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.f90

program search_binary_first
    implicit none
    integer :: arr(7) = [1, 2, 4, 4, 4, 7, 9]
    integer :: target, lo, hi, mid, result, value
    target = 4
    lo = 0
    hi = 6
    result = -1
    do while (lo <= hi)
        mid = lo + (hi - lo) / 2
        value = arr(mid + 1)
        if (value == target) then
            result = mid
            hi = mid - 1
        else if (value < target) then
            lo = mid + 1
        else
            hi = mid - 1
        end if
    end do
    print '(I0)', result
end program search_binary_first

Complexity

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

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-first`.

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