reduce
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.
Samples
import samples.*
import kotlin.test.*
fun main() {
//sampleStart
val strings = listOf("a", "b", "c", "d")
assertPrints(strings.reduce { acc, string -> acc + string }, "abcd")
assertPrints(strings.reduceIndexed { index, acc, string -> acc + string + index }, "ab1c2d3")
assertFails { emptyList<Int>().reduce { _, _ -> 0 } }
//sampleEnd
}Parameters
function that takes current accumulator value and an element, and calculates the next accumulator value.
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.
Samples
import samples.*
import kotlin.test.*
fun main() {
//sampleStart
val strings = listOf("a", "b", "c", "d")
assertPrints(strings.reduce { acc, string -> acc + string }, "abcd")
assertPrints(strings.reduceIndexed { index, acc, string -> acc + string + index }, "ab1c2d3")
assertFails { emptyList<Int>().reduce { _, _ -> 0 } }
//sampleEnd
}Parameters
function that takes current accumulator value and an element, and calculates the next accumulator value.
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
Samples
import samples.*
import kotlin.test.*
fun main() {
//sampleStart
val strings = listOf("a", "b", "c", "d")
assertPrints(strings.reduce { acc, string -> acc + string }, "abcd")
assertPrints(strings.reduceIndexed { index, acc, string -> acc + string + index }, "ab1c2d3")
assertFails { emptyList<Int>().reduce { _, _ -> 0 } }
//sampleEnd
}Parameters
function that takes current accumulator value and an element, and calculates the next accumulator value.
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.
Return
a Map associating the key of each group with the result of accumulating the group elements.
Since Kotlin
Samples
import 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) }
assertPrints(maxVowels, "{r=reindeer, c=camel, g=giraffe}")
//sampleEnd
}Parameters
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.