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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2014/imperative-programming/5/a/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 / a / Solution by

Codex  0 2026-10-31

Interpret step as a positive increment, consistent with the increasing sequence and exclusive upper bound. State this precondition explicitly; a zero increment would not define the intended finite progression. Use a wider cursor to avoid integer overflow on the update after the last valid Int result.

class RangeIter(lower: Int, upper: Int, step: Int) extends Iter[Int] {
  require(step > 0)
  private var current = lower.toLong
  def hasNext: Boolean = current < upper.toLong
  def next(): Int = {
    require(hasNext)
    val answer = current.toInt
    current += step.toLong
    answer
  }
}

After

q

calls to next, the abstraction function is the sequence of values

lower + t step < upper

with

t \geq q

, in increasing order. Repeated calls to hasNext do not change it; next removes precisely its first value. Therefore

1, 3, 5, 7, 9, 11 for RangeIter (1, 13, 2) .

(1)

If lower >= upper, the represented sequence is empty immediately. The paper does not specify a descending-range convention; this implementation exposes its positive-step precondition rather than assigning one implicitly.

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