inline fun <T, K> Iterable<T>.groupBy(
    keySelector: (T) -> K
): Map<K, List<T>>

Groups elements of the original collection by the key returned by the given keySelector function applied to each element and returns a map where each group key is associated with a list of corresponding elements.

The returned map preserves the entry iteration order of the keys produced from the original collection.

import kotlin.test.*
import java.util.*

fun main(args: Array<String>) {
//sampleStart
val words = listOf("a", "abc", "ab", "def", "abcd")
val byLength = words.groupBy { it.length }

println(byLength.keys) // [1, 3, 2, 4]
println(byLength.values) // [[a], [abc, def], [ab], [abcd]]

val mutableByLength: MutableMap<Int, MutableList<String>> = words.groupByTo(mutableMapOf()) { it.length }
// same content as in byLength map, but the map is mutable
println("mutableByLength == byLength is ${mutableByLength == byLength}") // true
//sampleEnd
}

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

Groups values returned by the valueTransform function applied to each element of the original collection by the key returned by the given keySelector function applied to the element and returns a map where each group key is associated with a list of corresponding values.

The returned map preserves the entry iteration order of the keys produced from the original collection.

import kotlin.test.*
import java.util.*

fun main(args: Array<String>) {
//sampleStart
val nameToTeam = listOf("Alice" to "Marketing", "Bob" to "Sales", "Carol" to "Marketing")
val namesByTeam = nameToTeam.groupBy({ it.second }, { it.first })
println(namesByTeam) // {Marketing=[Alice, Carol], Sales=[Bob]}

val mutableNamesByTeam = nameToTeam.groupByTo(HashMap(), { it.second }, { it.first })
// same content as in namesByTeam map, but the map is mutable
println("mutableNamesByTeam == namesByTeam is ${mutableNamesByTeam == namesByTeam}") // true
//sampleEnd
}

inline fun <T, K> Array<out T>.groupBy(
    keySelector: (T) -> K
): Map<K, List<T>>

inline fun <K> ByteArray.groupBy(
    keySelector: (Byte) -> K
): Map<K, List<Byte>>

inline fun <K> ShortArray.groupBy(
    keySelector: (Short) -> K
): Map<K, List<Short>>

inline fun <K> IntArray.groupBy(
    keySelector: (Int) -> K
): Map<K, List<Int>>

inline fun <K> LongArray.groupBy(
    keySelector: (Long) -> K
): Map<K, List<Long>>

inline fun <K> FloatArray.groupBy(
    keySelector: (Float) -> K
): Map<K, List<Float>>

inline fun <K> DoubleArray.groupBy(
    keySelector: (Double) -> K
): Map<K, List<Double>>

inline fun <K> BooleanArray.groupBy(
    keySelector: (Boolean) -> K
): Map<K, List<Boolean>>

inline fun <K> CharArray.groupBy(
    keySelector: (Char) -> K
): Map<K, List<Char>>

Groups elements of the original array by the key returned by the given keySelector function applied to each element and returns a map where each group key is associated with a list of corresponding elements.

The returned map preserves the entry iteration order of the keys produced from the original array.

import kotlin.test.*
import java.util.*

fun main(args: Array<String>) {
//sampleStart
val words = listOf("a", "abc", "ab", "def", "abcd")
val byLength = words.groupBy { it.length }

println(byLength.keys) // [1, 3, 2, 4]
println(byLength.values) // [[a], [abc, def], [ab], [abcd]]

val mutableByLength: MutableMap<Int, MutableList<String>> = words.groupByTo(mutableMapOf()) { it.length }
// same content as in byLength map, but the map is mutable
println("mutableByLength == byLength is ${mutableByLength == byLength}") // true
//sampleEnd
}

inline fun <T, K, V> Array<out T>.groupBy(
    keySelector: (T) -> K, 
    valueTransform: (T) -> V
): Map<K, List<V>>

inline fun <K, V> ByteArray.groupBy(
    keySelector: (Byte) -> K, 
    valueTransform: (Byte) -> V
): Map<K, List<V>>

inline fun <K, V> ShortArray.groupBy(
    keySelector: (Short) -> K, 
    valueTransform: (Short) -> V
): Map<K, List<V>>

inline fun <K, V> IntArray.groupBy(
    keySelector: (Int) -> K, 
    valueTransform: (Int) -> V
): Map<K, List<V>>

inline fun <K, V> LongArray.groupBy(
    keySelector: (Long) -> K, 
    valueTransform: (Long) -> V
): Map<K, List<V>>

inline fun <K, V> FloatArray.groupBy(
    keySelector: (Float) -> K, 
    valueTransform: (Float) -> V
): Map<K, List<V>>

inline fun <K, V> DoubleArray.groupBy(
    keySelector: (Double) -> K, 
    valueTransform: (Double) -> V
): Map<K, List<V>>

inline fun <K, V> BooleanArray.groupBy(
    keySelector: (Boolean) -> K, 
    valueTransform: (Boolean) -> V
): Map<K, List<V>>

inline fun <K, V> CharArray.groupBy(
    keySelector: (Char) -> K, 
    valueTransform: (Char) -> V
): Map<K, List<V>>

Groups values returned by the valueTransform function applied to each element of the original array by the key returned by the given keySelector function applied to the element and returns a map where each group key is associated with a list of corresponding values.

The returned map preserves the entry iteration order of the keys produced from the original array.

import kotlin.test.*
import java.util.*

fun main(args: Array<String>) {
//sampleStart
val nameToTeam = listOf("Alice" to "Marketing", "Bob" to "Sales", "Carol" to "Marketing")
val namesByTeam = nameToTeam.groupBy({ it.second }, { it.first })
println(namesByTeam) // {Marketing=[Alice, Carol], Sales=[Bob]}

val mutableNamesByTeam = nameToTeam.groupByTo(HashMap(), { it.second }, { it.first })
// same content as in namesByTeam map, but the map is mutable
println("mutableNamesByTeam == namesByTeam is ${mutableNamesByTeam == namesByTeam}") // true
//sampleEnd
}