kotlin-stdlib/kotlin.sequences/runningFold

runningFold

fun <T, R> Sequence<T>.runningFold(initial: R, operation: (acc: R, T) -> R): Sequence<R>(source)

Returns a sequence containing successive accumulation values generated by applying operation from left to right to each element and current accumulator value that starts with initial value.

Note that acc value passed to operation function should not be mutated; otherwise it would affect the previous value in resulting sequence. The initial value should also be immutable (or should not be mutated) as it may be passed to operation function later because of sequence's lazy nature.

Since Kotlin

1.4

Parameters

operation

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

The operation is intermediate and stateless.

Samples

import kotlin.test.*

fun main() { 
   //sampleStart 
   val strings = listOf("a", "b", "c", "d")
println(strings.runningFold("s") { acc, string -> acc + string }) // [s, sa, sab, sabc, sabcd]
println(strings.runningFoldIndexed("s") { index, acc, string -> acc + string + index }) // [s, sa0, sa0b1, sa0b1c2, sa0b1c2d3]

println(emptyList<String>().runningFold("s") { _, _ -> "X" }) // [s] 
   //sampleEnd
}