scalafor-comprehension

What is the output of the below Scala code snippet?

Why do I get the output of the below Scala code snippet as List((), (), (), ()) :

val ans1 = for(i3 <- 1 to 4) yield {
 if (i3 % 2 == 0)
  i3
}

I tried the below :

val ans1 = for(i3 <- 1 to 4) yield {
if (i3 % 2 == 0)
i3
}

Expected: List(2,4)

Solution

  • If thenExpr and elseExpr have type A then if (condition) thenExpr else elseExpr has type A too.

    If thenExpr and elseExpr have different types then if ... has their supertype.

    If elseExpr is omitted then if (condition) thenExpr is a shorthand for if (condition) thenExpr else (), where () has type Unit. So if thenExpr has type Unit then if (condition) thenExpr has type Unit, otherwise if (condition) thenExpr has a type, which is supertype of some type (the one of thenExpr) and Unit i.e. Any or AnyVal.

    That's where those () are from and why the return type is Seq[AnyVal] rather than Seq[Int].

    Just don't omit else part. Also you can use standard methods .map, .filter. For-comprehensions are desugared into them.

    As @Always_A_Learner and @Dima advised, you can try

    val ans1 = for(i3 <- 1 to 4 if i3 % 2 == 0) yield i3