kotlin-stdlib / kotlin.collections / reduce

reduce

Common
JVM
JS
Native
1.0
inline fun <S, T : S> Array<out T>.reduce(
    operation: (acc: S, T) -> S
): S
inline fun ByteArray.reduce(
    operation: (acc: Byte, Byte) -> Byte
): Byte
inline fun ShortArray.reduce(
    operation: (acc: Short, Short) -> Short
): Short
inline fun IntArray.reduce(
    operation: (acc: Int, Int) -> Int
): Int
inline fun LongArray.reduce(
    operation: (acc: Long, Long) -> Long
): Long
inline fun FloatArray.reduce(
    operation: (acc: Float, Float) -> Float
): Float
inline fun DoubleArray.reduce(
    operation: (acc: Double, Double) -> Double
): Double
inline fun BooleanArray.reduce(
    operation: (acc: Boolean, Boolean) -> Boolean
): Boolean
inline fun CharArray.reduce(
    operation: (acc: Char, Char) -> Char
): Char
inline fun <S, T : S> Iterable<T>.reduce(
    operation: (acc: S, T) -> S
): S
@ExperimentalUnsignedTypes inline fun UIntArray.reduce(
    operation: (acc: UInt, UInt) -> UInt
): UInt
@ExperimentalUnsignedTypes inline fun ULongArray.reduce(
    operation: (acc: ULong, ULong) -> ULong
): ULong
@ExperimentalUnsignedTypes inline fun UByteArray.reduce(
    operation: (acc: UByte, UByte) -> UByte
): UByte
@ExperimentalUnsignedTypes inline fun UShortArray.reduce(
    operation: (acc: UShort, UShort) -> UShort
): UShort

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val strings = listOf("a", "b", "c", "d")
println(strings.reduce { acc, string -> acc + string }) // abcd
println(strings.reduceIndexed { index, acc, string -> acc + string + index }) // ab1c2d3

// emptyList<Int>().reduce { _, _ -> 0 } //  will fail
//sampleEnd
}
Common
JVM
JS
Native
1.1
inline fun <S, T : S, K> Grouping<T, K>.reduce(
    operation: (key: K, accumulator: S, element: T) -> S
): Map<K, S>

Groups elements from the Grouping source by key and applies the reducing operation to the elements of each group sequentially starting from the second element of the group, passing the previously accumulated value and the current element as arguments, and stores the results in a new map. An initial value of accumulator is the first element of the group.



fun main(args: Array<String>) {
//sampleStart
val animals = listOf("raccoon", "reindeer", "cow", "camel", "giraffe", "goat")

// grouping by first char and collect only max of contains vowels
val compareByVowelCount = compareBy { s: String -> s.count { it in "aeiou" } }

val maxVowels = animals.groupingBy { it.first() }.reduce { _, a, b -> maxOf(a, b, compareByVowelCount) }

println(maxVowels) // {r=reindeer, c=camel, g=giraffe}
//sampleEnd
}

Parameters

operation - a function that is invoked on each subsequent element of the group with the following parameters:

Return a Map associating the key of each group with the result of accumulating the group elements.