Scala Language Collections Map and Filter Over A Collection
Example
Map
'Mapping' across a collection uses the map function to transform each element of that collection in a similar way. The general syntax is:
val someFunction: (A) => (B) = ???
collection.map(someFunction)
You can provide an anonymous function:
collection.map((x: T) => /*Do something with x*/)
Multiplying integer numbers by two
// Initialize
val list = List(1,2,3)
// list: List[Int] = List(1, 2, 3)
// Apply map
list.map((item: Int) => item*2)
// res0: List[Int] = List(2, 4, 6)
// Or in a more concise way
list.map(_*2)
// res1: List[Int] = List(2, 4, 6)
Filter
filter is used when you want to exclude or 'filter out' certain elements of a collection. As with map, the general syntax takes a function, but that function must return a Boolean:
val someFunction: (a) => Boolean = ???
collection.filter(someFunction)
You can provide an anonymous function directly:
collection.filter((x: T) => /*Do something that returns a Boolean*/)
Checking pair numbers
val list = 1 to 10 toList
// list: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
// Filter out all elements that aren't evenly divisible by 2
list.filter((item: Int) => item % 2==0)
// res0: List[Int] = List(2, 4, 6, 8, 10)
More Map and Filter examples
case class Person(firstName: String,
lastName: String,
title: String)
// Make a sequence of people
val people = Seq(
Person("Millie", "Fletcher", "Mrs"),
Person("Jim", "White", "Mr"),
Person("Jenny", "Ball", "Miss") )
// Make labels using map
val labels = people.map( person =>
s"${person.title}. ${person.lastName}"
)
// Filter the elements beginning with J
val beginningWithJ = people.filter(_.firstName.startsWith("J"))
// Extract first names and concatenate to a string
val firstNames = people.map(_.firstName).reduce( (a, b) => a + "," + b )
