Control Flow

Ordered Guards

Check ordered conditions from general to specific.

ordered-guards Ordered guards let later checks refine an earlier result when the conditions are simple and explicit.

Ordered Guards

score
NestedGuards.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val score = 82
    var result = "retry"

    if (score >= 60) {
      result = "pass"
    }
    if (score >= 90) {
      result = "honor"
    }

    println("score=" + score)
    println("result=" + result)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val score = 58
    var result = "retry"

    if (score >= 60) {
      result = "pass"
    }
    if (score >= 90) {
      result = "honor"
    }

    println("score=" + score)
    println("result=" + result)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val score = 95
    var result = "retry"

    if (score >= 60) {
      result = "pass"
    }
    if (score >= 90) {
      result = "honor"
    }

    println("score=" + score)
    println("result=" + result)
  }
}

  1. score ← 82, result ← retry

    1object Main {2  def main(args: Array[String]): Unit = {3    val score→ 82 = 82 //@score=58, 954    var result→ retry = "retry"56    if (score >= 60) {

  2. result ← pass

    6if (score82 >= 60) {7  result→ pass = "pass"8}9if (score >= 90) {

  3. println("score=" + score)

    13    println("score=" + score82)14    println("result=" + resultpass)15  }16}
    outputscore=82
result=pass

  1. score ← 58, result ← retry

    1object Main {2  def main(args: Array[String]): Unit = {3    val score→ 58 = 584    var result→ retry = "retry"56    if (score >= 60) {7      result = "pass"8    }9    if (score >= 90) {10      result = "honor"11    }1213    println("score=" + score58)14    println("result=" + resultretry)15  }16}
    outputscore=58
result=retry

  1. score ← 95, result ← retry

    1object Main {2  def main(args: Array[String]): Unit = {3    val score→ 95 = 954    var result→ retry = "retry"56    if (score >= 60) {

  2. result ← pass

    6if (score95 >= 60) {7  result→ pass = "pass"8}9if (score >= 90) {

  3. result ← honor

    8}9if (score95 >= 90) {10  result→ honor = "honor"11}

  4. println("score=" + score)

    13    println("score=" + score95)14    println("result=" + resulthonor)15  }16}
    outputscore=95
result=honor

Follow the Guards

  1. score starts at 82.
  2. result starts as "retry".
  3. The score >= 60 check is true, so result becomes "pass".
  4. The score >= 90 check is false, so result stays "pass".
  5. The program prints score=82 and result=pass. | score | score >= 60 | score >= 90 | final result | | --- | --- | --- | --- | | 58 | false | false | retry | | 82 | true | false | pass | | 95 | true | true | honor |

Exercise: NestedGuards.scala

Reproduce result=pass for score 82, then try score 58 and score 95 and predict the final result.