Kotlin·Kotlin from syntax to a coroutine app

Collections

Work with lists, sets, maps and functional operations.

Beginner 13 min read Lesson 4 of 300 Challenge included

What you will be able to do

Prefer listOf and mapOf for read-only data.
Use map and filter for transformations.
Use groupBy for grouping.
Use associateBy to index by a key.

In plain English

Kotlin separates read-only and mutable collection interfaces. Use map, filter, groupBy and associate to transform data. Read-only is the default; expose mutable state only where needed.

Collections.ktKotlin

data class Lesson(val title: String, val minutes: Int)

fun main() {
    val lessons = listOf(Lesson("HTML", 12), Lesson("CSS", 18), Lesson("SQL", 9))
    val long = lessons.filter { it.minutes >= 12 }.map { it.title }
    val total = lessons.sumOf { it.minutes }
    println(long)
    println(total)
}

Your turn

Filter and map

Easy

Return the titles of lessons with at least 12 minutes.

KOTLIN · starter

lessons

Check your understanding before moving on.

Knowledge checkQuestion 1

Which function groups elements by a key?

Select an answer to continue

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Lesson 4 of 300 · 0% of course