Scala - Lists

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
All operations on lists can be expressed in terms of the following three methods.
head
This method returns the first element of a list.
tail
This method returns a list consisting of all elements except the first.
isEmpty
This method returns true if the list is empty otherwise false.
The following example shows how to use the above methods.
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.
\>scalac Demo.scala
\>scala Demo

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

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 −
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.
\>scalac Demo.scala
\>scala Demo

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)

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

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

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.
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.
\>scalac Demo.scala
\>scala Demo

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))

You can use List.reverse method to reverse all elements of the list. The Following example shows the usage.
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.
\>scalac Demo.scala
\>scala Demo

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

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.
def +(elem: A): List[A]
Prepends an element to this list
def ::(x: A): List[A]
Adds an element at the beginning of this list.
def :::(prefix: List[A]): List[A]
Adds the elements of a given list in front of this list.
def ::(x: A): List[A]
Adds an element x at the beginning of the list
def addString(b: StringBuilder): StringBuilder
Appends all elements of the list to a string builder.
def addString(b: StringBuilder, sep: String): StringBuilder
Appends all elements of the list to a string builder using a separator string.
def apply(n: Int): A
Selects an element by its index in the list.
def contains(elem: Any): Boolean
Tests whether the list contains a given value as an element.
def copyToArray(xs: Array[A], start: Int, len: Int): Unit
Copies elements of the list to an array. Fills the given array xs with at most length (len) elements of this list, beginning at position start.
def distinct: List[A]
Builds a new list from the list without any duplicate elements.
def drop(n: Int): List[A]
Returns all elements except first n ones.
def dropRight(n: Int): List[A]
Returns all elements except last n ones.
def dropWhile(p: (A) => Boolean): List[A]
Drops longest prefix of elements that satisfy a predicate.
def endsWith[B](that: Seq[B]): Boolean
Tests whether the list ends with the given sequence.
def equals(that: Any): Boolean
The equals method for arbitrary sequences. Compares this sequence to some other object.
def exists(p: (A) => Boolean): Boolean
Tests whether a predicate holds for some of the elements of the list.
def filter(p: (A) => Boolean): List[A]
Returns all elements of the list which satisfy a predicate.
def forall(p: (A) => Boolean): Boolean
Tests whether a predicate holds for all elements of the list.
def foreach(f: (A) => Unit): Unit
Applies a function f to all elements of the list.
def head: A
Selects the first element of the list.
def indexOf(elem: A, from: Int): Int
Finds index of first occurrence value in the list, after the index position.
def init: List[A]
Returns all elements except the last.
def intersect(that: Seq[A]): List[A]
Computes the multiset intersection between the list and another sequence.
def isEmpty: Boolean
Tests whether the list is empty.
def iterator: Iterator[A]
Creates a new iterator over all elements contained in the iterable object.
def last: A
Returns the last element.
def lastIndexOf(elem: A, end: Int): Int
Finds index of last occurrence of some value in the list; before or at a given end index.
def length: Int
Returns the length of the list.
def map[B](f: (A) => B): List[B]
Builds a new collection by applying a function to all elements of this list.
def max: A
Finds the largest element.
def min: A
Finds the smallest element.
def mkString: String
Displays all elements of the list in a string.
def mkString(sep: String): String
Displays all elements of the list in a string using a separator string.
def reverse: List[A]
Returns new list with elements in reversed order.
def sorted[B >: A]: List[A]
Sorts the list according to an Ordering.
def startsWith[B](that: Seq[B], offset: Int): Boolean
Tests whether the list contains the given sequence at a given index.
def sum: A
Sums up the elements of this collection.
def tail: List[A]
Returns all elements except the first.
def take(n: Int): List[A]
Returns first "n" elements.
def takeRight(n: Int): List[A]
Returns last "n" elements.
def toArray: Array[A]
Converts the list to an array.
def toBuffer[B >: A]: Buffer[B]
Converts the list to a mutable buffer.
def toMap[T, U]: Map[T, U]
Converts this list to a map.
def toSeq: Seq[A]
Converts the list to a sequence.
def toSet[B >: A]: Set[B]
Converts the list to a set.
def toString(): String
Converts the list to a string.

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