For-Comprehensions

Guarded For

Keep only elements that pass a condition.

for-guard An `if` clause inside a `for` filters elements before they are yielded, the same result as `filter` followed by `map`.

Guarded For

limit
ForGuard.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val limit = 2
    val kept = for (n <- List(1, 2, 3, 4) if n > limit) yield n
    val text = kept.mkString(",")

    println("count=" + kept.length)
    println("text=" + text)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val limit = 1
    val kept = for (n <- List(1, 2, 3, 4) if n > limit) yield n
    val text = kept.mkString(",")

    println("count=" + kept.length)
    println("text=" + text)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val limit = 3
    val kept = for (n <- List(1, 2, 3, 4) if n > limit) yield n
    val text = kept.mkString(",")

    println("count=" + kept.length)
    println("text=" + text)
  }
}

  1. limit ← 2, kept ← List(3, 4), text ← 3,4

    1object Main {2  def main(args: Array[String]): Unit = {3    val limit→ 2 = 2 //@limit=1, 34    val kept→ List(3, 4) = for (n <- List(1, 2, 3, 4) if n > limit2) yield n5    val text→ 3,4 = keptList(3, 4).mkString(",")67    println("count=" + kept.length2)8    println("text=" + text3,4)9  }10}
    outputcount=2
text=3,4

  1. limit ← 1, kept ← List(2, 3, 4), text ← 2,3,4

    1object Main {2  def main(args: Array[String]): Unit = {3    val limit→ 1 = 14    val kept→ List(2, 3, 4) = for (n <- List(1, 2, 3, 4) if n > limit1) yield n5    val text→ 2,3,4 = keptList(2, 3, 4).mkString(",")67    println("count=" + kept.length3)8    println("text=" + text2,3,4)9  }10}
    outputcount=3
text=2,3,4

  1. limit ← 3, kept ← List(4), text ← 4

    1object Main {2  def main(args: Array[String]): Unit = {3    val limit→ 3 = 34    val kept→ List(4) = for (n <- List(1, 2, 3, 4) if n > limit3) yield n5    val text→ 4 = keptList(4).mkString(",")67    println("count=" + kept.length1)8    println("text=" + text4)9  }10}
    outputcount=1
text=4