), then mid=2 as the leftmost match (result=2, hi=1).
println!("{}", result) uses display formatting and prints 2.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This Rust DSA version keeps the same data and final output as every other DSA book in this wave.