Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2014/imperative-programming/5/b/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Imperative programming / 5 / b / Solution by Codex 0 2026-10-31
A demand-driven filtering iterator needs one cached accepted value. Otherwise hasNext would either lose an element or consume another accepted value every time it is queried.
def filterIter[T](p: T => Boolean, it: Iter[T]): Iter[T] =
  new Iter[T] {
    private var buffered: Option[T] = None
    private var finished = false
    private def fill(): Unit = {
      while (buffered.isEmpty && !finished) {
        if (!it.hasNext) finished = true
        else {
          val x = it.next()
          if (p(x)) buffered = Some(x)
        }
      }
    }
    def hasNext: Boolean = {
      fill()
      buffered.nonEmpty
    }
    def next(): T = {
      fill()
      require(buffered.nonEmpty)
      val answer = buffered.get
      buffered = None
      answer
    }
  }
The representation invariant says that buffered holds at most one accepted value. The represented sequence is the buffered value, if present, followed by the subsequence of the underlying remainder satisfying the predicate. Filling moves the next accepted value into the cache and discards only rejected values, so this abstract sequence is unchanged by hasNext. Clearing the cache in next removes its head. Thus
(1)
next works even when the client has not first called hasNext; exhaustion violates its stated precondition. As with ordinary iterator adapters, clients should give this wrapper exclusive consumption of it, and the predicate should give a stable answer for each element. One call may scan several rejected elements, but across a complete traversal each source element is consumed once. Cache storage is constant.
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