Collections

Code

val nums = listOf(1, 2, 3, 4, 5, 6)
val mixed = mutableListOf("a", "b", "c")

// map / filter / reduce
val doubled = nums.map { it * 2 }
val evens = nums.filter { it % 2 == 0 }
val total = nums.reduce { acc, n -> acc + n }
println("$doubled $evens $total")

// groupBy and associateBy
data class Person(val name: String, val team: String)
val people = listOf(Person("A", "x"), Person("B", "y"), Person("C", "x"))
println(people.groupBy { it.team })
println(people.associateBy { it.name })

// flatMap and distinct
val nested = listOf(listOf(1, 2), listOf(3, 2), listOf(5))
println(nested.flatMap { it }.distinct().sorted())

// zip and partition
val keys = listOf("a", "b", "c")
val values = listOf(1, 2, 3)
println(keys.zip(values).toMap())
val (yes, no) = nums.partition { it > 3 }
println("yes=$yes no=$no")

// chunked and windowed
println((1..10).chunked(3))
println((1..5).windowed(2))

// Sequences for lazy pipelines
val sumSq = (1..1_000_000).asSequence()
    .filter { it % 2 == 0 }
    .map { it * it }
    .take(5)
    .toList()
println(sumSq)

Explanation

Kotlin collections ship with functional operators like map, filter, groupBy, and partition that return new immutable lists. Sequences wrap collections for lazy, single-pass pipelines that avoid intermediate allocations. zip, chunked, and windowed handle pairing and sliding-window scenarios concisely.

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