associate and flatMap

Learn transformation with associate and flatMap.

associate and flatMap

associate produces a Map from a list. Each element is turned into a key-value pair. Example: words.associate { it to it.length } maps each word to its length. The to infix function creates a Pair.

flatMap transforms each element into a collection and then merges those collections into a single flat list. Example: lists.flatMap { it } flattens nested lists. While map replaces each element with one value, flatMap replaces each element with multiple values and concatenates the results.

associate and flatMap examples

associateBy selects only the key; the value is the element itself. associateWith produces a value for each key. Example: words.associateWith { it.length } maps words to their lengths.

flatMap can also unfold a string into characters: words.flatMap { it.toList() } produces characters from each word and merges them into one list. flatten only flattens nested lists without transformation. flatMap equals map followed by flatten.

Exercise Apply associate and flatMap

Define val words = listOf("apple", "pear", "cherry"). Produce a word-length Map with associate { it to it.length } and print it with println(lengths). Flatten all characters with flatMap { it.toList() } and print the count with println(chars.size).

Show example solution

fun main() {
    val words = listOf("apple", "pear", "cherry")
    val lengths = words.associate { it to it.length }
    val chars = words.flatMap { it.toList() }
    println(lengths)
    println(chars.size)
}

Test yourself

Match associate and flatMap

associate

Produces Map
flatMap

Flattens
{ it to it.length }

Key-value
List<List<T>>

Nested list

Complete the associate call

words.____ { it to it.length }

Show answer

words.associate { it to it.length }

What does flatMap return?
- A Set
- A nested list
- A Map
- A single flat list
Show answer

A single flat list — flatMap turns each element into a collection and merges into one flat list.

Next lesson: Summary - Advanced collections

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