Scala Collections

Examples

Sort A List

Supposing the following list we can sort a variety of ways.

val names = List("Kathryn", "Allie", "Beth", "Serin", "Alana")

The default behavior of sorted() is to use math.Ordering, which for strings results in a lexographic sort:

names.sorted
// results in: List(Alana, Allie, Beth, Kathryn, Serin)

sortWith allows you to provide your own ordering utilizing a comparison function:

names.sortWith(_.length < _.length)
// results in: List(Beth, Allie, Serin, Alana, Kathryn)

sortBy allows you to provide a transformation function:

//A set of vowels to use
val vowels = Set('a', 'e', 'i', 'o', 'u')
//A function that counts the vowels in a name
def countVowels(name: String) = name.count(l => vowels.contains(l.toLower))
//Sorts by the number of vowels
names.sortBy(countVowels)
//result is: List(Kathryn, Beth, Serin, Allie, Alana)

You can always reverse a list, or a sorted list, using `reverse:

names.sorted.reverse
//results in: List(Serin, Kathryn, Beth, Allie, Alana)

Lists can also be sorted using Java method java.util.Arrays.sort and its Scala wrapper scala.util.Sorting.quickSort

java.util.Arrays.sort(data)
scala.util.Sorting.quickSort(data)

Create a List containing n copies of x

To create a collection of n copies of some object x, use the fill method. This example creates a List, but this can work with other collections for which fill makes sense:

// List.fill(n)(x)
scala > List.fill(3)("Hello World")
res0: List[String] = List(Hello World, Hello World, Hello World)

List and Vector Cheatsheet

It is now a best-practice to use Vector instead of List because the implementations have better performance Performance characteristics can be found here. Vector can be used wherever List is used.

List creation

List[Int]() // Declares an empty list of type Int
List.empty[Int] // Uses `empty` method to declare empty list of type Int
Nil // A list of type Nothing that explicitly has nothing in it
List(1, 2, 3) // Declare a list with some elements
1 :: 2 :: 3 :: Nil // Chaining element prepending to an empty list, in a LISP-style

Take element

List(1, 2, 3).headOption // Some(1)
List(1, 2, 3).head // 1
List(1, 2, 3).lastOption // Some(3)
List(1, 2, 3).last // 3, complexity is O(n)
List(1, 2, 3)(1) // 2, complexity is O(n)
List(1, 2, 3)(3) // java.lang.IndexOutOfBoundsException: 4

Prepend Elements

0 :: List(1, 2, 3) // List(0, 1, 2, 3)

Append Elements

List(1, 2, 3) :+ 4 // List(1, 2, 3, 4), complexity is O(n)

Join (Concatenate) Lists

List(1, 2) ::: List(3, 4) // List(1, 2, 3, 4)
List.concat(List(1,2), List(3, 4)) // List(1, 2, 3, 4)
List(1, 2) ++ List(3, 4) // List(1, 2, 3, 4)

Common operations

List(1, 2, 3).find(_ == 3) // Some(3)
List(1, 2, 3).map(_ * 2) // List(2, 4, 6)
List(1, 2, 3).filter(_ % 2 == 1) // List(1, 3)
List(1, 2, 3).fold(0)((acc, i) => acc + i * i) // 1 * 1 + 2 * 2 + 3 * 3 = 14
List(1, 2, 3).foldLeft("Foo")(_ + _.toString) // "Foo123"
List(1, 2, 3).foldRight("Foo")(_ + _.toString) // "123Foo"