Scala Language

Collections

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)

These methods can improve performance when sorting larger collections if the collection conversions and unboxing/boxing can be avoided. For a more detailed discussion on the performance differences, read about Scala Collection sorted, sortWith and sortBy Performance.

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"

Map Collection Cheatsheet

Note that this deals with the creation of a collection of type Map, which is distinct from the map method.

Map Creation

Map[String, Int]() 
val m1: Map[String, Int] = Map()
val m2: String Map Int = Map()

A map can be considered a collection of tuples for most operations, where the first element is the key and the second is the value.

val l = List(("a", 1), ("b", 2), ("c", 3))
val m = l.toMap                               // Map(a -> 1, b -> 2, c -> 3)

Get element

val m = Map("a" -> 1, "b" -> 2, "c" -> 3)

m.get("a")  // Some(1)
m.get("d")  // None
m("a")      // 1
m("d")      // java.util.NoSuchElementException: key not found: d

m.keys      // Set(a, b, c)
m.values    // MapLike(1, 2, 3)

Add element(s)

Map("a" -> 1, "b" -> 2) + ("c" -> 3)               // Map(a -> 1, b -> 2, c -> 3)
Map("a" -> 1, "b" -> 2) + ("a" -> 3)               // Map(a -> 3, b -> 2)
Map("a" -> 1, "b" -> 2) ++ Map("b" -> 3, "c" -> 4) // Map(a -> 1, b -> 3, c -> 4)

Common operations

In operations where an iteration over a map occurs (map, find, forEach, etc), the elements of the collection are tuples. The function parameter can either use the tuple accessors (_1, _2), or a partial function with a case block:

m.find(_._1 == "a")  // Some((a,1))
m.map {
  case (key, value) => (value, key)
}                    // Map(1 -> a, 2 -> b, 3 -> c)
m.filter(_._2 == 2)  // Map(b -> 2)
m.foldLeft(0){
  case (acc, (key, value: Int)) => acc + value
}                    // 6

Map and Filter Over A Collection

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 )

Introduction to Scala Collections

The Scala Collections framework, according to its authors, is designed to be easy to use, concise, safe, fast, and universal.

The framework is made up of Scala traits that are designed to be building blocks for creating collections. For more information on these building blocks, read the official Scala collections overview.

These built-in collections are separated into the immutable and mutable packages. By default, the immutable versions are used. Constructing a List() (without importing anything) will construct an immutable list.

One of the most powerful features of the framework is the consistent and easy-to-use interface across like-minded collections. For example, summing all elements in a collection is the same for Lists, Sets, Vectors, Seqs and Arrays:

val numList = List[Int](1, 2, 3, 4, 5)
numList.reduce((n1, n2) => n1 + n2)  // 15

val numSet = Set[Int](1, 2, 3, 4, 5)
numSet.reduce((n1, n2) => n1 + n2)   // 15

val numArray = Array[Int](1, 2, 3, 4, 5)
numArray.reduce((n1, n2) => n1 + n2) // 15

These like-minded types inherit from the Traversable trait.

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.

Traversable types

Collection classes that have the Traversable trait implement foreach and inherit many methods for performing common operations to collections, which all function identically. The most common operations are listed here:

  • Map - map, flatMap, and collect produce new collections by applying a function to each element in the original collection.
List(1, 2, 3).map(num => num * 2) // double every number = List(2, 4, 6)

// split list of letters into individual strings and put them into the same list
List("a b c", "d e").flatMap(letters => letters.split(" ")) // = List("a", "b", "c", "d", "e")
  • Conversions - toList, toArray, and many other conversion operations change the current collection into a more specific kind of collection. These are usually methods prepended with ‘to’ and the more specific type (i.e. ‘toList’ converts to a List).
val array: Array[Int] = List[Int](1, 2, 3).toArray // convert list of ints to array of ints
  • Size info - isEmpty, nonEmpty, size, and hasDefiniteSize are all metadata about the set. This allows conditional operations on the collection, or for code to determine the size of the collection, including whether it’s infinite or discrete.
List().isEmpty // true
List(1).nonEmpty // true
  • Element retrieval - head, last, find, and their Option variants are used to retrieve the first or last element, or find a specific element in the collection.
val list = List(1, 2, 3)
list.head // = 1
list.last // = 3
List(-2, -1, 0, 1, 2).filter(num => num > 0) // = List(1, 2)
  • Subdivision operations - partition, splitAt, span, and groupBy split the current collection into different parts.
// split numbers into < 0 and >= 0
List(-2, -1, 0, 1, 2).partition(num => num < 0) // = (List(-2, -1), List(0, 1, 2))
  • Element tests - exists, forall, and count are operations used to check this collection to see if it satisfies a predicate.
List(1, 2, 3, 4).forall(num => num > 0) // = true, all numbers are positive
List(-3, -2, -1, 1).forall(num => num < 0) // = false, not all numbers are negative

Fold

The fold method iterates over a collection, using an initial accumulator value and applying a function that uses each element to update the accumulator successfully:

val nums = List(1,2,3,4,5)
var initialValue:Int = 0;
var sum = nums.fold(initialValue){
  (accumulator,currentElementBeingIterated) => accumulator + currentElementBeingIterated
}
println(sum) //prints 15 because 0+1+2+3+4+5 = 15

In the above example, an anonymous function was supplied to fold(). You can also use a named function that takes two arguments. Bearing this in my, the above example can be re-written thus:

def sum(x: Int, y: Int) = x+ y
val nums = List(1, 2, 3, 4, 5)
var initialValue: Int = 0
val sum = nums.fold(initialValue)(sum)
println(sum) // prints 15 because 0 + 1 + 2 + 3 + 4 + 5 = 15

Changing the initial value will affect the result:

initialValue = 2;
sum = nums.fold(initialValue){ 
 (accumulator,currentElementBeingIterated) => accumulator + currentElementBeingIterated
}
println(sum) //prints 17 because 2+1+2+3+4+5 = 17

The fold method has two variants - foldLeft and foldRight.

foldLeft() iterates from left to right (from the first element of the collection to the last in that order). foldRight() iterates from right to left (from the last element to the first element). fold() iterates from left to right like foldLeft(). In fact, fold() actually calls foldLeft() internally.

def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1 = foldLeft(z)(op)

fold(), foldLeft() and foldRight() will return a value that has the same type with the initial value it takes. However, unlike foldLeft() and foldRight(), the initial value given to fold() can only be of the same type or a supertype of the type of the collection.

In this example the order is not relevant, so you can change fold() to foldLeft() or foldRight() and the result will remain the same. Using a function that is sensitive to order will alter results.

If in doubt, prefer foldLeft() over foldRight(). foldRight() is less performant.

Foreach

foreach is unusual among the collections iterators in that it does not return a result. Instead it applies a function to each element that has only side effects. For example:

scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> x.foreach { println }
1
2
3

The function supplied to foreach can have any return type, but the result will be discarded. Typically foreach is used when side effects are desirable. If you want to transform data consider using map, filter, a for comprehension, or another option.

Example of discarding results

def myFunc(a: Int) : Int = a * 2
List(1,2,3).foreach(myFunc) // Returns nothing

Reduce

The reduce(), reduceLeft() and reduceRight methods are similar to folds. The function passed to reduce takes two values and yields a third. When operating on a list, the first two values are the first two values in the list. The result of the function and the next value in the list are then re-applied to the function, yielding a new result. This new result is applied with the next value of the list and so on until there are no more elements. The final result is returned.

val nums = List(1,2,3,4,5)
sum = nums.reduce({ (a, b) => a + b })
println(sum) //prints 15

val names = List("John","Koby", "Josh", "Matilda", "Zac", "Mary Poppins")

def findLongest(nameA:String, nameB:String):String = {
  if (nameA.length > nameB.length) nameA else nameB
}

def findLastAlphabetically(nameA:String, nameB:String):String = {
  if (nameA > nameB) nameA else nameB
}

val longestName:String = names.reduce(findLongest(_,_))
println(longestName) //prints Mary Poppins

//You can also omit the arguments if you want
val lastAlphabetically:String = names.reduce(findLastAlphabetically)
println(lastAlphabetically) //prints Zac

There are some differences in how the reduce functions work as compared to the fold functions. They are:

  1. The reduce functions have no initial accumulator value.
  2. Reduce functions cannot be called on empty lists.
  3. Reduce functions can only return the type or supertype of the list.
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