This document introduces Scala collections and their key features: - Collections provide a concise, safe, and fast way to process collections of data through built-in functions. - Collections can be mutable or immutable, with immutable being the default. Mutable collections require importing specific packages. - The core abstractions are Traversable, Iterable, and Seq, with traits like Set and Map defining specific collection types. - Common collection types include lists, arrays, buffers, and queues - each with their own performance characteristics for different usage cases.

1. Introducing Collections
in Scala
Rishi Khandelwal
Software Consultant
Knoldus Software LLP
Email : rishi@knoldus.com

2. Features

Easy to use

Concise

Safe

Fast

Universal

3. Continued...
e.g. val (minors, adults) = people partition (_.age < 18)

It partitions a collection of people into minors and adults
depending on their age.

Much more concise than the one to three loops required for traditional
collection processing.

Writing this code is much easier, once we learn the basic
collection vocabulary.

Safer than writing explicit loops.

The partition operation is quite fast.

4. Mutable and Immutable collections
Mutable :

can change, add, or remove elements of a collection

import scala.collection.mutable
Immutable :

never change

updation return a new collection and leave the old collection
unchanged.

By default collections are immutable

5. Continued...

To use mutable collections, just import
scala.collection.mutable.

To use both mutable and immutable versions of collections is
to import just the package collection.mutable.
e.g. scala> import scala.collection.mutable
import scala.collection.mutable
scala> val immutSet=Set(1,2,3)
immutSet: scala.collection.immutable.Set[Int] = Set(1, 2, 3)
scala> val mutSet=mutable.Set(1,2,3)
mutSet: scala.collection.mutable.Set[Int] = Set(2, 1, 3)

6. Collections consistency

Quite a bit of commonality shared by all these collections.

Every kind of collection can be created by the same uniform
syntax writing collection class name followed by its elements:
e.g. Traversable(1, 2, 3)
Iterable("x", "y", "z")
Map("x" -> 24, "y" -> 25, "z" -> 26)
Set(Color.Red, Color.Green, Color.Blue)
Same principle also applies for specific collection implementations
e.g. List(1, 2, 3)
HashMap("x" -> 24, "y" -> 25, "z" -> 26)

7. Collection hierarchy
scala
Traversable
«trait»
scala
Iterable
«trait»
scala scala.collection scala.collection
Seq Set Map
«trait» «trait» «trait»

8. Trait Traversable

At the top of the collection hierarchy.

Only abstract operation is foreach:
def foreach[U](f: Elem =>U)
foreach method is meant to traverse all elements of the collection.
apply the given operation f, to each element.
Elem => U= the type of the operation.
Elem = the type of the collection’s elements.
U = an arbitrary result type.

It also defines many concrete methods

9. Trait Iterable

Next trait from the top.

All methods are defined in terms of an abstract method, iterator, which
yields the collection’s elements one by one.

Implementation of foreach :
def foreach[U](f: Elem => U): Unit = {
val it = iterator
while (it.hasNext) f(it.next())
}

Two more methods exist in Iterable that return iterators:
grouped and sliding.

These iterators do not return single elements but whole subsequences of
elements of the original collection.

10. scala> val xs = List(1, 2, 3, 4, 5)
xs: List[Int] = List(1, 2, 3, 4, 5)
grouped :
scala> val git = xs grouped 3
git: Iterator[List[Int]] = non-empty iterator
scala> git.next()
res2: List[Int] = List(1, 2, 3)
scala> git.next()
res3: List[Int] = List(4, 5)
sliding:
scala> val sit = xs sliding 3
sit: Iterator[List[Int]] = non-empty iterator
scala> sit.next()
res4: List[Int] = List(1, 2, 3)
scala> sit.next()
res5: List[Int] = List(2, 3, 4)
scala> sit.next()
res6: List[Int] = List(3, 4, 5)

11. Why have both Traversable and Iterable?
sealed abstract class Tree
case class Branch(left: Tree, right: Tree) extends Tree
case class Node(elem: Int) extends Tree

Using Traversable
sealed abstract class Tree extends Traversable[Int] {
def foreach[U](f: Int => U) = this match {
case Node(elem) => f(elem)
case Branch(l, r) => l foreach f; r foreach f
}
}

Traversing a balanced tree takes time proportional to the number of
elements in the tree.

A balanced tree with N leaves will have N - 1 interior nodes of
class branch. So the total number of steps to traverse the tree is N + N - 1.

12. Continued...

Using Iterable :
sealed abstract class Tree extends Iterable[Int] {
def iterator: Iterator[Int] = this match {
case Node(elem) => Iterator.single(elem)
case Branch(l, r) => l.iterator ++ r.iterator
}
}

There’s an efficiency problem that has to do with the implementation of the
iterator concatenation method, ++

The computation needs to follow one indirection to get at the right iterator
(either l.iterator,or r.iterator).

Overall, that makes log(N) indirections to get at a leaf of a balanced tree
with N leaves.

13. Trait Seq

Seq trait represents sequences.

A sequence is a kind of iterable that has a length and whose elements have
fixed index positions, starting from 0.

Each Seq trait has two subtraits, LinearSeq and IndexedSeq

A linear sequence has efficient head and tail operations
e.g. List, Stream

An indexed sequence has efficient apply, length, and (if mutable)
update operations. e.g. Array, ArrayBuffer

14. Sequences

Classes that inherit from trait Seq
Lists :

Always Immutable

Support fast addition and removal of items to the beginning of
the list
scala> val colors = List("red", "blue", "green")
colors: List[java.lang.String] = List(red, blue, green)
scala> colors.head
res0: java.lang.String = red
scala> colors.tail
res1: List[java.lang.String] = List(blue, green)

15. Continued...
Array :

Efficiently access an element at an arbitrary position.

Scala arrays are represented in the same way as Java arrays

Create an array whose size is known but don’t yet know the
element values:
e.g. scala> val fiveInts = new Array[Int](5)
fiveInts: Array[Int] = Array(0, 0, 0, 0, 0)

Initialize an array when we do know the element values:
e.g. scala> val fiveToOne = Array(5, 4, 3, 2, 1)
fiveToOne: Array[Int] = Array(5, 4, 3, 2, 1)

Accessing and updating an array element:
e.g. scala> fiveInts(0) = fiveToOne(4)
scala> fiveInts
res1: Array[Int] = Array(1, 0, 0, 0, 0)

16. Continued...
List buffers :

It is a mutable object which can help you build lists more
efficiently when you need to append.

Provides constant time append and prepend operations.

Append elements with the += operator,and prepend them with
the +: operator.

Obtain a List by invoking toList on the ListBuffer.

To use it, just import scala.collection.mutable.ListBuffer

17. Continued...
scala> import scala.collection.mutable.ListBuffer
import scala.collection.mutable.ListBuffer
scala> val buf = new ListBuffer[Int]
buf: scala.collection.mutable.ListBuffer[Int] = ListBuffer()
scala> buf += 1
scala> buf += 2
scala> buf
res11: scala.collection.mutable.ListBuffer[Int]= ListBuffer(1, 2)
scala> 3 +: buf
res12: scala.collection.mutable.Buffer[Int]= ListBuffer(3, 1, 2)
scala> buf.toList
res13: List[Int] = List(3, 1, 2)

18. Continued...
Array buffers :

It is like an array, except that you can additionally add and remove
elements from the beginning and end of the sequence.

To use it just import scala.collection.mutable.ArrayBuffer
e.g. scala> import scala.collection.mutable.ArrayBuffer
import scala.collection.mutable.ArrayBuffer

To create an ArrayBuffer, only specify a type parameter, no need not
specify a length.
e.g. scala> val buf = new ArrayBuffer[Int]()
buf: scala.collection.mutable.ArrayBuffer[Int] =
ArrayBuffer()

19. Continued...

Append to an ArrayBuffer using the += method:
e.g. scala> buf += 12
scala> buf += 15
scala> buf
res16: scala.collection.mutable.ArrayBuffer[Int] =
ArrayBuffer(12, 15).

All the normal array methods are available
e.g. scala> buf.length
res17: Int = 2
scala> buf(0)
res18: Int = 12

20. Continued...
Queue :

first-in-first-out sequence.

Both mutable and immutable variants of Queue.

Create an empty immutable queue:
e.g. scala> import scala.collection.immutable.Queue
import scala.collection.immutable.Queue
scala> val empty = Queue[Int]()
empty: scala.collection.immutable.Queue[Int] = Queue()

Note :
scala> val empty=new Queue[Int]
<console>:8: error: constructor Queue in class Queue cannot be
accessed in object $iw Access to protected constructor Queue not permitted
because enclosing class object $iw in object $iw is not a subclass ofclass
Queue in package immutable where target is defined
val empty=new Queue[Int]
^

21. Continued...

Append an element to an immutable queue with enqueue:
e.g. scala> val has1 = empty.enqueue(1)
has1: scala.collection.immutable.Queue[Int] = Queue(1)

To append multiple elements to a queue, call enqueue with a collection as
its argument:
e.g. scala> val has123 = has1.enqueue(List(2, 3))
has123: scala.collection.immutable.Queue[Int] =
Queue(1,2,3)

To remove an element from the head of the queue,use dequeue:
scala> val (element, has23) = has123.dequeue
element: Int = 1
has23: scala.collection.immutable.Queue[Int] = Queue(2,3)

22. Continued...

Use mutable Queue
scala> import scala.collection.mutable.Queue
import scala.collection.mutable.Queue
scala> val queue = new Queue[String]
queue: scala.collection.mutable.Queue[String] = Queue()
scala> queue += "a"
scala> queue ++= List("b", "c")
scala> queue
res21: scala.collection.mutable.Queue[String] = Queue(a, b, c)
scala> queue.dequeue
res22: String = a
scala> queue
res23: scala.collection.mutable.Queue[String] = Queue(b, c)

23. Continued...
Stack :

last-in-first-out sequence.

Both mutable and immutable variants..

push an element onto a stack with push,

pop an element with pop,

peek at the top of the stack without removing it with top
scala> import scala.collection.mutable.Stack
import scala.collection.mutable.Stack
scala> val stack = new Stack[Int]
stack: scala.collection.mutable.Stack[Int] = Stack()

24. Continued...
scala> stack.push(1)
scala> stack
res1: scala.collection.mutable.Stack[Int] = Stack(1)
scala> stack.push(2)
scala> stack
res3: scala.collection.mutable.Stack[Int] = Stack(1, 2)
scala> stack.top
res8: Int = 2
scala> stack
res9: scala.collection.mutable.Stack[Int] = Stack(1, 2)
scala> stack.pop
res10: Int = 2
scala> stack
res11: scala.collection.mutable.Stack[Int] = Stack(1)

25. Continued...
Strings (via StringOps) :

It implements many sequence methods..

Predef has an implicit conversion from String to StringOps,we can treat any
string as a Seq[Char].
scala> def hasUpperCase(s: String) = s.exists(_.isUpperCase)
hasUpperCase: (String)Boolean
scala> hasUpperCase("Robert Frost")
res14: Boolean = true
scala> hasUpperCase("e e cummings")
res15: Boolean = false

26. Trait Set

Sets are Iterables that contain no duplicate elements

Both mutable and immutable
scala.collection
Set
«trait»
scala.collection.immutable scala.collection.mutable
Set Set
«trait» «trait»
scala.collection.immutable Scala.collection.mutable
HashSet HashSet

27. Continued...
scala> val text = "See Spot run. Run, Spot. Run!"
text: java.lang.String = See Spot run. Run, Spot. Run!
scala> val wordsArray = text.split("[ !,.]+")
wordsArray: Array[java.lang.String] =
Array(See, Spot, run, Run, Spot, Run)
scala> import scala.collection.mutable
import scala.collection.mutable
scala>val words = mutable.Set.empty[String]
words: scala.collection.mutable.Set[String] = Set()
scala> for (word <- wordsArray)
words += word.toLowerCase
scala> words
res25: scala.collection.mutable.Set[String] = Set(spot, run, see)

28. Continued...

Two Set subtraits are SortedSet and BitSet
SortedSet :

No matter what order elements were added to the set, the elements are
traversed in sorted order.

Default representation of a SortedSet is an ordered binary tree

Define ordering :
scala> val myOrdering = Ordering.fromLessThan[String](_ > _)
myOrdering: scala.math.Ordering[String] = ...

Create an empty tree set with that ordering, use:
scala> import scala.collection.immutable.TreeSet
import scala.collection.immutable.TreeSet
scala> val mySet=TreeSet.empty(myOrdering)
mySet: scala.collection.immutable.TreeSet[String] = TreeSet()

29. Continued...

Default ordering Set :
scala> val set = TreeSet.empty[String]
set: scala.collection.immutable.TreeSet[String] = TreeSet()

Creating new sets from a tree set by concatenation
scala> val numbers = set + ("one", "two", "three", "four")
numbers: scala.collection.immutable.TreeSet[String] =TreeSet(four, one,
three, two)
scala> val myNumbers=mySet + ("one","two","three","four")
myNumbers: scala.collection.immutable.TreeSet[String] = TreeSet(two,
three, one, four)

Sorted sets also support ranges of elements.
scala> numbers range ("one", "two")
res13: scala.collection.immutable.TreeSet[String]= TreeSet(one, three)
scala> numbers from "three"
res14: scala.collection.immutable.TreeSet[String] = TreeSet(three, two)

30. Continued...
Bit Set :

Bit sets are sets of non-negative integer elements that are implemented in
one or more words of packed bits.

The internal representation of a bit set uses an array of Longs.

The first Long covers elements from 0 to 63, the second from 64 to 127, and
so on

For every Long, each of its 64 bits is set to 1 if the corresponding element is
contained in the set, and is unset otherwise.

It follows that the size of a bit set depends on the largest integer that’s
stored in it. If N is that largest integer, then the size of the set is N/64 Long
words,or N/8 bytes, plus a small number of extra bytes for status
information.

31. Trait Map

Maps are Iterables of pairs of keys and values.

Both mutable and immutable
scala.collection
Map
«trait»
scala.collection.immutable scala.collection.mutable
Map Map
«trait» «trait»
scala.collection.immutable Scala.collection.mutable
HashMap HashMap

32. Continued...
scala> import scala.collection.mutable
import scala.collection.mutable
scala> val map = mutable.Map.empty[String, Int]
map: scala.collection.mutable.Map[String,Int] = Map()
scala> map("hello") = 1
scala> map("there") = 2
scala> map
res2: scala.collection.mutable.Map[String,Int] = Map(there -> 2, hello -> 1)
scala> map("hello")
res3: Int = 1

33. Continued...
Sorted Map

Trait SortedMap are implemented by class TreeMap

Order is determined by Ordered trait on key element type
scala> import scala.collection.immutable.TreeMap
import scala.collection.immutable.TreeMap
scala> var tm = TreeMap(3 -> 'x', 1 -> 'x', 4 -> 'x')
tm: scala.collection.immutable.SortedMap[Int,Char] =
Map(1 -> x, 3 -> x, 4 -> x)
scala> tm += (2 -> 'x')
scala> tm
res38: scala.collection.immutable.SortedMap[Int,Char] =
Map(1 -> x, 2 -> x, 3 -> x, 4 -> x)

34. Default sets and maps

scala.collection.mutable.Set() factory returns a
scala.collection.mutable.HashSet

Similarly, the scala.collection.mutable.Map() factory returns a
scala.collection.mutable.HashMap.

The class returned by the scala.collection.immutable.Set() factory method
& scala.collection.immutable.Map() depends on how many elements you
pass to it

Number of elements Implementation
0 scala.collection.immutable.EmptySet
1 scala.collection.immutable.Set1
2 scala.collection.immutable.Set2
3 scala.collection.immutable.Set3
4 scala.collection.immutable.Set4
5 or more scala.collection.immutable.HashSet

35. Continued...
Similarily for Map

Number of elements Implementation
0 scala.collection.immutable.EmptyMap
1 scala.collection.immutable.Map1
2 scala.collection.immutable.Map2
3 scala.collection.immutable.Map3
4 scala.collection.immutable.Map4
5 or more scala.collection.immutable.HashMap

36. Synchronized sets and maps

For a thread-safe map,mix the SynchronizedMap trait into particular map
implementation
import scala.collection.mutable.{Map,SynchronizedMap, HashMap}
object MapMaker {
def makeMap: Map[String, String] = {
new HashMap[String, String] with SynchronizedMap[String, String] {
override def default(key: String) =“Why do you want to know?"
}
}
}

Similarily for sets
import scala.collection.mutable
val synchroSet =new mutable.HashSet[Int] with mutable.SynchronizedSet[Int]

37. Continued...
scala> val capital = MapMaker.makeMap
capital: scala.collection.mutable.Map[String,String] = Map()
scala> capital ++ List("US" -> "Washington",
"Paris" -> "France", "Japan" -> "Tokyo")
res0: scala.collection.mutable.Map[String,String] =
Map(Paris -> France, US -> Washington, Japan -> Tokyo)
scala> capital("Japan")
res1: String = Tokyo
scala> capital("New Zealand")
res2: String = Why do you want to know?
scala> capital += ("New Zealand" -> "Wellington")
scala> capital("New Zealand")
res3: String = Wellington

38. Selecting mutable versus immutable
collections

It is better to start with an immutable collection and change it later if you
need to.

Immutable collections can usually be stored more compactly than mutable
ones if the number of eements stored in the collection is small.

An empty mutable map in its default representation of HashMap takes up
about 80 bytes and about 16 more are added for each entry that’s added to
it.

Scala collections library currently stores immutable maps and sets with up
to four entries in a single object, which typically takes up between 16 and 40
bytes, depending on the number of entries stored in the collection.

39. Initializing collections

Most common way to create and initialize a collection is to pass the initial
elements to a factory method on the companion object of collection.
scala> List(1, 2, 3)
res0: List[Int] = List(1, 2, 3)
scala> Set('a', 'b', 'c')
res1: scala.collection.immutable.Set[Char] = Set(a, b, c)
scala> import scala.collection.mutable
import scala.collection.mutable
scala> mutable.Map("hi" -> 2, "there" -> 5)
res2: scala.collection.mutable.Map[java.lang.String,Int] =
Map(hi -> 2, there -> 5)
scala> Array(1.0, 2.0, 3.0)
res3: Array[Double] = Array(1.0, 2.0, 3.0)

40. Continued...
Initialize a collection with another collection.
scala> val colors = List("blue", "yellow", "red", "green")
colors: List[java.lang.String] = List(blue, yellow, red, green)
scala> import scala.collection.immutable.TreeSet
import scala.collection.immutable.TreeSet

Cannot pass the colors list to the factory method for TreeSet
scala> val treeSet = TreeSet(colors)
<console>:9: error: No implicit Ordering defined for List[java.lang.String].
val treeSet = TreeSet(colors)
^

Create an empty TreeSet[String] and add to it the elements of the list with
the TreeSet’s ++ operator:
scala> val treeSet = TreeSet[String]() ++ colors
treeSet: scala.collection.immutable.TreeSet[String] = TreeSet(blue, green,
red, yellow)

41. Continued...

Converting to array or list
scala> treeSet.toList
res54: List[String] = List(blue, green, red, yellow)
scala> treeSet.toArray
res55: Array[String] = Array(blue, green, red, yellow)

Converting between mutable and immutable sets and maps
scala> import scala.collection.mutable
import scala.collection.mutable
scala> treeSet
res5: scala.collection.immutable.SortedSet[String] =Set(blue, green, red,
yellow)
scala> val mutaSet = mutable.Set.empty ++ treeSet
mutaSet: scala.collection.mutable.Set[String] =Set(yellow, blue, red, green)
scala> val immutaSet = Set.empty ++ mutaSet
immutaSet: scala.collection.immutable.Set[String] =Set(yellow, blue, red,
green)

42. Tuples

A tuple can hold objects with different types. e.g. (1, "hello", Console)

Tuples do not inherit from Iterable.
e.g.
def longestWord(words: Array[String]) = {
var word = words(0)
var idx = 0
for (i <- 1 until words.length)
if (words(i).length > word.length) {
word = words(i)
idx = i
}
(word, idx)
}
scala> val longest =longestWord("The quick brown fox".split(" "))
longest: (String, Int) = (quick,1)

43. Continued...

To access elements of a tuple
scala> longest._1
res56: String = quick
scala> longest._2
res57: Int = 1

Assign each element of the tuple to its own variable
scala> val (word, idx) = longest
word: String = quick
idx: Int = 1
scala> word
res58: String = quick

Leave off the parentheses can give a different result:
scala> val word, idx = longest
word: (String, Int) = (quick,1)
idx: (String, Int) = (quick,1)