kotlin-stdlib / kotlin.collections / reduceIndexed

reduceIndexed

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

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element with its index in the original array.

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

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
}

Parameters

operation - function that takes the index of an element, current accumulator value and the element itself, and calculates the next accumulator value.

Common
JVM
JS
Native
1.0
inline fun <S, T : S> Iterable<T>.reduceIndexed(
    operation: (index: Int, 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 with its index in the original collection.

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

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
}

Parameters

operation - function that takes the index of an element, current accumulator value and the element itself, and calculates the next accumulator value.