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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/8/d/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) / 8 / d / Solution by

Codex  0 2026-10-25

Choose the following explicit policy: an edit of pounds recomputes euros; an edit of euros recomputes pounds; an edit of the rate keeps pounds fixed and recomputes euros. Amounts may be any finite signed values, while a rate must be finite and strictly positive. Invalid text, non-finite numbers and overflowing conversions leave the last accepted state intact and restore its display.

import scala.swing._
import scala.swing.event.EditDone
import scala.util.Try

object CurrencyConverter extends SimpleSwingApplication {
  def top: Frame = new MainFrame {
    title = "Currency converter"
    val pounds = new TextField(8)
    val euros = new TextField(8)
    val rate = new TextField(8)
    private var poundsValue = 10.0
    private var rateValue = 1.15
    private var updating = false

    private def finite(x: Double): Boolean =
      java.lang.Double.isFinite(x)

    private def money(x: Double): String =
      java.lang.String.format(java.util.Locale.ROOT, "%.2f",
                              java.lang.Double.valueOf(x))

    private def showValues(): Unit = {
      updating = true
      try {
        pounds.text = money(poundsValue)
        euros.text = money(poundsValue * rateValue)
        rate.text = java.lang.Double.toString(rateValue)
      } finally updating = false
    }

    private def commit(field: TextField): Unit = {
      if (!updating) {
        val proposed = Try(field.text.trim.toDouble).toOption
          .filter(finite).map { value =>
            if (field eq pounds) (value, rateValue)
            else if (field eq euros) (value / rateValue, rateValue)
            else (poundsValue, value)
          }
        proposed.foreach { case (newPounds, newRate) =>
          if (finite(newPounds) && finite(newRate) && newRate > 0.0 &&
              finite(newPounds * newRate)) {
            poundsValue = newPounds
            rateValue = newRate
          }
        }
        showValues()
      }
    }

    contents = new FlowPanel {
      contents += new Label("Pounds")
      contents += pounds
      contents += new Label("Euros")
      contents += euros
      contents += new Label("Euros per pound")
      contents += rate
    }
    listenTo(pounds, euros, rate)
    reactions += {
      case EditDone(source) if source eq pounds => commit(pounds)
      case EditDone(source) if source eq euros => commit(euros)
      case EditDone(source) if source eq rate => commit(rate)
    }
    showValues()
  }
}

The numeric state has the invariant

e u ros Va l u e = p o u n d s Va l u e \cdot r a t e Va l u e

and

r a t e Va l u e > 0

; euros are derived instead of being stored independently. A proposal is validated before either stored value changes. The update flag prevents a synchronous programmatic display update from recursively committing another edit. Currency amounts display two decimal places, while the rate retains its parsed precision; display rounding is not repeatedly fed back into the numeric model. Locale.ROOT ensures that formatted decimal points match the input parser.

The code handles the EditDone events used by the paper rather than trying to parse every intermediate keystroke. It uses Scala 2 style and Java 8-or-newer numeric helpers. No Scala compiler or Swing runtime execution was available in the checking environment; the state transitions were checked independently, and the API usage was compared with the supplied contract and official Scala/Swing documentation.^[ref]

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