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.rb

arr = [1, 2, 4, 4, 4, 7, 9]
target = 4
lo = 0
hi = arr.length - 1
result = -1
while lo <= hi
	mid = lo + (hi - lo) / 2
	if arr[mid] == target
		result = mid
		hi = mid - 1
	elsif arr[mid] < target
		lo = mid + 1
	else
		hi = mid - 1
	end
end
puts result

Complexity

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

Implementation notes

arr is a Ruby Array read by index; the search never mutates it.
lo, hi, mid, and result are local integer variables reassigned during the while lo <= hi loop.
The midpoint uses mid = lo + (hi - lo) / 2; Ruby integer division keeps mid as an integer index.
result starts at -1, so a miss would keep the sentinel value.
On arr[mid] == target, the code records result = mid and then sets hi = mid - 1 to keep searching for an earlier copy.
If arr[mid] < target, lo = mid + 1; otherwise hi = mid - 1.
The trace first matches at index 3, moves left, skips value 2 at index 1, then matches index 2 as the first occurrence.
puts result prints the final scalar index 2.

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

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