How to convert List to Map in Kotlin?

问题:

For example I have a list of strings like:

val list = listOf("a", "b", "c", "d")

and I want to convert it to a map, where the strings are the keys.

I know I should use the .toMap() function, but I don't know how, and I haven't seen any examples of it.

回答1:

You have two choices:

The first and most performant is to use associateBy function that takes two lambdas for generating the key and value, and inlines the creation of the map:

val map = friends.associateBy({it.facebookId}, {it.points})

The second, less performant, is to use the standard map function to create a list of Pair which can be used by toMap to generate the final map:

val map = friends.map { it.facebookId to it.points }.toMap()

回答2:

#1. From List to Map with associate function

With Kotlin, List has a function called associate. associate has the following declaration:

fun <T, K, V> Iterable<T>.associate(transform: (T) -> Pair<K, V>): Map<K, V>

Returns a Map containing key-value pairs provided by transform function applied to elements of the given collection.

Usage:

class Person(val name: String, val id: Int)

fun main(args: Array<String>) {
    val friends = listOf(Person("Sue Helen", 1), Person("JR", 2), Person("Pamela", 3))
    val map = friends.associate({ Pair(it.id, it.name) })
    //val map = friends.associate({ it.id to it.name }) // also works

    println(map) // prints: {1=Sue Helen, 2=JR, 3=Pamela}
}    

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#2. From List to Map with associateBy function

With Kotlin, List has a function called associateBy. associateBy has the following declaration:

fun <T, K, V> Iterable<T>.associateBy(keySelector: (T) -> K, valueTransform: (T) -> V): Map<K, V>

Returns a Map containing the values provided by valueTransform and indexed by keySelector functions applied to elements of the given collection.

Usage:

class Person(val name: String, val id: Int)

fun main(args: Array<String>) {
    val friends = listOf(Person("Sue Helen", 1), Person("JR", 2), Person("Pamela", 3))
    val map = friends.associateBy(keySelector = { person -> person.id }, valueTransform = { person -> person.name })
    //val map = friends.associateBy({ it.id }, { it.name }) // also works

    println(map) // prints: {1=Sue Helen, 2=JR, 3=Pamela}
}

回答3:

You can use associate for this task:

val list = listOf("a", "b", "c", "d")
val m: Map<String, Int> = list.associate { it to it.length }

In this example, the strings from list become the keys and their corresponding lengths (as an example) become the values inside the map.

回答4:

That have changed on the RC version.

I am using val map = list.groupByTo(destinationMap, {it.facebookId}, { it -> it.point })

回答5:

For example, you have a list of strings like:

val list = listOf("a", "b", "c", "d")

and you need to convert it to a map, where the strings are the keys.

There are two ways to do this:

The first and most performant is to use associateBy function that takes two lambdas for generating the key and value, and inlines the creation of the map:

val map = friends.associateBy({it.facebookId}, {it.points})

The second, less performant, is to use the standard map function to create a list of Pair which can be used by toMap to generate the final map:

val map = friends.map { it.facebookId to it.points }.toMap()

Source: https://hype.codes/how-convert-list-map-kotlin