Kotlin Tutorial · Functional Kotlin and Collections · Lesson 20 of 31

Kotlin Collection Operations

Transform and query collections with map, filter, sortedBy, groupBy, associate, fold, partition and more.

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Transforming and filtering

The standard library has a rich set of operations that return new collections, so you rarely write loops.

map, filter, sort and sum

data class Order(val id: Int, val customer: String, val total: Double, val paid: Boolean)

val orders = listOf(
    Order(1, "Ana", 40.0, true), Order(2, "Ben", 15.5, false),
    Order(3, "Ana", 22.0, true), Order(4, "Caro", 99.9, true),
)

fun main() {
    println(orders.filter { it.paid }.map { it.id })
    println(orders.sortedByDescending { it.total }.take(2).map { it.customer })
    println(orders.sumOf { it.total })
    println(orders.maxBy { it.total }.id)
    println(orders.any { !it.paid })
    println(orders.map { it.customer }.distinct().joinToString(", "))
}

Output

[1, 3, 4]
[Caro, Ana]
177.4
4
true
Ana, Ben, Caro

Run this on your computer — see “Get Started” for setup.

Grouping and aggregating

groupBy builds a map of lists, associateBy a map keyed by a property, fold reduces with an initial value, and partition splits into two lists.

groupBy, associateBy, partition and fold

data class Order(val id: Int, val customer: String, val total: Double, val paid: Boolean)

val orders = listOf(
    Order(1, "Ana", 40.0, true), Order(2, "Ben", 15.5, false),
    Order(3, "Ana", 22.0, true), Order(4, "Caro", 99.9, true),
)

fun main() {
    val byCustomer = orders.groupBy { it.customer }
    println(byCustomer.mapValues { (_, list) -> list.sumOf { it.total } })
    println(orders.associateBy { it.id }[3])
    val (paid, unpaid) = orders.partition { it.paid }
    println("${paid.size} paid, ${unpaid.size} unpaid")
    println(orders.fold(0) { count, o -> if (o.total > 20) count + 1 else count })
    println(orders.chunked(2).map { chunk -> chunk.map { it.id } })
}

Output

{Ana=62.0, Ben=15.5, Caro=99.9}
Order(id=3, customer=Ana, total=22.0, paid=true)
3 paid, 1 unpaid
3
[[1, 2], [3, 4]]

Run this on your computer — see “Get Started” for setup.

Test yourself

What does groupBy return?

A sorted list
A map from each key to the list of elements with that key
A set of keys
A pair of lists

Show answer

Answer: A map from each key to the list of elements with that key

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