Scala Cookbook

10.14. Transforming One Collection to Another with map

Problem

Like the previous recipe, you want to transform one collection into another by applying an algorithm to every element in the original collection.

Solution

Rather than using the for/yield combination shown in the previous recipe, call the map method on your collection, passing it a function, an anonymous function, or method to transform each element. This is shown in the following examples, where each String in a List is converted to begin with a capital letter:

scala> val helpers = Vector("adam", "kim", "melissa")
helpers: scala.collection.immutable.Vector[java.lang.String] =
    Vector(adam, kim, melissa)

// the long form
scala> val caps = helpers.map(e => e.capitalize)
caps: scala.collection.immutable.Vector[String] = Vector(Adam, Kim, Melissa)

// the short form
scala> val caps = helpers.map(_.capitalize)
caps: scala.collection.immutable.Vector[String] = Vector(Adam, Kim, Melissa)

The next example shows that an array of String can be converted to an array of Int:

scala> val names = Array("Fred", "Joe", "Jonathan")
names: Array[java.lang.String] = Array(Fred, Joe, Jonathan)

scala> val lengths = names.map(_.length)
lengths: Array[Int] = Array(4, 3, 8)

The map method comes in handy if you want to convert a collection to a list of XML elements:

scala> val nieces = List("Aleka", "Christina", "Molly")
nieces: List[String] = List(Aleka, Christina, Molly)

scala> val elems = nieces.map(niece => <li>{niece}</li>) elems: List[scala.xml.Elem] ...