Scala - Lists

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Scala Lists are quite similar to arrays which means, all the elements of a list have the same type but there are two important differences. First, lists are immutable, which means elements of a list cannot be changed by assignment. Second, lists represent a linked list whereas arrays are flat.

The type of a list that has elements of type T is written as List[T].

Try the following example, here are few lists defined for various data types.

// List of Strings
val fruit: List[String] = List("apples", "oranges", "pears")

// List of Integers
val nums: List[Int] = List(1, 2, 3, 4)

// Empty List.
val empty: List[Nothing] = List()

// Two dimensional list
val dim: List[List[Int]] =
   List(
      List(1, 0, 0),
      List(0, 1, 0),
      List(0, 0, 1)
   )

All lists can be defined using two fundamental building blocks, a tail Nil and ::, which is pronounced cons. Nil also represents the empty list. All the above lists can be defined as follows.

// List of Strings
val fruit = "apples" :: ("oranges" :: ("pears" :: Nil))

// List of Integers
val nums = 1 :: (2 :: (3 :: (4 :: Nil)))

// Empty List.
val empty = Nil

// Two dimensional list
val dim = (1 :: (0 :: (0 :: Nil))) ::
          (0 :: (1 :: (0 :: Nil))) ::
          (0 :: (0 :: (1 :: Nil))) :: Nil

Basic Operations on Lists

All operations on lists can be expressed in terms of the following three methods.

The following example shows how to use the above methods.

Example

object Demo {
   def main(args: Array[String]) {
      val fruit = "apples" :: ("oranges" :: ("pears" :: Nil))
      val nums = Nil

      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 )
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

Head of fruit : apples
Tail of fruit : List(oranges, pears)
Check if fruit is empty : false
Check if nums is empty : true

Concatenating Lists

You can use either ::: operator or List.:::() method or List.concat() method to add two or more lists. Please find the following example given below −

Example

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

      // use two or more lists with ::: operator
      var fruit = fruit1 ::: fruit2
      println( "fruit1 ::: fruit2 : " + fruit )
      
      // use two lists with Set.:::() method
      fruit = fruit1.:::(fruit2)
      println( "fruit1.:::(fruit2) : " + fruit )

      // pass two or more lists as arguments
      fruit = List.concat(fruit1, fruit2)
      println( "List.concat(fruit1, fruit2) : " + fruit  )
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

fruit1 ::: fruit2 : List(apples, oranges, pears, mangoes, banana)
fruit1.:::(fruit2) : List(mangoes, banana, apples, oranges, pears)
List.concat(fruit1, fruit2) : List(apples, oranges, pears, mangoes, banana)

Creating Uniform Lists

You can use List.fill() method creates a list consisting of zero or more copies of the same element. Try the following example program.

Example

object Demo {
   def main(args: Array[String]) {
      val fruit = List.fill(3)("apples") // Repeats apples three times.
      println( "fruit : " + fruit  )

      val num = List.fill(10)(2)         // Repeats 2, 10 times.
      println( "num : " + num  )
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

fruit : List(apples, apples, apples)
num : List(2, 2, 2, 2, 2, 2, 2, 2, 2, 2)

Tabulating a Function

You can use a function along with List.tabulate() method to apply on all the elements of the list before tabulating the list. Its arguments are just like those of List.fill: the first argument list gives the dimensions of the list to create, and the second describes the elements of the list. The only difference is that instead of the elements being fixed, they are computed from a function.

Try the following example program.

Example

object Demo {
   def main(args: Array[String]) {
      // Creates 5 elements using the given function.
      val squares = List.tabulate(6)(n => n * n)
      println( "squares : " + squares  )

      val mul = List.tabulate( 4,5 )( _ * _ )      
      println( "mul : " + mul  )
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

squares : List(0, 1, 4, 9, 16, 25)
mul : List(List(0, 0, 0, 0, 0), List(0, 1, 2, 3, 4), 
   List(0, 2, 4, 6, 8), List(0, 3, 6, 9, 12))

Reverse List Order

You can use List.reverse method to reverse all elements of the list. The Following example shows the usage.

Example

object Demo {
   def main(args: Array[String]) {
      val fruit = "apples" :: ("oranges" :: ("pears" :: Nil))
      
      println( "Before reverse fruit : " + fruit )
      println( "After reverse fruit : " + fruit.reverse )
   }
}

Save the above program in Demo.scala. The following commands are used to compile and execute this program.

Command

\>scalac Demo.scala
\>scala Demo

Output

Before reverse fruit : List(apples, oranges, pears)
After reverse fruit : List(pears, oranges, apples)

Scala List Methods

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