Scala - Arrays

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Scala provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. This tutorial introduces how to declare array variables, create arrays, and process arrays using indexed variables. The index of the first element of an array is the number zero and the index of the last element is the total number of elements minus one.

Declaring Array Variables

To use an array in a program, you must declare a variable to reference the array and you must specify the type of array the variable can reference.

The following is the syntax for declaring an array variable.

Syntax

var z:Array[String] = new Array[String](3)

or

var z = new Array[String](3)

Here, z is declared as an array of Strings that may hold up to three elements. Values can be assigned to individual elements or get access to individual elements, it can be done by using commands like the following −

Command

z(0) = "Zara"; z(1) = "Nuha"; z(4/2) = "Ayan"

Here, the last example shows that in general the index can be any expression that yields a whole number. There is one more way of defining an array −

var z = Array("Zara", "Nuha", "Ayan")

Following picture represents an array myList. Here, myList holds ten double values and the indices are from 0 to 9.

Processing Arrays

When processing array elements, we often use loop contol structures because all of the elements in an array are of the same type and the size of the array is known.

Below is an example program of showing how to create, initialize and process arrays −

Example

object Demo {
   def main(args: Array[String]) {
      var myList = Array(1.9, 2.9, 3.4, 3.5)
      
      // Print all the array elements
      for ( x <- myList ) {
         println( x )
      }

      // Summing all elements
      var total = 0.0;
      
      for ( i <- 0 to (myList.length - 1)) {
         total += myList(i);
      }
      println("Total is " + total);

      // Finding the largest element
      var max = myList(0);
      
      for ( i <- 1 to (myList.length - 1) ) {
         if (myList(i) > max) max = myList(i);
      }
      
      println("Max is " + max);
   }
}

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

1.9
2.9
3.4
3.5
Total is 11.7
Max is 3.5

Scala does not directly support various array operations and provides various methods to process arrays in any dimension. If you want to use the different methods then it is required to import Array._ package.

Multi-Dimensional Arrays

There are many situations where you would need to define and use multi-dimensional arrays (i.e., arrays whose elements are arrays). For example, matrices and tables are examples of structures that can be realized as two-dimensional arrays.

The following is the example of defining a two-dimensional array −

var myMatrix = ofDim[Int](3,3)

This is an array that has three elements each being an array of integers that has three elements.

Try the following example program to process a multi-dimensional array −

Example

import Array._

object Demo {
   def main(args: Array[String]) {
      var myMatrix = ofDim[Int](3,3)
      
      // build a matrix
      for (i <- 0 to 2) {
         for ( j <- 0 to 2) {
            myMatrix(i)(j) = j;
         }
      }
      
      // Print two dimensional array
      for (i <- 0 to 2) {
         for ( j <- 0 to 2) {
            print(" " + myMatrix(i)(j));
         }
         println();
      }
   }
}

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

0 1 2
0 1 2
0 1 2

Concatenate Arrays

Try the following example which makes use of concat() method to concatenate two arrays. You can pass more than one array as arguments to concat() method.

Example

import Array._

object Demo {
   def main(args: Array[String]) {
      var myList1 = Array(1.9, 2.9, 3.4, 3.5)
      var myList2 = Array(8.9, 7.9, 0.4, 1.5)

      var myList3 =  concat( myList1, myList2)
      
      // Print all the array elements
      for ( x <- myList3 ) {
         println( x )
      }
   }
}

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

1.9
2.9
3.4
3.5
8.9
7.9
0.4
1.5

Create Array with Range

Use of range() method to generate an array containing a sequence of increasing integers in a given range. You can use final argument as step to create the sequence; if you do not use final argument, then step would be assumed as 1.

Let us take an example of creating an array of range (10, 20, 2): It means creating an array with elements between 10 and 20 and range difference 2. Elements in the array are 10, 12, 14, 16, and 18.

Another example: range (10, 20). Here range difference is not given so by default it assumes 1 element. It create an array with the elements in between 10 and 20 with range difference 1. Elements in the array are 10, 11, 12, 13, …, and 19.

The following example program shows how to create an array with ranges.

Example

import Array._

object Demo {
   def main(args: Array[String]) {
      var myList1 = range(10, 20, 2)
      var myList2 = range(10,20)

      // Print all the array elements
      for ( x <- myList1 ) {
         print( " " + x )
      }
      
      println()
      for ( x <- myList2 ) {
         print( " " + x )
      }
   }
}

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

10 12 14 16 18
10 11 12 13 14 15 16 17 18 19

Scala Array Methods

Following are the important methods, which you can use while playing with array. As shown above, you would have to import Array._ package before using any of the mentioned methods. For a complete list of methods available, please check official documentation of Scala.

Sr.No	Methods with Description
1	def apply( x: T, xs: T* ): Array[T] Creates an array of T objects, where T can be Unit, Double, Float, Long, Int, Char, Short, Byte, Boolean.
2	def concat[T]( xss: Array[T]* ): Array[T] Concatenates all arrays into a single array.
3	def copy( src: AnyRef, srcPos: Int, dest: AnyRef, destPos: Int, length: Int ): Unit Copy one array to another. Equivalent to Java's System.arraycopy(src, srcPos, dest, destPos, length).
4	def empty[T]: Array[T] Returns an array of length 0
5	def iterate[T]( start: T, len: Int )( f: (T) => T ): Array[T] Returns an array containing repeated applications of a function to a start value.
6	def fill[T]( n: Int )(elem: => T): Array[T] Returns an array that contains the results of some element computation a number of times.
7	def fill[T]( n1: Int, n2: Int )( elem: => T ): Array[Array[T]] Returns a two-dimensional array that contains the results of some element computation a number of times.
8	def iterate[T]( start: T, len: Int)( f: (T) => T ): Array[T] Returns an array containing repeated applications of a function to a start value.
9	def ofDim[T]( n1: Int ): Array[T] Creates array with given dimensions.
10	def ofDim[T]( n1: Int, n2: Int ): Array[Array[T]] Creates a 2-dimensional array
11	def ofDim[T]( n1: Int, n2: Int, n3: Int ): Array[Array[Array[T]]] Creates a 3-dimensional array
12	def range( start: Int, end: Int, step: Int ): Array[Int] Returns an array containing equally spaced values in some integer interval.
13	def range( start: Int, end: Int ): Array[Int] Returns an array containing a sequence of increasing integers in a range.
14	def tabulate[T]( n: Int )(f: (Int)=> T): Array[T] Returns an array containing values of a given function over a range of integer values starting from 0.
15	def tabulate[T]( n1: Int, n2: Int )( f: (Int, Int ) => T): Array[Array[T]] Returns a two-dimensional array containing values of a given function over ranges of integer values starting from 0.