kotlin-stdlib/kotlin.collections/reduce

reduce

inline fun <S, T : S> Array<out T>.reduce(operation: (acc: S, T) -> S): S(source)
inline fun ByteArray.reduce(operation: (acc: Byte, Byte) -> Byte): Byte(source)
inline fun ShortArray.reduce(operation: (acc: Short, Short) -> Short): Short(source)
inline fun IntArray.reduce(operation: (acc: Int, Int) -> Int): Int(source)
inline fun LongArray.reduce(operation: (acc: Long, Long) -> Long): Long(source)
inline fun FloatArray.reduce(operation: (acc: Float, Float) -> Float): Float(source)
inline fun DoubleArray.reduce(operation: (acc: Double, Double) -> Double): Double(source)
inline fun BooleanArray.reduce(operation: (acc: Boolean, Boolean) -> Boolean): Boolean(source)
inline fun CharArray.reduce(operation: (acc: Char, Char) -> Char): Char(source)

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

Throws an exception if this array is empty. If the array can be empty in an expected way, please use reduceOrNull instead. It returns null when its receiver is empty.

Since Kotlin

1.0

Parameters

operation

function that takes current accumulator value and an element, and calculates the next accumulator value.

Samples

import kotlin.test.*

fun main() { 
   //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
}
inline fun <S, T : S> Iterable<T>.reduce(operation: (acc: S, T) -> S): S(source)

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

Throws an exception if this collection is empty. If the collection can be empty in an expected way, please use reduceOrNull instead. It returns null when its receiver is empty.

Since Kotlin

1.0

Parameters

operation

function that takes current accumulator value and an element, and calculates the next accumulator value.

Samples

import kotlin.test.*

fun main() { 
   //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
}
inline fun <S, T : S, K> Grouping<T, K>.reduce(operation: (key: K, accumulator: S, element: T) -> S): Map<K, S>(source)

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.

Since Kotlin

1.1

Return

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

Parameters

operation

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

  • key: the key of the group this element belongs to;

  • accumulator: the current value of the accumulator of the group;

  • element: the element from the source being accumulated.

Samples


fun main() { 
   //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
}