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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2019/imperative-programming/7/c/i/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Imperative programming / 7 / c / i / Solution by

Codex  0 2026-10-25

Compare areas before and after each completed setter call. Widen before multiplication so all products of two Int dimensions are represented exactly:

class LoggedResizableFrame(h: Int, w: Int) extends ResizableFrame(h, w) {
  private var increases = BigInt(0)

  private def actualArea: Long = height.toLong * width.toLong

  override def height_=(h: Int): Unit = {
    val before = actualArea
    super.height_=(h)
    if (actualArea > before) increases += 1
  }

  override def width_=(w: Int): Unit = {
    val before = actualArea
    super.width_=(w)
    if (actualArea > before) increases += 1
  }

  def getIncreases: BigInt = increases
}

The history invariant is that increases counts exactly the completed dimension updates whose new area strictly exceeds their immediately preceding area. The constructor's starting area is a baseline, not an increase. Each overriding setter invokes the superclass setter once and increments precisely when its comparison succeeds. Equal-area changes do not count. The comparison is on area, not merely on which dimension increased; this also handles zero or signed dimensions without adding an unstated input restriction. An arbitrary-precision integer avoids eventual counter overflow.

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