Case Classes and Pattern Matching

Pattern Matching

Choose a result by matching a value against cases.

pattern-match A `match` expression tests a value against ordered cases and returns the first match. The `_` case is the catch-all default.

Pattern Matching

code
PatternMatch.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val code = 2
    val label = code match {
      case 1 => "one"
      case 2 => "two"
      case 3 => "three"
      case _ => "other"
    }

    println("code=" + code)
    println("label=" + label)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val code = 1
    val label = code match {
      case 1 => "one"
      case 2 => "two"
      case 3 => "three"
      case _ => "other"
    }

    println("code=" + code)
    println("label=" + label)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val code = 3
    val label = code match {
      case 1 => "one"
      case 2 => "two"
      case 3 => "three"
      case _ => "other"
    }

    println("code=" + code)
    println("label=" + label)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val code = 9
    val label = code match {
      case 1 => "one"
      case 2 => "two"
      case 3 => "three"
      case _ => "other"
    }

    println("code=" + code)
    println("label=" + label)
  }
}

  1. code ← 2, label ← two

    1object Main {2  def main(args: Array[String]): Unit = {3    val code→ 2 = 2 //@code=1, 3, 94    val label→ two = code2 match {5      case 1 => "one"6      case 2 => "two"7      case 3 => "three"8      case _ => "other"9    }1011    println("code=" + code2)12    println("label=" + labeltwo)13  }14}
    outputcode=2
label=two

  1. code ← 1, label ← one

    1object Main {2  def main(args: Array[String]): Unit = {3    val code→ 1 = 14    val label→ one = code1 match {5      case 1 => "one"6      case 2 => "two"7      case 3 => "three"8      case _ => "other"9    }1011    println("code=" + code1)12    println("label=" + labelone)13  }14}
    outputcode=1
label=one

  1. code ← 3, label ← three

    1object Main {2  def main(args: Array[String]): Unit = {3    val code→ 3 = 34    val label→ three = code3 match {5      case 1 => "one"6      case 2 => "two"7      case 3 => "three"8      case _ => "other"9    }1011    println("code=" + code3)12    println("label=" + labelthree)13  }14}
    outputcode=3
label=three

  1. code ← 9, label ← other

    1object Main {2  def main(args: Array[String]): Unit = {3    val code→ 9 = 94    val label→ other = code9 match {5      case 1 => "one"6      case 2 => "two"7      case 3 => "three"8      case _ => "other"9    }1011    println("code=" + code9)12    println("label=" + labelother)13  }14}
    outputcode=9
label=other

Follow the Match

  1. code starts at 2.
  2. The match checks cases from top to bottom.
  3. case 1 does not match 2.
  4. case 2 matches and returns two.
  5. The program prints code=2 and label=two. | code | matching case | label | | ---: | --- | --- | | 1 | case 1 | one | | 2 | case 2 | two | | 3 | case 3 | three | | 9 | case _ | other |

Exercise: PatternMatch.scala

Reproduce code=2 and label=two, then use codes 1, 3, and 9 to predict labels one, three, and other.