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What are concrete immutable collection classes in Scala?

Sarvech Qadir

There are several immutable collection classes in Scala. They are different in their implementation of maps, sets, and sequences. The collection classes are as follows:

  1. Lists
  2. LazyLists
  3. Immutable ArraySeqs
  4. Vectors
  5. Immutable Queues
  6. Ranges
  7. Compressed Hash-Array Mapped Prefix-trees
  8. Red-Black Trees
  9. Immutable BitSets
  10. VectorMaps
  11. ListMaps

Let’s cover some important immutable collection classes in this shot.

Lists

Lists are finite sized sequences. Lists offer O(1) accessibility. Most operations in a list like adding an element, removing an element are O(n) complexity.

LazyLists

A LazyList is similar to a List. Only difference is that the elements and methods are computer lazily.

By lazily, it means that all elements are constructed only as a proxy of resultant collection.

The elements are constructed whenever they are demanded. This means that a lazylist can be infinitely long.

Let’s look at the code:

scala> val lazyList = 1 #:: 2 #:: 3 #:: LazyList.empty
lazyList: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

In order to create a lazy list, use the #:: operator.

Immutable ArraySeqs

Unlike Lists, ArraySeq does not cater to the head of the collection to modify random elements from it. In lists, it is inefficient to access random locations. ArraySeqs can easily access elements at any location in a constant time O(1).

Let’s have a look at the syntax:

// create an array
scala> val arr1 = scala.collection.immutable.ArraySeq(1, 2, 3)

// append an element
scala> val arr2 = arr1 :+ 10
arr2: scala.collection.immutable.ArraySeq[Int] = ArraySeq(1, 2, 3, 10)

Since ArraySeqs are immutable, the updated, appended and prepended operations create new ArraySeqs.

Vectors

Vectors are one the most widely used collections in Scala. Unlike ArraySeq and Lists, vectors provide good performance for prepending and appending an element. Vectors have the ability to access elements in “effectively” constant time O(1).

Also, vectors provide easy modifications at any arbitrary location. Let’s see how to write the code for vectors:

// creating empty vector
scala> val vect1 = scala.collection.immutable.Vector.empty

appending 
scala> val vec2 = vec1 :+ 3 :+ 4
vec2: scala.collection.immutable.Vector[Int] = Vector(3, 4)

Immutable Queues

A queue is a FIFO (First in first out) based collection of data. Key operations include enqueue which involves adding a new element at the end of the queue, and dequeue which removes the element at the front of the queue.

// Creating an empty queue
scala> val empty = scala.collection.immutable.Queue[Int]()

// enqueue/add an element
scala> val queue1 = empty.enqueue(1)

// Removing an element
scala> queue2 = queue1.dequeue

Ranges

A Range refers to an ordered sequence of data like “1, 2, 3”, “10, 20, 30” etc. In Scala, ranges are easily defined using the start value, end value, and the stepping value. To define the range, the keywords to and by are used.

scala> 1 to 3
res2: scala.collection.immutable.Range.Inclusive = Range(1, 2, 3)
scala> 10 to 40 by 10
res3: scala.collection.immutable.Range = Range(10, 20, 30, 40)

You can use until to keep the range below the upper limit.

scala> 1 until 3
res2: scala.collection.immutable.Range = Range(1, 2)

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Sarvech Qadir
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