Learn Scala Map with Examples Quickly & Effectively

1. Scala Map – Objective

In this tutorial on Scala Map, we will see how to define and process maps, and what methods to call on them. We will learn to declare a Scala Map, Operations on a Map in Scala, Concatenating Maps, Printing Keys and Values from a Scala Map, Searching for a Key in a Map, Methods to Call on a Map etc.

So, let’s begin Scala Map Tutorial.

2. An Introduction to Maps in Scala

A Map in Scala is a collection of key-value pairs, and is also called a hash table. We can use a key to access a value. These keys are unique; however, the values may be common. The default Scala Map is immutable. To use a mutable Map, we use the scala.collection.mutable.Map class. To use both in the same place, refer to the immutable Map as Map, and to the mutable Map as mutable.Map. I also recommend you refer our latest blog on Scala Closures which is explained in detail with Examples. For now let’s jump to learn to declare a Scala Map.

3. Declaring a Scala Map

We can either declare an empty Scala map or one with values.

a. Declaring an Empty Scala Map

scala> var m:Map[String,Int]=Map()
m: Map[String,Int] = Map()

Here, we must include the type annotation so it can assign proper types to the variables.

b. Declaring a Scala Map with Values

When we provide values, Scala will use those to infer the type of variables. So, we don’t need to include the type annotations.

scala> var m=Map("Ayushi"->0,"Megha"->1)
m: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 1)

Now, to add a key-value pair to this, we do the following:

scala> m+=("Ruchi"->2)
scala> m
res1: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 1, Ruchi -> 2)

Learn: Scala Arrays and Multidimensional Arrays in Scala

4. Operations on a Map in Scala

These are the basic operations we can carry out on a Map:

a. keys

This returns an iterable with each key in the Map.

scala> m.keys
res2: Iterable[String] = Set(Ayushi, Megha, Ruchi)
scala> m("Megha")
res56: Int = 1

b. values

This returns an iterable with each value in the Scala Map.

scala> m.values
res3: Iterable[Int] = MapLike.DefaultValuesIterable(0, 1, 2)

c. isEmpty

If the Map is empty, this returns true; otherwise, false.

scala> m.isEmpty
res4: Boolean = false
scala> Map().isEmpty
res5: Boolean = true

5. Concatenating Maps in Scala

We can concatenate/joing two Maps in more than one way. Let’s take another Scala Map for this.

scala> var m1=Map("Megha"->3,"Ruchi"->2,"Becky"->4)
m1: scala.collection.immutable.Map[String,Int] = Map(Megha -> 3, Ruchi -> 2, Becky -> 4)

The ++ Operator

scala> m++m1
res6: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 3, Ruchi -> 2, Becky -> 4)
scala> m1++m
res7: scala.collection.immutable.Map[String,Int] = Map(Megha -> 1, Ruchi -> 2, Becky -> 4, Ayushi -> 0)

See the difference in the values for “Megha” in both cases?
Learn: Scala Operator

6. Printing Keys and Values from a Map

We can use a foreach loop to walk through the keys and values of a Scala Map:

scala> m.keys.foreach{i=>println(i+" "+m(i))}

Ayushi 0
Megha 1
Ruchi 2

7. Searching for a Key in a Scala Map

The Map.contains() method will tell us if a certain key exists in the Map.

scala> m.contains("Ayushi")
res10: Boolean = true
scala> m.contains("Fluffy")
res11: Boolean = false

Any Doubt yet in Scala Map? Please Comment.

8. Methods to Call on a Scala Map

We can call the following methods on a Map. (Note that they don’t modify the original Scala Map)

a. def ++(xs: Map[(A, B)]): Map[A, B]

This concatenates two Maps.

scala> m.++(m1)
res15: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 3, Ruchi -> 2, Becky -> 4)
scala> m1.++(m)
res16: scala.collection.immutable.Map[String,Int] = Map(Megha -> 1, Ruchi -> 2, Becky -> 4, Ayushi -> 0)

b. def -(elem1: A, elem2: A, elems: A*): Map[A, B]

This returns a new Map eliding the pairs for the keys mentioned in the arguments.

scala> m.-("Ayushi","Ruchi")
res21: scala.collection.immutable.Map[String,Int] = Map(Megha -> 1)

c. def get(key: A): Option[B]

This returns the value associated with the key; it returns this as an Option.

scala> m.get("Megha")
res27: Option[Int] = Some(1)
scala> m.get("Fluffy")
res28: Option[Int] = None

d. def iterator: Iterator[(A, B)]

This returns an iterator over the Scala Map.

scala> m.iterator
res29: Iterator[(String, Int)] = non-empty iterator

Learn: Scala Arrays and Multidimensional Arrays in Scala

e. def addString(b: StringBuilder): StringBuilder

This appends all elements of the Map to the String Builder.

scala> m.addString(new StringBuilder())
res30: StringBuilder = Ayushi -> 0Megha -> 1Ruchi -> 2

f. def addString(b: StringBuilder, sep: String): StringBuilder

This does what the above method does, except it introduces a separator between the pairs.

scala> m.addString(new StringBuilder(),"*")
res31: StringBuilder = Ayushi -> 0*Megha -> 1*Ruchi -> 2

g. def apply(key: A): B

This searches for a key in the Scala Map.

scala> m.apply("Fluffy")
java.util.NoSuchElementException: key not found: Fluffy
 at scala.collection.immutable.Map$Map3.apply(Map.scala:167)
 ... 28 elided
scala> m.apply("Ayushi")
res33: Int = 0

h. def clear(): Unit

This actually removes all bindings from the Map in Scala.

scala> import scala.collection.mutable.Map
import scala.collection.mutable.Map
scala> var m2=scala.collection.mutable.Map("One"->1,"Two"->2,"Three"->3)
m2: scala.collection.mutable.Map[String,Int] = Map(One -> 1, Two -> 2, Three -> 3)
scala> m2.clear()
scala> m2
res36: scala.collection.mutable.Map[String,Int] = Map()

Doing this to m1 will result in the following error:

scala> m1.clear()
<console>:13: error: value clear is not a member of scala.collection.immutable.Map[String,Int]
      m1.clear()
         ^

i. def clone(): Map[A, B]

This creates a clone/copy of the receiver object.

We can’t clone an immutable Map in Scala. So, let’s revive m2.

scala> var m2=scala.collection.mutable.Map("One"->1,"Two"->2,"Three"->3)
m2: scala.collection.mutable.Map[String,Int] = Map(One -> 1, Two -> 2, Three -> 3)
scala> m2.clone()
res38: scala.collection.mutable.Map[String,Int] = Map(One -> 1, Two -> 2, Three -> 3)

j. def contains(key: A): Boolean

If the Map contains this key, this returns true; otherwise, false.

scala> m.contains("Megha")
res40: Boolean = true
scala> m.contains("Fluffy")
res41: Boolean = false

k. def copyToArray(xs: Array[(A, B)]): Unit

This fills key-value pairs from the Map into an Array.

scala> var arr:Array[Any]=Array(0,0,0,0,0,0,0,0)
arr: Array[Any] = Array(0, 0, 0, 0, 0, 0, 0, 0)
scala> m.copyToArray(arr)
scala> arr
res47: Array[Any] = Array((Ayushi,0), (Megha,1), (Ruchi,2), 0, 0, 0, 0, 0)

l. def count(p: ((A, B)) => Boolean): Int

This returns the number of key-value pairs in the Scala Map that satisfy the given predicate.

scala> m.count(x=>true)
res49: Int = 3

Learn: Tuples in Scala – A Quick and Easy Tutorial

m. def drop(n: Int): Map[A, B]

This returns all elements except the first n.

scala> m.drop(2)
res59: scala.collection.immutable.Map[String,Int] = Map(Ruchi -> 2)

n. def dropRight(n: Int): Map[A, B]

This returns all elements except the last n.

scala> m.dropRight(2)
res60: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0)

o. def dropWhile(p: ((A, B)) => Boolean): Map[A, B]

This drops pairs until the predicate becomes false for a pair.

scala> m.dropWhile(x=>false)
res61: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 1, Ruchi -> 2)

p. def empty: Map[A, B]

This returns an empty Map of the same kind.

scala> m.empty
res68: scala.collection.immutable.Map[String,Int] = Map()

q. def equals(that: Any): Boolean

This returns true if both Maps contain the same key-value pairs; otherwise, false.

scala> m.equals(m1)
res69: Boolean = false

r. def exists(p: ((A, B)) => Boolean): Boolean

If the predicate holds true for some elements of the  Scala Map, this returns true; otherwise, false.

s. def filter(p: ((A, B))=> Boolean): Map[A, B]

This returns all such elements (see above).

t. def filterKeys(p: (A) => Boolean): Map[A, B]

This returns all pairs where the key satisfies the predicate.

u. def find(p: ((A, B)) => Boolean): Option[(A, B)]

This returns the first element that satisfies the predicate.

v. def foreach(f: ((A, B)) => Unit): Unit

This applies the function to all elements of the Scala Map.

w. def init: Map[A, B]

This returns all elements except the last.

scala> m.init
res82: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 1)

x. def isEmpty: Boolean

If the Map is empty, this returns true; otherwise, false.

scala> m.isEmpty
res83: Boolean = false

y. def keys: Iterable[A]

This returns an iterator over all keys in the Map.

scala> m.keys
res84: Iterable[String] = Set(Ayushi, Megha, Ruchi)

z. def last: (A, B)

This returns the last element from the Map in Scala.

scala> m.last
res85: (String, Int) = (Ruchi,2)

aa. def max: (A, B)

This returns the largest element.

scala> m.max
res86: (String, Int) = (Ruchi,2)

ab. def min: (A, B)

This returns the smallest element.

scala> m.min
res87: (String, Int) = (Ayushi,0)

ac. def mkString: String

This represents the elements of the Map as a String.

scala> m.mkString
res88: String = Ayushi -> 0Megha -> 1Ruchi -> 2

ad. def product: (A, B)

This returns the product of all elements of the Scala Map.

ae. def remove(key: A): Option[B]

This removes a key from the Map and returns the value. Let’s take a new Map for this.

scala> var d=Map(1->2,3->4,5->6)
d: scala.collection.mutable.Map[Int,Int] = Map(5 -> 6, 1 -> 2, 3 -> 4)
scala> d.remove(3)
res93: Option[Int] = Some(4)

af. def retain(p: (A, B) => Boolean): Map.this.type

This retains, in the Map, only the pairs that satisfy the predicate.

Learn: Scala Functions: Quick and Easy Tutorial

ag. def size: Int

This returns the number of elements in the Map.

scala> m.size
res94: Int = 3

ah. def sum: (A, B)

This returns the sum of all elements in the Map in Scala.

ai. def tail: Map[A, B]

This returns all elements except the first.

scala> m.tail
res97: scala.collection.immutable.Map[String,Int] = Map(Megha -> 1, Ruchi -> 2)

aj. def take(n: Int): Map[A, B]

This returns the first n elements from the Map in Scala.

scala> m.take(2)
res98: scala.collection.immutable.Map[String,Int] = Map(Ayushi -> 0, Megha -> 1)

ak. def takeRight(n: Int): Map[A, B]

This returns the last n elements.

scala> m.takeRight(2)
res99: scala.collection.immutable.Map[String,Int] = Map(Megha -> 1, Ruchi -> 2)

al. def takeWhile(p: ((A, B)) => Boolean): Map[A, B]

This returns elements from the Map as long as the predicate is satisfied.

am. def toArray: Array[(A, B)]

This returns an Array from the Map.

scala> m.toArray
res100: Array[(String, Int)] = Array((Ayushi,0), (Megha,1), (Ruchi,2))

an. def toList: List[A]

This returns a List from the Map.

scala> m.toList
res101: List[(String, Int)] = List((Ayushi,0), (Megha,1), (Ruchi,2))

ao. def toSeq: Seq[A]

This returns a Sequence from the Scala Map.

scala> m.toSeq
res102: Seq[(String, Int)] = Vector((Ayushi,0), (Megha,1), (Ruchi,2))

ap. def toSet: Set[A]

This returns a Set from the Map.

scala> m.toSet
res103: scala.collection.immutable.Set[(String, Int)] = Set((Ayushi,0), (Megha,1), (Ruchi,2))

aq. def toString(): String

This returns a String from the Map.

scala> m.toString
res104: String = Map(Ayushi -> 0, Megha -> 1, Ruchi -> 2)

So, this was all on Scala Map. Hope you like our explanation,

9. Conclusion

A Map in Scala is a collection holding key-value pairs. In this tutorial, we saw how to create, process, and call methods on them.

For Reference