kotlin-stdlib/kotlin.collections/reduceOrNull

reduceOrNull

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

Returns null if the array is empty.

Since Kotlin

1.4

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.reduceOrNull { acc, string -> acc + string }) // abcd
println(strings.reduceIndexedOrNull { index, acc, string -> acc + string + index }) // ab1c2d3

println(emptyList<String>().reduceOrNull { _, _ -> "" }) // null 
   //sampleEnd
}
inline fun <S, T : S> Iterable<T>.reduceOrNull(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.

Returns null if the collection is empty.

Since Kotlin

1.4

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.reduceOrNull { acc, string -> acc + string }) // abcd
println(strings.reduceIndexedOrNull { index, acc, string -> acc + string + index }) // ab1c2d3

println(emptyList<String>().reduceOrNull { _, _ -> "" }) // null 
   //sampleEnd
}