groupingBy所在位置是kotlin.collections.groupingBy，其相关用法介绍如下。

用法一

inline fun <T, K> Array<out T>.groupingBy(
    crossinline keySelector: (T) -> K
): Grouping<T, K>

从数组中创建一个 Grouping 源，以便稍后与 group-and-fold 操作之一使用指定的 keySelector 函数从每个元素中提取一个键。

例子：



fun main(args: Array<String>) {
//sampleStart
val words = "one two three four five six seven eight nine ten".split(' ')
val frequenciesByFirstChar = words.groupingBy { it.first() }.eachCount()
println("Counting first letters:")
println(frequenciesByFirstChar) // {o=1, t=3, f=2, s=2, e=1, n=1}

val moreWords = "eleven twelve".split(' ')
val moreFrequencies = moreWords.groupingBy { it.first() }.eachCountTo(frequenciesByFirstChar.toMutableMap())
println(moreFrequencies) // {o=1, t=4, f=2, s=2, e=2, n=1}
//sampleEnd
}

输出：

Counting first letters:
{o=1, t=3, f=2, s=2, e=1, n=1}
{o=1, t=4, f=2, s=2, e=2, n=1}

用法二

inline fun <T, K> Iterable<T>.groupingBy(
    crossinline keySelector: (T) -> K
): Grouping<T, K>

从集合中创建一个 Grouping 源，以便稍后与 group-and-fold 操作之一使用指定的 keySelector 函数从每个元素中提取一个键。

例子：



fun main(args: Array<String>) {
//sampleStart
val words = "one two three four five six seven eight nine ten".split(' ')
val frequenciesByFirstChar = words.groupingBy { it.first() }.eachCount()
println("Counting first letters:")
println(frequenciesByFirstChar) // {o=1, t=3, f=2, s=2, e=1, n=1}

val moreWords = "eleven twelve".split(' ')
val moreFrequencies = moreWords.groupingBy { it.first() }.eachCountTo(frequenciesByFirstChar.toMutableMap())
println(moreFrequencies) // {o=1, t=4, f=2, s=2, e=2, n=1}
//sampleEnd
}

输出：

Counting first letters:
{o=1, t=3, f=2, s=2, e=1, n=1}
{o=1, t=4, f=2, s=2, e=2, n=1}