kotlin-stdlib / kotlin.collections / Grouping

Grouping

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1.1
interface Grouping<T, out K>

Represents a source of elements with a keyOf function, which can be applied to each element to get its key.

A Grouping structure serves as an intermediate step in group-and-fold operations: they group elements by their keys and then fold each group with some aggregating operation.

It is created by attaching keySelector: (T) -> K function to a source of elements. To get an instance of Grouping use one of groupingBy extension functions:

For the list of group-and-fold operations available, see the extension functions for Grouping.

Functions

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1.0

keyOf

Extracts the key of an element.

abstract fun keyOf(element: T): K
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sourceIterator

Returns an Iterator over the elements of the source of this grouping.

abstract fun sourceIterator(): Iterator<T>

Extension Functions

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aggregate

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in a new map.

fun <T, K, R> Grouping<T, K>.aggregate(
    operation: (key: K, accumulator: R?, element: T, first: Boolean) -> R
): Map<K, R>
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aggregateTo

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in the given destination map.

fun <T, K, R, M : MutableMap<in K, R>> Grouping<T, K>.aggregateTo(
    destination: M,
    operation: (key: K, accumulator: R?, element: T, first: Boolean) -> R
): M
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eachCountTo

Groups elements from the Grouping source by key and counts elements in each group to the given destination map.

fun <T, K, M : MutableMap<in K, Int>> Grouping<T, K>.eachCountTo(
    destination: M
): M
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fold

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, 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 provided by initialValueSelector function.

fun <T, K, R> Grouping<T, K>.fold(
    initialValueSelector: (key: K, element: T) -> R,
    operation: (key: K, accumulator: R, element: T) -> R
): Map<K, R>

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, 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 same initialValue for each group.

fun <T, K, R> Grouping<T, K>.fold(
    initialValue: R,
    operation: (accumulator: R, element: T) -> R
): Map<K, R>
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foldTo

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in the given destination map. An initial value of accumulator is provided by initialValueSelector function.

fun <T, K, R, M : MutableMap<in K, R>> Grouping<T, K>.foldTo(
    destination: M,
    initialValueSelector: (key: K, element: T) -> R,
    operation: (key: K, accumulator: R, element: T) -> R
): M

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in the given destination map. An initial value of accumulator is the same initialValue for each group.

fun <T, K, R, M : MutableMap<in K, R>> Grouping<T, K>.foldTo(
    destination: M,
    initialValue: R,
    operation: (accumulator: R, element: T) -> R
): M
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1.1

reduce

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.

fun <S, T : S, K> Grouping<T, K>.reduce(
    operation: (key: K, accumulator: S, element: T) -> S
): Map<K, S>
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reduceTo

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 the given destination map. An initial value of accumulator is the first element of the group.

fun <S, T : S, K, M : MutableMap<in K, S>> Grouping<T, K>.reduceTo(
    destination: M,
    operation: (key: K, accumulator: S, element: T) -> S
): M