groupBy
Groups elements of the original sequence 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 sequence.
The operation is terminal.
Since Kotlin
1.0Samples
import kotlin.test.*
fun main() {
//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> Sequence<T>.groupBy(keySelector: (T) -> K, valueTransform: (T) -> V): Map<K, List<V>>(source)
Groups values returned by the valueTransform function applied to each element of the original sequence 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 sequence.
The operation is terminal.
Since Kotlin
1.0Samples
import kotlin.test.*
fun main() {
//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
}