Scala-flatten-flatMap

GenTraversable

GenTraversable 可理解為可以被loop...被transverse(look over, examine) {% quote %} GenTraversable: A trait for all traversable collections which may possibly have their operations implemented in parallel. {% endquote %}

Common sub-classes: List, Map, Option GenTraversable內的super class是GenTraversableOnce (多了Once)

Flatten

Flatten 的作用是將GenTraversable裡面的elements的elements 取出, 變做one dimension GenTraversable裡面的elements 也需要是GenTraversable

例如 {% codeblock %} scala> List(List(4, 6), List(3,4)).flatten res40: List[Int] = List(4, 6, 3, 4) {% endcodeblock %} List 是GenTraversable,而他也包含了兩個List elements(也是GenTraversable) flatten 就是將List(4, 6)抽出4, 6 和List(3,4) 抽出3, 4 變做one dimension

List

{% codeblock %} List(1, 2, 3).flatten :11: error: No implicit view available from Int => scala.collection.GenTraversableOnce[B]. List(1, 2, 3).flatten {% endcodeblock %}
如果裡面的element 不是GenTraversableOnce, 會發生錯誤 1,2,3 都是Int, 不是GenTraversableOnce

{% codeblock %} private def sequential: TraversableOnce[A] = this.asInstanceOf[GenTraversableOnce[A]].seq

def flattenB: CC[B] = { val b = genericBuilder[B] for (xs <- sequential) b ++= asTraversable(xs).seq b.result() } {% endcodeblock %} From source code: 如果是instance of GenTraversableOnce巳經可以被flatten

Option

{% quote %} trait Iterable[+A] extends Traversable[A] with GenIterable[A] with GenericTraversableTemplate[A, Iterable] with IterableLike[A, Iterable[A]] {% endquote %}

{% codeblock %} scala> val l = List(Some(4), Some(5), None) l: List[Option[Int]] = List(Some(4), Some(5), None)

scala> l.flatten res28: List[Int] = List(4, 5) {% endcodeblock %} 有趣的Option 可以implicit converted into Iterable which is subclass of GenTraversable 可以直接flatten 出value, ignore None

FlatMap

FlatMap 類似Map, 但接收的function 需要return GenTraversable,然後被flatten {% codeblock %} scala> val l = List(List(3, 4), (3, 4)).map (x => List(x)).flatten l: List[Product with java.io.Serializable] = List(List(3, 4), (3,4)) {% endcodeblock %}

To combine with Option {% codeblock %} res87: List[Option[Int]] = List(Some(4), Some(5), None) scala> l.flatMap( x => x) res88: List[Int] = List(4, 5) {% endcodeblock %}

It may looks very useless, but it could be used with exception return Some(value) if everthing is OK otherwise just return None {% codeblock %} scala> val n = List(1,2,3,4) n: List[Int] = List(1, 2, 3, 4)

scala> n.flatMap( x => if (x > 2) Some (x) else Nne)

def toInt(in: String): Option[Int] = { try { Some(Integer.parseInt(in.trim)) } catch { case e: Exception => None } }

scala> val s = List("1", "2", "god", "jason") s: List[String] = List(1, 2, god, jason)

scala> s.flatMap(toInt(_)) res93: List[Int] = List(1, 2) {% endcodeblock %}

Reference

Option Cheat SHeet FlatMap