Kotlin groupBy用法及代碼示例

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>>

@ExperimentalUnsignedTypes inline fun <K> UIntArray.groupBy(
    keySelector: (UInt) -> K
): Map<K, List<UInt>>

@ExperimentalUnsignedTypes inline fun <K> ULongArray.groupBy(
    keySelector: (ULong) -> K
): Map<K, List<ULong>>

@ExperimentalUnsignedTypes inline fun <K> UByteArray.groupBy(
    keySelector: (UByte) -> K
): Map<K, List<UByte>>

@ExperimentalUnsignedTypes inline fun <K> UShortArray.groupBy(
    keySelector: (UShort) -> K
): Map<K, List<UShort>>

通過應用於每個元素的給定 keySelector 函數返回的鍵對原始數組的元素進行分組，並返回一個映射，其中每個組鍵與對應元素的列表相關聯。

返回的映射保留從原始數組生成的鍵的條目迭代順序。

例子：

import kotlin.test.*

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
}

[1, 3, 2, 4]
[[a], [abc, def], [ab], [abcd]]
mutableByLength == byLength is true

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>>

@ExperimentalUnsignedTypes inline fun <K, V> UIntArray.groupBy(
    keySelector: (UInt) -> K, 
    valueTransform: (UInt) -> V
): Map<K, List<V>>

@ExperimentalUnsignedTypes inline fun <K, V> ULongArray.groupBy(
    keySelector: (ULong) -> K, 
    valueTransform: (ULong) -> V
): Map<K, List<V>>

@ExperimentalUnsignedTypes inline fun <K, V> UByteArray.groupBy(
    keySelector: (UByte) -> K, 
    valueTransform: (UByte) -> V
): Map<K, List<V>>

@ExperimentalUnsignedTypes inline fun <K, V> UShortArray.groupBy(
    keySelector: (UShort) -> K, 
    valueTransform: (UShort) -> V
): Map<K, List<V>>

通過應用於元素的給定keySelector 函數返回的鍵對應用於原始數組的每個元素的valueTransform 函數返回的值進行分組，並返回一個映射，其中每個組鍵與對應值的列表相關聯。

返回的映射保留從原始數組生成的鍵的條目迭代順序。

例子：

import kotlin.test.*

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
}

{Marketing=[Alice, Carol], Sales=[Bob]}
mutableNamesByTeam == namesByTeam is true

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

通過應用於每個元素的給定 keySelector 函數返回的鍵對原始集合的元素進行分組，並返回一個映射，其中每個組鍵與對應元素的列表相關聯。

返回的映射保留從原始集合生成的鍵的條目迭代順序。

例子：

import kotlin.test.*

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
}

[1, 3, 2, 4]
[[a], [abc, def], [ab], [abcd]]
mutableByLength == byLength is true

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

通過應用於元素的給定keySelector 函數返回的鍵對應用於原始集合的每個元素的valueTransform 函數返回的值進行分組，並返回一個映射，其中每個組鍵與對應值的列表相關聯。

返回的映射保留從原始集合生成的鍵的條目迭代順序。

例子：

import kotlin.test.*

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
}

{Marketing=[Alice, Carol], Sales=[Bob]}
mutableNamesByTeam == namesByTeam is true