Scala Sets | Learn About Sets in Scala Collections
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In this Scala Sets tutorial, we will learn about sets, how to define Scala Sets, how to process them, and what methods to call on sets in Scala. We will also learn Declaring a set, Operations on sets in Scala collections, Concatenating Sets, Max and Min in a Scala set, and Finding Values common to two sets in Scala.
So, let’s start the discussion on Scala Sets.
An Introduction to Sets in Scala
A Scala Set is a collection that won’t take duplicates. By default, Scala uses immutable sets. But if you want, you can import the scala.collection.mutable.Set class. To be able to refer to both of these in the same collection, we can refer to the immutable set as Set and the mutable set as mutable.Set. You can also learn Scala Array from our tutorial, once you are done with Scala sets.
Declaring a Set in Scala
You can either declare an empty set in Scala or one with values.
a. Declaring an Empty Set
We must include the type annotation when declaring an empty set, so it can be decided what concrete type to assign to a variable.
scala> var s:Set[Int]=Set() s: Set[Int] = Set()
b. Declaring a Set with Values
We may choose to elide the type annotation. When we do so, Scala will infer that from the type of values inside the set.
scala> var s=Set(1,4,4,3) s: scala.collection.immutable.Set[Int] = Set(1, 4, 3) scala> var s:Set[Int]=Set(1,4,4,3) s: Set[Int] = Set(1, 4, 3)
Note that this does not rearrange values as {1,3,4} as Python would do.
Operations on Scala Sets
Scala has the following three operations on a set:
a. head
This will return the first element of a Scala set.
scala> s.head res0: Int = 1
b. tail
This will return all elements of a set except the first.
scala> s.tail res1: scala.collection.immutable.Set[Int] = Set(4, 3)
c. isEmpty
If the set is empty, this will return a Boolean true; otherwise, false.
scala> s.isEmpty res2: Boolean = false
Concatenating Scala Sets
There are two ways to concatenate sets. Let’s take another Scala set for this:
scala> var s1=Set(7,9,8,9) s1: scala.collection.immutable.Set[Int] = Set(7, 9, 8)
a. The ++ Operator
scala> s++s1 res3: scala.collection.immutable.Set[Int] = Set(1, 9, 7, 3, 8, 4) scala> s++s1 res4: scala.collection.immutable.Set[Int] = Set(1, 9, 7, 3, 8, 4) scala> s1++s res5: scala.collection.immutable.Set[Int] = Set(1, 9, 7, 3, 8, 4)
b. The Set.++() Method
We can call the ++() method to either set and pass the other to it.
scala> s.++(s1) res6: scala.collection.immutable.Set[Int] = Set(1, 9, 7, 3, 8, 4) scala> s1.++(s) res7: scala.collection.immutable.Set[Int] = Set(1, 9, 7, 3, 8, 4)
Read: Scala Operators with Syntax and Examples
Max and Min in a Set
We can find the maximum and minimum values in a set.
a. max
This returns the maximum value from a set.
scala> s.max res8: Int = 4
b. min
This returns the minimum value from a set.
scala> s.min res9: Int = 1
Finding Values Common to Two Sets
To determine the values common to two certain sets, we can use one of two methods. Let’s take two new sets for this:
scala> var a=Set(1,4,3,4,2) a: scala.collection.immutable.Set[Int] = Set(1, 4, 3, 2) scala> var b=Set(7,9,8,2,8) b: scala.collection.immutable.Set[Int] = Set(7, 9, 8, 2)
a. The Set.&() Method
We can call &() on one set, and pass another to it.
scala> a.&(b) res10: scala.collection.immutable.Set[Int] = Set(2)
b. Set.intersect() Method
We can use the intersect() method.
scala> a.intersect(b) res11: scala.collection.immutable.Set[Int] = Set(2) scala> b.intersect(a) res12: scala.collection.immutable.Set[Int] = Set(2)
Read: Scala Functions with Syntax and Examples
Methods to Call on a Set
Playing around with sets in Scala, you can call these methods on them. (Note that they do not modify the set itself)
a. def +(elem: A): Set[A]
This adds an element to the set and returns it. (But doesn’t modify the original set)
scala> a.+("6")
res15: String = Set(1, 4, 3, 2)6b. def -(elem: A): Set[A]
This removes the element from the set and then returns it. Note that this takes an Int instead of a string for our set of Ints.
scala> a.-(2) res19: scala.collection.immutable.Set[Int] = Set(1, 4, 3) scala> a.-(6) res21: scala.collection.immutable.Set[Int] = Set(1, 4, 3, 2)
c. def contains(elem: A): Boolean
If the set contains that element, this returns true; otherwise, false.
scala> a.contains(2) res22: Boolean = true scala> a.contains(6) res23: Boolean = false
d. def &(that: Set[A]): Set[A]
This returns an intersection of two sets, as we just saw.
scala> a.&(b) res24: scala.collection.immutable.Set[Int] = Set(2)
e. def &~(that: Set[A]): Set[A]
&~ stands for set difference.
scala> a.&~(b) #All elements that are in a, but not in b res25: scala.collection.immutable.Set[Int] = Set(1, 4, 3) scala> b.&~(a) #All elements that are in b, but not in a res26: scala.collection.immutable.Set[Int] = Set(7, 9, 8)
f. def +(elem1: A, elem2: A, elems: A*): Set[A]
This adds multiple elements to a Scala set and returns it.
scala> a.+(0,6,7) res27: scala.collection.immutable.Set[Int] = Set(0, 1, 6, 2, 7, 3, 4)
g. def ++(elems: A): Set[A]
This concatenates a set with another collection.
scala> a.++(List(7,9,8)) res29: scala.collection.immutable.Set[Int] = Set(1, 9, 2, 7, 3, 8, 4) scala> a.++(b) res30: scala.collection.immutable.Set[Int] = Set(1, 9, 2, 7, 3, 8, 4)
h. def -(elem1: A, elem2: A, elems: A*): Set[A]
This removes, each element mentioned from the set.
scala> a.-(2,3,6) res31: scala.collection.immutable.Set[Int] = Set(1, 4)
i. def addString(b: StringBuilder): StringBuilder
This adds all elements of the set to the String Builder.
scala> a.addString(new StringBuilder()) res32: StringBuilder = 1432
j. def addString(b: StringBuilder, sep: String): StringBuilder
This uses a separator to the above functionality.
scala> a.addString(new StringBuilder(),"*") res33: StringBuilder = 1*4*3*2
k. def apply(elem: A)
This checks whether the element is part of the set.
scala> a.apply(1) res34: Boolean = true scala> a.apply(7) res35: Boolean = false
Read: Scala DataTypes
l. def count(p: (A) => Boolean): Int
This returns the count of elements that satisfy the predicate.
scala> a.count(x=>{x%2!=0})
res36: Int = 2m. def copyToArray(xs: Array[A], start: Int, len: Int): Unit
These copies len elements from the set to the Array xs, starting at position start.
scala> var c=Array(0,0,0,0,0,0,0,0) c: Array[Int] = Array(0, 0, 0, 0, 0, 0, 0, 0) scala> a.copyToArray(c,3,2) scala> c res39: Array[Int] = Array(0, 0, 0, 1, 4, 0, 0, 0)
n. def diff(that: Set[A]): Set[A]
This returns the set difference(elements existing in one set, but not in another)
scala> a.diff(b) res40: scala.collection.immutable.Set[Int] = Set(1, 4, 3) scala> b.diff(a) res42: scala.collection.immutable.Set[Int] = Set(7, 9, 8)
o. def drop(n: Int): Set[A]]
This returns all elements except the first n.
scala> a.drop(2) res45: scala.collection.immutable.Set[Int] = Set(3, 2)
p. def dropRight(n: Int): Set[A]
This Scala set returns all elements except the last n.
scala> a.dropRight(2) res46: scala.collection.immutable.Set[Int] = Set(1, 4)
q. def dropWhile(p: (A) => Boolean): Set[A]
This drops elements until the first element that doesn’t satisfy the predicate.
scala> a.dropWhile(x=>{x%2!=0})
res48: scala.collection.immutable.Set[Int] = Set(4, 3, 2)r. def equals(that: Any): Boolean
This set in Scala compares the set to another sequence.
scala> a.equals(List(1,4,3,2)) res50: Boolean = false
s. def exists(p: (A) => Boolean): Boolean
If the predicate holds true for some elements in the set, this returns true; otherwise, false.
scala> a.exists(x=>{x%4==0})
res52: Boolean = true
scala> a.exists(x=>{x%5==0})
res53: Boolean = falset. def filter(p: (A) => Boolean): Set[A]
This filters such elements (see the previous method)
scala> a.filter(x=>{x%4==0})
res54: scala.collection.immutable.Set[Int] = Set(4)u. def find(p: (A) => Boolean): Option[A]
This Scala set returns the first element that satisfies the predicate.
scala> a.find(x=>{x%2==0})
res55: Option[Int] = Some(4)v. def forall(p: (A) => Boolean): Boolean
This returns true if all elements of the set satisfy the predicate; otherwise, false.
scala> a.forall(x=>{x%2==0})
res56: Boolean = falsew. def head: A
This Scala Set returns the first element from the set.
scala> a.head res59: Int = 1
x. def init: Set[A]
This returns all elements from the set, except the last.
scala> a.init res62: scala.collection.immutable.Set[Int] = Set(1, 4, 3)
y. def intersect(that: Set[A]): Set[A]
This returns the intersection of two sets(elements common to both).
scala> a.intersect(b) res64: scala.collection.immutable.Set[Int] = Set(2) scala> b.intersect(a) res65: scala.collection.immutable.Set[Int] = Set(2)
z. def isEmpty: Boolean
This set in Scala returns true if the set is empty; otherwise, false.
scala> a.isEmpty res66: Boolean = false
Read: Scala String: Creating String, Concatenation, String Length
aa. def iterator: Iterator[A]
This creates a new iterator over the set.
scala> a.iterator res67: Iterator[Int] = non-empty iterator
ab. def last: A
This returns the last element from a set.
scala> a.last res68: Int = 2
ac. def map[B](f: (A) => B): immutable.Set[B]
This Scala Set applies the function to all elements of the set and returns it.
scala> a.map(x=>x*2) res69: scala.collection.immutable.Set[Int] = Set(2, 8, 6, 4)
ad. def max: A
This returns the highest value from the set.
scala> a.max res70: Int = 4
ae. def min: A
This returns the lowest element.
scala> a.min res71: Int = 1
af. def mkString: String
This Scala set represents all elements of the set as a String.
scala> a.mkString res72: String = 1432
ag. def mkString(sep: String): String
This lets us define a separator for the above method’s functionality.
scala> a.mkString("*")
res73: String = 1*4*3*2ah. def product: A
This returns the algebraic product of all elements in the set.
scala> a.product res74: Int = 24
ai. def size: Int
This returns the size of the set.
scala> a.size res75: Int = 4
aj. def splitAt(n: Int): (Set[A], Set[A])
This splits the set at the given index and returns the two resulting subsets.
scala> a.splitAt(2) res76: (scala.collection.immutable.Set[Int], scala.collection.immutable.Set[Int]) = (Set(1, 4),Set(3, 2)) scala> a.splitAt(3) res77: (scala.collection.immutable.Set[Int], scala.collection.immutable.Set[Int]) = (Set(1, 4, 3),Set(2))
ak. def subsetOf(that: Set[A]): Boolean
If the set passed as argument is a subset of this set, this returns true; else, false.
scala> Set(3,2).subsetOf(a) res79: Boolean = true scala> Set(2,3).subsetOf(a) res80: Boolean = true scala> Set(1,3,4).subsetOf(a) res82: Boolean = true
al. def sum: A
This returns the sum of all elements of the set.
scala> a.sum res83: Int = 10
am. def tail: Set[A]
This returns all elements of the set except the first.
scala> a.tail res84: scala.collection.immutable.Set[Int] = Set(4, 3, 2)
an. def take(n: Int): Set[A]
This Scala set returns the first n elements from the set.
scala> a.take(3) res85: scala.collection.immutable.Set[Int] = Set(1, 4, 3)
ao. def takeRight(n: Int):Set[A]
This returns the last n elements.
scala> a.takeRight(3) res86: scala.collection.immutable.Set[Int] = Set(4, 3, 2)
ap. def toArray: Array[A]
This returns an Array holding elements from the set.
scala> a.toArray res87: Array[Int] = Array(1, 4, 3, 2)
aq. def toList: List[A]
This returns a List from elements of the set.
scala> a.toList res88: List[Int] = List(1, 4, 3, 2)
ar. def toSeq: Seq[A]
This returns a sequence from the set.
scala> a.toSeq res90: Seq[Int] = Vector(1, 4, 3, 2)
as. def toString(): String
This represents the elements of the set as a String.
scala> a.toString res91: String = Set(1, 4, 3, 2)
This was all about Scala Sets.
Conclusion
This is all about how to declare and process Scala sets. Next, we will discuss Scala Maps. Furthermore, if you have any query, feel free to ask in the comment section.
For reference
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