Scala - Iterators

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An iterator is not a collection, but rather a way to access the elements of a collection one by one. The two basic operations on an iterator it are next and hasNext. A call to it.next() will return the next element of the iterator and advance the state of the iterator. You can find out whether there are more elements to return using Iterator's it.hasNext method.

The most straightforward way to "step through" all the elements returned by an iterator is to use a while loop. Let us follow the following example program.

Example

object Demo {
   def main(args: Array[String]) {
      val it = Iterator("a", "number", "of", "words")
      
      while (it.hasNext){
         println(it.next())
      }
   }
}

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

a
number
of
words

Find Min & Max Valued Element

You can use it.min and it.max methods to find out the minimum and maximum valued elements from an iterator. Here, we used ita and itb to perform two different operations because iterator can be traversed only once. Following is the example program.

Example

object Demo {
   def main(args: Array[String]) {
      val ita = Iterator(20,40,2,50,69, 90)
      val itb = Iterator(20,40,2,50,69, 90)
      
      println("Maximum valued element " + ita.max )
      println("Minimum valued element " + itb.min )
   }
}

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

Maximum valued element 90
Minimum valued element 2

Find the Length of the Iterator

You can use either it.size or it.length methods to find out the number of elements available in an iterator. Here, we used ita and itb to perform two different operations because iterator can be traversed only once. Following is the example program.

Example

object Demo {
   def main(args: Array[String]) {
      val ita = Iterator(20,40,2,50,69, 90)
      val itb = Iterator(20,40,2,50,69, 90)
      
      println("Value of ita.size : " + ita.size )
      println("Value of itb.length : " + itb.length )
   }
}

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

Value of ita.size : 6
Value of itb.length : 6

Scala Iterator Methods

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

Duplicate an Iterator

You can duplicate an iterator using the duplicate method. This will create two new iterators from the original iterator. So you can traverse the same elements multiple times.

Example

Consider following example for duplicate an iterator -

object Demo {
   def main(args: Array[String]) = {
      val it = Iterator(1, 2, 3, 4, 5)
      val (it1, it2) = it.duplicate
      println("First iterator elements:")
      it1.foreach(println)
      println("Second iterator elements:")
      it2.foreach(println)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

First iterator elements:
1
2
3
4
5
Second iterator elements:
1
2
3
4
5

Convert Iterator to List

You can convert an iterator to a list using the toList method. It is used to store the elements of an iterator in a collection that can be traversed multiple times.

Example

Consider following example to convert iterator to a list -

object Demo {
   def main(args: Array[String]) = {
      val it = Iterator(6, 7, 8, 9, 10)
      val list = it.toList
      println("List: " + list)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

List: List(6, 7, 8, 9, 10)

Using Map with Iterators

You can apply a function to each element of an iterator using the map method. This will create a new iterator where each element is the result of applying the function to the corresponding element of the original iterator.

Example

Consider following example for using map with iterators -

object Demo {
   def main(args: Array[String]) = {
      val it = Iterator(1, 2, 3, 4)
      val mappedIt = it.map(_ * 2)
      println("Mapped iterator elements:")
      mappedIt.foreach(println)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

Mapped iterator elements:
2
4
6
8

Filtering Elements of an Iterator

You can filter elements of an iterator using the filter method. This creates a new iterator that only contains the elements which satisfy a given predicate.

Example

Consider following example for filtering elements of an iterator -

object Demo {
   def main(args: Array[String]) = {
      val it = Iterator(1, 2, 3, 4, 5, 6)
      val filteredIt = it.filter(_ % 2 == 0)
      println("Filtered iterator elements (even numbers):")
      filteredIt.foreach(println)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

Filtered iterator elements (even numbers):
2
4
6

Reducing Elements of an Iterator

You can reduce the elements of an iterator to a single value using the reduce method. This is used for operations, like summing all elements and finding the product of all elements.

Example

Consider following example for reducing elements of an iterator -

 object Demo {
   def main(args: Array[String]) = {
      val it = Iterator(1, 2, 3, 4)
      val sum = it.reduce(_ + _)
      println("Sum of elements: " + sum)
   }
}

Save the above program in Demo.scala. Use the following commands to compile and execute this program.

Command

> scalac Demo.scala
> scala Demo

Output

Sum of elements: 10

Scala Iterators Summary

• Iterators in Scala are used to access elements of a collection one by one.
• Basic operations on iterators include next and hasNext.
• Iterators can be traversed using a while loop.
• Methods like min, max, size, and length provide more functionalities.
• You can perform various operations on iterators, like duplicating, converting to list, mapping, filtering, and reducing elements.