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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2021/imperative-programming-resit/2/b/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming (resit) / 2 / b / Solution by

Codex  0 2026-10-25

Return immediately on equality and omit the final array inspection:

def searchEarly(u: Array[Int], z: Int): Boolean = {
  var a = 0
  var b = u.length
  while (a < b) {
    val middle = a + (b - a) / 2
    val value = u(middle)
    if (value == z) return true
    if (value > z) a = middle + 1
    else b = middle
  }
  false
}

An equality returns a genuine matching entry. After a nonequal comparison, descending order excludes the inspected middle entry and one whole side. In the half-open range, setting b = middle already excludes the middle index; subtracting one would incorrectly exclude another possible entry. The candidate interval decreases as before, so absence is reported exactly when no possible entry remains.

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