Scala : Unable to match the pattern

Question

I am new to Scala and I am trying to execute = following code in Scala:

scala> case class FoldExp(firstname : String, lname : String, age : Int, sex : String)

defined class FoldExp

scala> object Foo{
 | def apply(firstname : String, lname : String, age : Int, sex : String) = new FoldExp(firstname,lname,age,sex)
 | }

defined object Foo

scala> val foldList = List(Foo("Hugh", "Jass", 25, "male"),Foo("Biggus"," Dickus", 43, "male"),Foo("Incontinentia", "Buttocks", 37, "female"))

foldList: List[FoldExp] = List(FoldExp(Hugh,Jass,25,male), FoldExp(Biggus, Dickus,43,male), FoldExp(Incontinentia,Buttocks,37,female))

val secTry = foldList.foldLeft(List[String]()){(w,f) =>
       val comment = f.age match{
       case (f.age == 25) => "Band A"
       case (f.age > 30) => "Band B"
       case (f.age > 50) => "Band D"
       }
       w:+ s"${f.firstname},${f.age} $comment"

  }

The above block of code threw following error:

<console>:11: error: not found: value foldList
   val secTry = foldList.foldLeft(List[String]()){(w,f) =>
                ^
<console>:13: error: not found: value ==
              case (f.age == 25) => "Band A"
                          ^
<console>:14: error: not found: value >
              case (f.age > 30) => "Band B"
                          ^
<console>:15: error: not found: value >
              case (f.age > 50) => "Band D"

I want to categorize people in the list into their respective bands based on their age. But iam not able to achieve this using pattern matching. Could anyone tell me why the above approach is wrong and what is the method to be followed to achieve my objective. Any attempt to find solution to the above problem is appreciated. Thanks in advance.

Answer 1

You can't put expressions in case clauses directly. Something like this should work:

 val comment = f.age match {
     case 25 => "Band A"
     case a if a > 50 => "Band D"
     case a if a > 30 => "Band B"

 }

Note, that I swapped around the >30 and >50 cases, because match statements are evaluated in order, and evaluation is stopped as soon as the first match found. So, if a > 30 comes before a>50, then the latter will never end up being executed, because everything that matches it, would match the previous one as well.

Also note, that this will throw a MatchError if age is less than 25 or if it is between 26 and 29. To avoid that, you need a default "catch all" case at the end, something like case _ => "Unknown band"