Scala Sets

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Scala Set is a collection of pairwise different elements of the same type. In other words, a Set is a collection that contains no duplicate elements. There are two kinds of Sets, the immutable and the mutable. The difference between mutable and immutable objects is that when an object is immutable, the object itself can't be changed.

By default, Scala uses the immutable Set. If you want to use the mutable Set, you'll have to import scala.collection.mutable.Set class explicitly. If you want to use both mutable and immutable sets in the same, then you can continue to refer to the immutable Set as Set but you can refer to the mutable Set as mutable.Set. Following is the example to declare immutable Sets as follows:

// Empty set of integer type
var s : Set[Int] = Set()

// Set of integer type
var s : Set[Int] = Set(1,3,5,7)

or 

var s = Set(1,3,5,7)

While defining empty set, the type annotation is necessary as the system needs to assign a concrete type to variable.

Basic Operations on Set:

All operations on sets can be expressed in terms of the following three methods:

MethodsDescription
headThis method returns the first element of a set.
tailThis method returns a set consisting of all elements except the first.
isEmptyThis method returns true if the set is empty otherwise false.

Following is the example showing usage of the above methods:

object Test {
   def main(args: Array[String]) {
      val fruit = Set("apples", "oranges", "pears")
      val nums: Set[Int] = Set()

      println( "Head of fruit : " + fruit.head )
      println( "Tail of fruit : " + fruit.tail )
      println( "Check if fruit is empty : " + fruit.isEmpty )
      println( "Check if nums is empty : " + nums.isEmpty )
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
Head of fruit : apples
Tail of fruit : Set(oranges, pears)
Check if fruit is empty : false
Check if nums is empty : true

C:/>

Concatenating Sets:

You can use either ++ operator or Set.++() method to concatenate two or more sets, but while adding sets it will remove duplicate elements. Following is the example to concatenate two sets:

object Test {
   def main(args: Array[String]) {
      val fruit1 = Set("apples", "oranges", "pears")
      val fruit2 = Set("mangoes", "banana")

      // use two or more sets with ++ as operator
      var fruit = fruit1 ++ fruit2
      println( "fruit1 ++ fruit2 : " + fruit )

      // use two sets with ++ as method
      fruit = fruit1.++(fruit2)
      println( "fruit1.++(fruit2) : " + fruit )
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
fruit1 ++ fruit2 : Set(banana, apples, mangoes, pears, oranges)
fruit1.++(fruit2) : Set(banana, apples, mangoes, pears, oranges)

C:/>

Find max, min elements in Set:

You can use Set.min method to find out the minimum and Set.max method to find out the maximum of the elements available in a set. Following is the example to show the usage:

object Test {
   def main(args: Array[String]) {
      val num = Set(5,6,9,20,30,45)

      // find min and max of the elements
      println( "Min element in Set(5,6,9,20,30,45) : " + num.min )
      println( "Max element in Set(5,6,9,20,30,45) : " + num.max )
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
Min element in Set(5,6,9,20,30,45) : 5
Max element in Set(5,6,9,20,30,45) : 45

C:/>

Find common values in Sets:

You can use either Set.& method or Set.intersect method to find out the common values between two sets. Following is the example to show the usage:

object Test {
   def main(args: Array[String]) {
      val num1 = Set(5,6,9,20,30,45)
      val num2 = Set(50,60,9,20,35,55)

      // find common elements between two sets
      println( "num1.&(num2) : " + num1.&(num2) )
      println( "num1.intersect(num2) : " + num1.intersect(num2) )
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
num1.&(num2) : Set(20, 9)
num1.intersect(num2) : Set(20, 9)

C:/>

Scala Set Methods:

Following are the important methods which you can use while playing with Sets. For a complete list of methods available, please check official documentation of Scala.

SNMethods with Description
1def +(elem: A): Set[A]
Creates a new set with an additional element, unless the element is already present.
2def -(elem: A): Set[A]
Creates a new set with a given element removed from this set.
3def contains(elem: A): Boolean
Returns true if elem is contained in this set, false otherwise.
4def &(that: Set[A]): Set[A]
Returns a new set consisting of all elements that are both in this set and in the given set that.
5def &~(that: Set[A]): Set[A]
Returns the difference of this set and another set.
6def +(elem1: A, elem2: A, elems: A*): Set[A]
Creates a new immutable set with additional elements from the passed sets
7def ++(elems: A): Set[A]
Concatenates this immutable set with the elements of another collection to this immutable set.
8def -(elem1: A, elem2: A, elems: A*): Set[A]
Returns a new immutable set that contains all elements of the current immutable set except one less occurrence of each of the given argument elements.
9def addString(b: StringBuilder): StringBuilder
Appends all elements of this immutable set to a string builder.
10def addString(b: StringBuilder, sep: String): StringBuilder
Appends all elements of this immutable set to a string builder using a separator string.
11def apply(elem: A)
Tests if some element is contained in this set.
12def count(p: (A) => Boolean): Int
Counts the number of elements in the immutable set which satisfy a predicate.
13def copyToArray(xs: Array[A], start: Int, len: Int): Unit
Copies elements of this immutable set to an array.
14def diff(that: Set[A]): Set[A]
Computes the difference of this set and another set.
15def drop(n: Int): Set[A]]
Returns all elements except first n ones.
16def dropRight(n: Int): Set[A]
Returns all elements except last n ones.
17def dropWhile(p: (A) => Boolean): Set[A]
Drops longest prefix of elements that satisfy a predicate.
18def equals(that: Any): Boolean
The equals method for arbitrary sequences. Compares this sequence to some other object.
19def exists(p: (A) => Boolean): Boolean
Tests whether a predicate holds for some of the elements of this immutable set.
20def filter(p: (A) => Boolean): Set[A]
Returns all elements of this immutable set which satisfy a predicate.
21def find(p: (A) => Boolean): Option[A]
Finds the first element of the immutable set satisfying a predicate, if any.
22def forall(p: (A) => Boolean): Boolean
Tests whether a predicate holds for all elements of this immutable set.
23def foreach(f: (A) => Unit): Unit
Applies a function f to all elements of this immutable set.
24def head: A
Returns the first element of this immutable set.
25def init: Set[A]
Returns all elements except the last.
26def intersect(that: Set[A]): Set[A]
Computes the intersection between this set and another set.
27def isEmpty: Boolean
Tests if this set is empty.
28def iterator: Iterator[A]
Creates a new iterator over all elements contained in the iterable object.
29def last: A
Returns the last element.
30def map[B](f: (A) => B): immutable.Set[B]
Builds a new collection by applying a function to all elements of this immutable set.
31def max: A
Finds the largest element.
32def min: A
Finds the smallest element.
33def mkString: String
Displays all elements of this immutable set in a string.
34def mkString(sep: String): String
Displays all elements of this immutable set in a string using a separator string.
35def product: A
Returns the product of all elements of this immutable set with respect to the * operator in num.
36def size: Int
Returns the number of elements in this immutable set.
37def splitAt(n: Int): (Set[A], Set[A])
Returns a pair of immutable sets consisting of the first n elements of this immutable set, and the other elements.
38def subsetOf(that: Set[A]): Boolean
Returns true if this set is a subset of that, i.e. if every element of this set is also an element of that.
39def sum: A
Returns the sum of all elements of this immutable set with respect to the + operator in num.
40def tail: Set[A]
Returns a immutable set consisting of all elements of this immutable set except the first one.
41def take(n: Int): Set[A]
Returns first n elements.
42def takeRight(n: Int):Set[A]
Returns last n elements.
43def toArray: Array[A]
Returns an array containing all elements of this immutable set.
44def toBuffer[B >: A]: Buffer[B]
Returns a buffer containing all elements of this immutable set.
45def toList: List[A]
Returns a list containing all elements of this immutable set.
46def toMap[T, U]: Map[T, U]
Converts this immutable set to a map
47def toSeq: Seq[A]
Returns a seq containing all elements of this immutable set.
48def toString(): String
Returns a String representation of the object.


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