Sequences

Sequences are a type in Kotlin that provide similar features to other collections, but process data in a lazy and efficient manner. This lazy approach can be useful for processing collections, particularly when working with large amounts of data.

Unlike a List or an Array, which store all their elements in memory, a Sequence computes elements on-the-fly as they are requested.

Syntax

Sequences can be created by multiple methods, the primary options are:

val numbers = sequenceOf(2, 8, 3, 7, 10)

val letters = listOf("A", "B", "C")
val lettersSequence = letters.asSequence()

val squaresSequence = generateSequence(2) { if(it < 20) it * it else null}

• A sequence can be created by passing elements to the sequenceOf() method.
• An existing collection, such as an array or a list, can be converted into a sequence using the asSequence() extension function.
• A sequence can also be generated using the generateSequence() function. This function takes a starting value or seed, and a lambda function.

Note: In the example above the lambda expression terminates the sequence using null. A sequence generated without null is infinite and will result in an error if a function is called on the entire sequence.

Example

In this example, generateSequence() function starts with an initial value of 2 and repeatedly applies a function that increments the value by 2 in each iteration. As a result, an infinite sequence of even numbers can be generated. However, to print the sequence’s initial ten elements to the console, these elements are extracted using the take(10) function and stored in the variable firstTenEven.

fun main() {
  val evenNumbers = generateSequence(2) { it + 2 }
  val firstTenEven = evenNumbers.take(10).toList()
  println(firstTenEven)
}

The example will result in the following output:

[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]