Method collect on Scala 2.7

Question

I'm looking for a collect method in scala 2.7 but I can't seem to find an applicable call. Is there something equivalent to collect that I can use in scala?

To be clear I'm looking to filter elements from a list and map those filtered to a new type.

Answer 1

You can use flatMap (the full method signature in 2.7 is def flatMap[B](f : (A) => Iterable[B]) : List[B] ). It is declared on both Iterable and Iterator (with slightly different signatures):

scala> val l = List("1", "Hello", "2")
l: List[java.lang.String] = List(1, Hello, 2)

scala> val ints = l.flatMap { s => try { Some(s.toInt) } catch { case _ => None } }
ints: List[Int] = List(1, 2)

In the above example I'm taking advantage of an explicit conversion option2iterable in Predef. It's declared in 2.8 on TraversableLike:

def flatMap[B, That](f: A => Traversable[B])(implicit bf: CanBuildFrom[Repr, B, That]): That

Answer 2

scala> List(-1,-2, 1,2,3).filter{i => (i % 2) == 0}.map{i => i / 2} 

line10: scala.List[scala.Int] = List(-1,1)

You need to seperate out the two calls

From a very useful blog

Answer 3

Using flatMap/Option, as described by Chris is the way I do it usually, but there is a more involved alternative that is, sometimes, more pleasing to the eyes:

class Filter[A](f: Any => Option[A]) {
  def unapply(a: Any) = f(a)
}

object Filter {
  def apply[A](f: Any => Option[A]) = new Filter(f)
}

val evens = Filter {
  case n: Int if n % 2 == 0 => Some(n)
  case _ => None
}

Usage:

scala> for (evens(n) <- List.range(0,10)) yield n
res0: List[Int] = List(0, 2, 4, 6, 8)