Sequence creation methods in Kotlin

Introduction

Sequences let you work with lists of items one by one without making a big list all at once. This saves memory and can be faster.

When you want to process a large list of numbers without using too much memory.

When you want to create a list of values that are calculated on the fly.

When you want to generate an infinite list of items and take only some of them.

When you want to chain many operations like filtering and mapping without creating intermediate lists.

Syntax

Kotlin

val sequence = sequenceOf(item1, item2, item3)

val sequence = generateSequence(seed) { nextValue }

val sequence = (start..end).asSequence()

sequenceOf creates a sequence from given items.

generateSequence creates a sequence by starting from a seed and generating next items until null.

You can convert ranges or collections to sequences using asSequence().

Examples

This creates a sequence with four numbers.

Kotlin

val seq1 = sequenceOf(1, 2, 3, 4)

This creates an infinite sequence starting at 1 and adding 1 each time.

Kotlin

val seq2 = generateSequence(1) { it + 1 }

This converts the range 1 to 5 into a sequence.

Kotlin

val seq3 = (1..5).asSequence()

Sample Program

This program creates a sequence of odd numbers starting at 1, takes the first 5, and prints them as a list.

Kotlin

fun main() {
    val seq = generateSequence(1) { it + 2 } // odd numbers starting at 1
    val firstFive = seq.take(5).toList()
    println(firstFive)
}

Important Notes

Sequences are lazy, so they only calculate items when needed.

Use toList() or similar to get a real list from a sequence.

Infinite sequences must be limited with operations like take() to avoid endless loops.

Summary

Sequences help process items one by one without storing all at once.

You can create sequences from items, ranges, or by generating values.

Sequences are lazy and efficient for big or infinite data.