Scala Maps


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Scala map is a collection of key/value pairs. Any value can be retrieved based on its key. Keys are unique in the Map, but values need not be unique. Maps are also called Hash tables. There are two kinds of Maps, 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 Map. If you want to use the mutable Set, you'll have to import scala.collection.mutable.Map class explicitly. If you want to use both mutable and immutable Maps in the same, then you can continue to refer to the immutable Map as Map but you can refer to the mutable set as mutable.Map. Following is the example to declare immutable Maps as follows:

// Empty hash table whose keys are strings and values are integers:
var A:Map[Char,Int] = Map()

// A map with keys and values.
val colors = Map("red" -> "#FF0000", "azure" -> "#F0FFFF")

While defining empty map, the type annotation is necessary as the system needs to assign a concrete type to variable. If we want to add a key-value pair to a Map, we can use the operator + as follows:

A += ('I' -> 1)
A += ('J' -> 5)
A += ('K' -> 10)
A += ('L' -> 100)

Basic Operations on Map:

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

Methods Description
keys This method returns an iterable containing each key in the map.
values This method returns an iterable containing each value in the map.
isEmpty This method returns true if the map is empty otherwise false.

Following is the example showing usage of the above methods:

object Test {
   def main(args: Array[String]) {
      val colors = Map("red" -> "#FF0000",
                       "azure" -> "#F0FFFF",
                       "peru" -> "#CD853F")

      val nums: Map[Int, Int] = Map()

      println( "Keys in colors : " + colors.keys )
      println( "Values in colors : " + colors.values )
      println( "Check if colors is empty : " + colors.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
Keys in colors : Set(red, azure, peru)
Values in colors : MapLike(#FF0000, #F0FFFF, #CD853F)
Check if colors is empty : false
Check if nums is empty : true

C:/>

Concatenating Maps

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

object Test {
   def main(args: Array[String]) {
      val colors1 = Map("red" -> "#FF0000",
                        "azure" -> "#F0FFFF",
                        "peru" -> "#CD853F")
      val colors2 = Map("blue" -> "#0033FF",
                        "yellow" -> "#FFFF00",
                        "red" -> "#FF0000")

      // use two or more Maps with ++ as operator
      var colors = colors1 ++ colors2
      println( "colors1 ++ colors2 : " + colors )

      // use two maps with ++ as method
      colors = colors1.++(colors2)
      println( "colors1.++(colors2)) : " + colors )

   }
}

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

C:/>scalac Test.scala
C:/>scala Test
colors1 ++ colors2 : Map(blue -> #0033FF, azure -> #F0FFFF, 
                     peru -> #CD853F, yellow -> #FFFF00, red -> #FF0000)
colors1.++(colors2)) : Map(blue -> #0033FF, azure -> #F0FFFF, 
                     peru -> #CD853F, yellow -> #FFFF00, red -> #FF0000)

C:/>

Print Keys and Values from a Map:

You can iterate through the keys and values of a Map using foreach loop. Following is the example to show the usage:

object Test {
   def main(args: Array[String]) {
      val colors = Map("red" -> "#FF0000",
                       "azure" -> "#F0FFFF",
                       "peru" -> "#CD853F")

      colors.keys.foreach{ i =>  
                           print( "Key = " + i )
                           println(" Value = " + colors(i) )}
   }
}

Here, we used method foreach associated with iterator to walk through the keys. When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
Key = red Value = #FF0000
Key = azure Value = #F0FFFF
Key = peru Value = #CD853F

C:/>

Check for a Key in Map:

You can use either Map.contains method to test if a given key exists in the map or not. Following is the example to show the usage:

object Test {
   def main(args: Array[String]) {
      val colors = Map("red" -> "#FF0000",
                       "azure" -> "#F0FFFF",
                       "peru" -> "#CD853F")

      if( colors.contains( "red" )){
           println("Red key exists with value :"  + colors("red"))
      }else{
           println("Red key does not exist")
      }
      if( colors.contains( "maroon" )){
           println("Maroon key exists with value :"  + colors("maroon"))
      }else{
           println("Maroon key does not exist")
      }
   }
}

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

C:/>scalac Test.scala
C:/>scala Test
Red key exists with value :#FF0000
Maroon key does not exist

C:/>

Scala Map Methods:

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

SN Methods with Description
1

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

Returns a new map containing mappings of this map and those provided by xs.

2

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

Returns a new map containing all the mappings of this map except mappings with a key equal to elem1, elem2 or any of elems.

3

def --(xs: GTO[A]): Map[A, B]

Returns a new map with all the key/value mappings of this map except mappings with a key equal to a key from the traversable object xs.

4

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

Optionally returns the value associated with a key.

5

def iterator: Iterator[(A, B)]

Creates a new iterator over all key/value pairs of this map

6

def addString(b: StringBuilder): StringBuilder

Appends all elements of this shrinkable collection to a string builder.

7

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

Appends all elements of this shrinkable collection to a string builder using a separator string.

8

def apply(key: A): B

Returns the value associated with the given key, or the result of the map's default method, if none exists.

9

def clear(): Unit

Removes all bindings from the map. After this operation has completed, the map will be empty.

10

def clone(): Map[A, B]

Creates a copy of the receiver object.

11

def contains(key: A): Boolean

Returns true if there is a binding for key in this map, false otherwise.

12

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

Copies values of this shrinkable collection to an array. Fills the given array xs with values of this shrinkable collection.

13

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

Counts the number of elements in the shrinkable collection which satisfy a predicate.

14

def default(key: A): B

Defines the default value computation for the map, returned when a key is not found.

15

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

Returns all elements except first n ones.

16

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

Returns all elements except last n ones

17

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

Drops longest prefix of elements that satisfy a predicate.

18

def empty: Map[A, B]

Returns the empty map of the same type as this map.

19

def equals(that: Any): Boolean

Returns true if both maps contain exactly the same keys/values, false otherwise.

20

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

Returns true if the given predicate p holds for some of the elements of this shrinkable collection, otherwise false.

21

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

Returns all elements of this shrinkable collection which satisfy a predicate.

22

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

Returns an immutable map consisting only of those key value pairs of this map where the key satisfies the predicate p.

23

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

Finds the first element of the shrinkable collection satisfying a predicate, if any.

24

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

Applies a function f to all elements of this shrinkable collection.

25

def init: Map[A, B]

Returns all elements except the last.

26

def isEmpty: Boolean

Tests whether the map is empty.

27

def keys: Iterable[A]

Returns an iterator over all keys.

28

def last: (A, B)

Returns the last element.

29

def max: (A, B)

Finds the largest element.

30

def min: (A, B)

Finds the smallest element.

31

def mkString: String

Displays all elements of this shrinkable collection in a string.

32

def product: (A, B)

Returns the product of all elements of this shrinkable collection with respect to the * operator in num.

33

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

Removes a key from this map, returning the value associated previously with that key as an option.

34

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

Retains only those mappings for which the predicate p returns true.

35

def size: Int

Return the number of elements in this map.

36

def sum: (A, B)

Returns the sum of all elements of this shrinkable collection with respect to the + operator in num.

37

def tail: Map[A, B]

Returns all elements except the first.

38

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

Returns first n elements.

39

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

Returns last n elements.

40

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

Takes longest prefix of elements that satisfy a predicate.

41

def toArray: Array[(A, B)]

Converts this shrinkable collection to an array.

42

def toBuffer[B >: A]: Buffer[B]

Returns a buffer containing all elements of this map.

43

def toList: List[A]

Returns a list containing all elements of this map.

44

def toSeq: Seq[A]

Returns a seq containing all elements of this map.

45

def toSet: Set[A]

Returns a set containing all elements of this map.

46

def toString(): String

Returns a String representation of the object.


scala_collections.htm

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