Sequence

interface Sequence<out T> (source)

A sequence that returns values through its iterator. The values are evaluated lazily, and the sequence is potentially infinite.

Sequences can be iterated multiple times, however some sequence implementations might constrain themselves to be iterated only once. That is mentioned specifically in their documentation (e.g. generateSequence overload). The latter sequences throw an exception on an attempt to iterate them the second time.

Sequence operations, like Sequence.map, Sequence.filter etc, generally preserve that property of a sequence, and again it's documented for an operation if it doesn't.

Parameters

T - the type of elements in the sequence.

Functions

iterator

abstract operator fun iterator(): Iterator<T>

Returns an Iterator that returns the values from the sequence.

Inherited Functions

equals

open operator fun equals(other: Any?): Boolean

Indicates whether some other object is "equal to" this one. Implementations must fulfil the following requirements:

hashCode

open fun hashCode(): Int

Returns a hash code value for the object. The general contract of hashCode is:

toString

open fun toString(): String

Returns a string representation of the object.

Extension Functions

all

fun <T> Sequence<T>.all(predicate: (T) -> Boolean): Boolean

Returns true if all elements match the given predicate.

any

fun <T> Sequence<T>.any(): Boolean

Returns true if sequence has at least one element.

fun <T> Sequence<T>.any(predicate: (T) -> Boolean): Boolean

Returns true if at least one element matches the given predicate.

asIterable

fun <T> Sequence<T>.asIterable(): Iterable<T>

Creates an Iterable instance that wraps the original sequence returning its elements when being iterated.

asSequence

fun <T> Sequence<T>.asSequence(): Sequence<T>

Returns this sequence as a Sequence.

asStream

fun <T> Sequence<T>.asStream(): Stream<T>

Creates a sequential Stream instance that produces elements from the original sequence.

associate

fun <T, K, V> Sequence<T>.associate(
    transform: (T) -> Pair<K, V>
): Map<K, V>

Returns a Map containing key-value pairs provided by transform function applied to elements of the given sequence.

associateBy

fun <T, K> Sequence<T>.associateBy(
    keySelector: (T) -> K
): Map<K, T>

Returns a Map containing the elements from the given sequence indexed by the key returned from keySelector function applied to each element.

fun <T, K, V> Sequence<T>.associateBy(
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): Map<K, V>

Returns a Map containing the values provided by valueTransform and indexed by keySelector functions applied to elements of the given sequence.

associateByTo

fun <T, K, M : MutableMap<in K, in T>> Sequence<T>.associateByTo(
    destination: M,
    keySelector: (T) -> K
): M

Populates and returns the destination mutable map with key-value pairs, where key is provided by the keySelector function applied to each element of the given sequence and value is the element itself.

fun <T, K, V, M : MutableMap<in K, in V>> Sequence<T>.associateByTo(
    destination: M,
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): M

Populates and returns the destination mutable map with key-value pairs, where key is provided by the keySelector function and and value is provided by the valueTransform function applied to elements of the given sequence.

associateTo

fun <T, K, V, M : MutableMap<in K, in V>> Sequence<T>.associateTo(
    destination: M,
    transform: (T) -> Pair<K, V>
): M

Populates and returns the destination mutable map with key-value pairs provided by transform function applied to each element of the given sequence.

average

fun Sequence<Byte>.average(): Double
fun Sequence<Short>.average(): Double
fun Sequence<Int>.average(): Double
fun Sequence<Long>.average(): Double
fun Sequence<Float>.average(): Double
fun Sequence<Double>.average(): Double

Returns an average value of elements in the sequence.

constrainOnce

fun <T> Sequence<T>.constrainOnce(): Sequence<T>

Returns a wrapper sequence that provides values of this sequence, but ensures it can be iterated only one time.

contains

operator fun <T> Sequence<T>.contains(element: T): Boolean

Returns true if element is found in the sequence.

count

fun <T> Sequence<T>.count(): Int

Returns the number of elements in this sequence.

fun <T> Sequence<T>.count(predicate: (T) -> Boolean): Int

Returns the number of elements matching the given predicate.

distinct

fun <T> Sequence<T>.distinct(): Sequence<T>

Returns a sequence containing only distinct elements from the given sequence.

distinctBy

fun <T, K> Sequence<T>.distinctBy(
    selector: (T) -> K
): Sequence<T>

Returns a sequence containing only elements from the given sequence having distinct keys returned by the given selector function.

drop

fun <T> Sequence<T>.drop(n: Int): Sequence<T>

Returns a sequence containing all elements except first n elements.

dropWhile

fun <T> Sequence<T>.dropWhile(
    predicate: (T) -> Boolean
): Sequence<T>

Returns a sequence containing all elements except first elements that satisfy the given predicate.

elementAt

fun <T> Sequence<T>.elementAt(index: Int): T

Returns an element at the given index or throws an IndexOutOfBoundsException if the index is out of bounds of this sequence.

elementAtOrElse

fun <T> Sequence<T>.elementAtOrElse(
    index: Int,
    defaultValue: (Int) -> T
): T

Returns an element at the given index or the result of calling the defaultValue function if the index is out of bounds of this sequence.

elementAtOrNull

fun <T> Sequence<T>.elementAtOrNull(index: Int): T?

Returns an element at the given index or null if the index is out of bounds of this sequence.

filter

fun <T> Sequence<T>.filter(
    predicate: (T) -> Boolean
): Sequence<T>

Returns a sequence containing only elements matching the given predicate.

filterIndexed

fun <T> Sequence<T>.filterIndexed(
    predicate: (index: Int, T) -> Boolean
): Sequence<T>

Returns a sequence containing only elements matching the given predicate.

filterIndexedTo

fun <T, C : MutableCollection<in T>> Sequence<T>.filterIndexedTo(
    destination: C,
    predicate: (index: Int, T) -> Boolean
): C

Appends all elements matching the given predicate to the given destination.

filterIsInstance

fun <R> Sequence<*>.filterIsInstance(): Sequence<R>

Returns a sequence containing all elements that are instances of specified type parameter R.

fun <R> Sequence<*>.filterIsInstance(
    klass: Class<R>
): Sequence<R>

Returns a sequence containing all elements that are instances of specified class.

filterIsInstanceTo

fun <R, C : MutableCollection<in R>> Sequence<*>.filterIsInstanceTo(
    destination: C
): C

Appends all elements that are instances of specified type parameter R to the given destination.

fun <C : MutableCollection<in R>, R> Sequence<*>.filterIsInstanceTo(
    destination: C,
    klass: Class<R>
): C

Appends all elements that are instances of specified class to the given destination.

filterNot

fun <T> Sequence<T>.filterNot(
    predicate: (T) -> Boolean
): Sequence<T>

Returns a sequence containing all elements not matching the given predicate.

filterNotNull

fun <T : Any> Sequence<T?>.filterNotNull(): Sequence<T>

Returns a sequence containing all elements that are not null.

filterNotNullTo

fun <C : MutableCollection<in T>, T : Any> Sequence<T?>.filterNotNullTo(
    destination: C
): C

Appends all elements that are not null to the given destination.

filterNotTo

fun <T, C : MutableCollection<in T>> Sequence<T>.filterNotTo(
    destination: C,
    predicate: (T) -> Boolean
): C

Appends all elements not matching the given predicate to the given destination.

filterTo

fun <T, C : MutableCollection<in T>> Sequence<T>.filterTo(
    destination: C,
    predicate: (T) -> Boolean
): C

Appends all elements matching the given predicate to the given destination.

find

fun <T> Sequence<T>.find(predicate: (T) -> Boolean): T?

Returns the first element matching the given predicate, or null if no such element was found.

findLast

fun <T> Sequence<T>.findLast(predicate: (T) -> Boolean): T?

Returns the last element matching the given predicate, or null if no such element was found.

first

fun <T> Sequence<T>.first(): T

Returns first element.

fun <T> Sequence<T>.first(predicate: (T) -> Boolean): T

Returns the first element matching the given predicate.

firstOrNull

fun <T> Sequence<T>.firstOrNull(): T?

Returns the first element, or null if the sequence is empty.

fun <T> Sequence<T>.firstOrNull(
    predicate: (T) -> Boolean
): T?

Returns the first element matching the given predicate, or null if element was not found.

flatMap

fun <T, R> Sequence<T>.flatMap(
    transform: (T) -> Sequence<R>
): Sequence<R>

Returns a single sequence of all elements from results of transform function being invoked on each element of original sequence.

flatMapTo

fun <T, R, C : MutableCollection<in R>> Sequence<T>.flatMapTo(
    destination: C,
    transform: (T) -> Sequence<R>
): C

Appends all elements yielded from results of transform function being invoked on each element of original sequence, to the given destination.

flatten

fun <T> Sequence<Sequence<T>>.flatten(): Sequence<T>

Returns a sequence of all elements from all sequences in this sequence.

fun <T> Sequence<Iterable<T>>.flatten(): Sequence<T>

Returns a sequence of all elements from all iterables in this sequence.

fold

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

Accumulates value starting with initial value and applying operation from left to right to current accumulator value and each element.

foldIndexed

fun <T, R> Sequence<T>.foldIndexed(
    initial: R,
    operation: (index: Int, acc: R, T) -> R
): R

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

forEach

fun <T> Sequence<T>.forEach(action: (T) -> Unit)

Performs the given action on each element.

forEachIndexed

fun <T> Sequence<T>.forEachIndexed(
    action: (index: Int, T) -> Unit)

Performs the given action on each element, providing sequential index with the element.

groupBy

fun <T, K> Sequence<T>.groupBy(
    keySelector: (T) -> K
): Map<K, List<T>>

Groups elements of the original sequence by the key returned by the given keySelector function applied to each element and returns a map where each group key is associated with a list of corresponding elements.

fun <T, K, V> Sequence<T>.groupBy(
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): Map<K, List<V>>

Groups values returned by the valueTransform function applied to each element of the original sequence by the key returned by the given keySelector function applied to the element and returns a map where each group key is associated with a list of corresponding values.

groupByTo

fun <T, K, M : MutableMap<in K, MutableList<T>>> Sequence<T>.groupByTo(
    destination: M,
    keySelector: (T) -> K
): M

Groups elements of the original sequence by the key returned by the given keySelector function applied to each element and puts to the destination map each group key associated with a list of corresponding elements.

fun <T, K, V, M : MutableMap<in K, MutableList<V>>> Sequence<T>.groupByTo(
    destination: M,
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): M

Groups values returned by the valueTransform function applied to each element of the original sequence by the key returned by the given keySelector function applied to the element and puts to the destination map each group key associated with a list of corresponding values.

groupingBy

fun <T, K> Sequence<T>.groupingBy(
    keySelector: (T) -> K
): Grouping<T, K>

Creates a Grouping source from a sequence to be used later with one of group-and-fold operations using the specified keySelector function to extract a key from each element.

indexOf

fun <T> Sequence<T>.indexOf(element: T): Int

Returns first index of element, or -1 if the sequence does not contain element.

indexOfFirst

fun <T> Sequence<T>.indexOfFirst(
    predicate: (T) -> Boolean
): Int

Returns index of the first element matching the given predicate, or -1 if the sequence does not contain such element.

indexOfLast

fun <T> Sequence<T>.indexOfLast(
    predicate: (T) -> Boolean
): Int

Returns index of the last element matching the given predicate, or -1 if the sequence does not contain such element.

joinTo

fun <T, A> Sequence<T>.joinTo(
    buffer: A,
    separator: CharSequence = ", ",
    prefix: CharSequence = "",
    postfix: CharSequence = "",
    limit: Int = -1,
    truncated: CharSequence = "...",
    transform: (T) -> CharSequence = null
): A

Appends the string from all the elements separated using separator and using the given prefix and postfix if supplied.

joinToString

fun <T> Sequence<T>.joinToString(
    separator: CharSequence = ", ",
    prefix: CharSequence = "",
    postfix: CharSequence = "",
    limit: Int = -1,
    truncated: CharSequence = "...",
    transform: (T) -> CharSequence = null
): String

Creates a string from all the elements separated using separator and using the given prefix and postfix if supplied.

last

fun <T> Sequence<T>.last(): T

Returns the last element.

fun <T> Sequence<T>.last(predicate: (T) -> Boolean): T

Returns the last element matching the given predicate.

lastIndexOf

fun <T> Sequence<T>.lastIndexOf(element: T): Int

Returns last index of element, or -1 if the sequence does not contain element.

lastOrNull

fun <T> Sequence<T>.lastOrNull(): T?

Returns the last element, or null if the sequence is empty.

fun <T> Sequence<T>.lastOrNull(predicate: (T) -> Boolean): T?

Returns the last element matching the given predicate, or null if no such element was found.

map

fun <T, R> Sequence<T>.map(transform: (T) -> R): Sequence<R>

Returns a sequence containing the results of applying the given transform function to each element in the original sequence.

mapIndexed

fun <T, R> Sequence<T>.mapIndexed(
    transform: (index: Int, T) -> R
): Sequence<R>

Returns a sequence containing the results of applying the given transform function to each element and its index in the original sequence.

mapIndexedNotNull

fun <T, R : Any> Sequence<T>.mapIndexedNotNull(
    transform: (index: Int, T) -> R?
): Sequence<R>

Returns a sequence containing only the non-null results of applying the given transform function to each element and its index in the original sequence.

mapIndexedNotNullTo

fun <T, R : Any, C : MutableCollection<in R>> Sequence<T>.mapIndexedNotNullTo(
    destination: C,
    transform: (index: Int, T) -> R?
): C

Applies the given transform function to each element and its index in the original sequence and appends only the non-null results to the given destination.

mapIndexedTo

fun <T, R, C : MutableCollection<in R>> Sequence<T>.mapIndexedTo(
    destination: C,
    transform: (index: Int, T) -> R
): C

Applies the given transform function to each element and its index in the original sequence and appends the results to the given destination.

mapNotNull

fun <T, R : Any> Sequence<T>.mapNotNull(
    transform: (T) -> R?
): Sequence<R>

Returns a sequence containing only the non-null results of applying the given transform function to each element in the original sequence.

mapNotNullTo

fun <T, R : Any, C : MutableCollection<in R>> Sequence<T>.mapNotNullTo(
    destination: C,
    transform: (T) -> R?
): C

Applies the given transform function to each element in the original sequence and appends only the non-null results to the given destination.

mapTo

fun <T, R, C : MutableCollection<in R>> Sequence<T>.mapTo(
    destination: C,
    transform: (T) -> R
): C

Applies the given transform function to each element of the original sequence and appends the results to the given destination.

max

fun Sequence<Double>.max(): Double?
fun Sequence<Float>.max(): Float?
fun <T : Comparable<T>> Sequence<T>.max(): T?

Returns the largest element or null if there are no elements.

maxBy

fun <T, R : Comparable<R>> Sequence<T>.maxBy(
    selector: (T) -> R
): T?

Returns the first element yielding the largest value of the given function or null if there are no elements.

maxWith

fun <T> Sequence<T>.maxWith(comparator: Comparator<in T>): T?
fun <T> Sequence<T>.maxWith(comparator: Comparator<in T>): T?

Returns the first element having the largest value according to the provided comparator or null if there are no elements.

min

fun Sequence<Double>.min(): Double?
fun Sequence<Float>.min(): Float?
fun <T : Comparable<T>> Sequence<T>.min(): T?

Returns the smallest element or null if there are no elements.

minBy

fun <T, R : Comparable<R>> Sequence<T>.minBy(
    selector: (T) -> R
): T?

Returns the first element yielding the smallest value of the given function or null if there are no elements.

minWith

fun <T> Sequence<T>.minWith(comparator: Comparator<in T>): T?
fun <T> Sequence<T>.minWith(comparator: Comparator<in T>): T?

Returns the first element having the smallest value according to the provided comparator or null if there are no elements.

minus

operator fun <T> Sequence<T>.minus(element: T): Sequence<T>

Returns a sequence containing all elements of the original sequence without the first occurrence of the given element.

operator fun <T> Sequence<T>.minus(
    elements: Array<out T>
): Sequence<T>

Returns a sequence containing all elements of original sequence except the elements contained in the given elements array.

operator fun <T> Sequence<T>.minus(
    elements: Iterable<T>
): Sequence<T>

Returns a sequence containing all elements of original sequence except the elements contained in the given elements collection.

operator fun <T> Sequence<T>.minus(
    elements: Sequence<T>
): Sequence<T>

Returns a sequence containing all elements of original sequence except the elements contained in the given elements sequence.

minusElement

fun <T> Sequence<T>.minusElement(element: T): Sequence<T>

Returns a sequence containing all elements of the original sequence without the first occurrence of the given element.

none

fun <T> Sequence<T>.none(): Boolean

Returns true if the sequence has no elements.

fun <T> Sequence<T>.none(predicate: (T) -> Boolean): Boolean

Returns true if no elements match the given predicate.

onEach

fun <T> Sequence<T>.onEach(action: (T) -> Unit): Sequence<T>

Returns a sequence which performs the given action on each element of the original sequence as they pass though it.

partition

fun <T> Sequence<T>.partition(
    predicate: (T) -> Boolean
): Pair<List<T>, List<T>>

Splits the original sequence into pair of lists, where first list contains elements for which predicate yielded true, while second list contains elements for which predicate yielded false.

plus

operator fun <T> Sequence<T>.plus(element: T): Sequence<T>

Returns a sequence containing all elements of the original sequence and then the given element.

operator fun <T> Sequence<T>.plus(
    elements: Array<out T>
): Sequence<T>

Returns a sequence containing all elements of original sequence and then all elements of the given elements array.

operator fun <T> Sequence<T>.plus(
    elements: Iterable<T>
): Sequence<T>

Returns a sequence containing all elements of original sequence and then all elements of the given elements collection.

operator fun <T> Sequence<T>.plus(
    elements: Sequence<T>
): Sequence<T>

Returns a sequence containing all elements of original sequence and then all elements of the given elements sequence.

plusElement

fun <T> Sequence<T>.plusElement(element: T): Sequence<T>

Returns a sequence containing all elements of the original sequence and then the given element.

reduce

fun <S, T : S> Sequence<T>.reduce(
    operation: (acc: S, T) -> S
): S

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element.

reduceIndexed

fun <S, T : S> Sequence<T>.reduceIndexed(
    operation: (index: Int, acc: S, T) -> S
): S

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 sequence.

requireNoNulls

fun <T : Any> Sequence<T?>.requireNoNulls(): Sequence<T>

Returns an original collection containing all the non-null elements, throwing an IllegalArgumentException if there are any null elements.

single

fun <T> Sequence<T>.single(): T

Returns the single element, or throws an exception if the sequence is empty or has more than one element.

fun <T> Sequence<T>.single(predicate: (T) -> Boolean): T

Returns the single element matching the given predicate, or throws exception if there is no or more than one matching element.

singleOrNull

fun <T> Sequence<T>.singleOrNull(): T?

Returns single element, or null if the sequence is empty or has more than one element.

fun <T> Sequence<T>.singleOrNull(
    predicate: (T) -> Boolean
): T?

Returns the single element matching the given predicate, or null if element was not found or more than one element was found.

sorted

fun <T : Comparable<T>> Sequence<T>.sorted(): Sequence<T>

Returns a sequence that yields elements of this sequence sorted according to their natural sort order.

sortedBy

fun <T, R : Comparable<R>> Sequence<T>.sortedBy(
    selector: (T) -> R?
): Sequence<T>

Returns a sequence that yields elements of this sequence sorted according to natural sort order of the value returned by specified selector function.

sortedByDescending

fun <T, R : Comparable<R>> Sequence<T>.sortedByDescending(
    selector: (T) -> R?
): Sequence<T>

Returns a sequence that yields elements of this sequence sorted descending according to natural sort order of the value returned by specified selector function.

sortedDescending

fun <T : Comparable<T>> Sequence<T>.sortedDescending(): Sequence<T>

Returns a sequence that yields elements of this sequence sorted descending according to their natural sort order.

sortedWith

fun <T> Sequence<T>.sortedWith(
    comparator: Comparator<in T>
): Sequence<T>
fun <T> Sequence<T>.sortedWith(
    comparator: Comparator<in T>
): Sequence<T>

Returns a sequence that yields elements of this sequence sorted according to the specified comparator.

sum

fun Sequence<Byte>.sum(): Int
fun Sequence<Short>.sum(): Int
fun Sequence<Int>.sum(): Int
fun Sequence<Long>.sum(): Long
fun Sequence<Float>.sum(): Float
fun Sequence<Double>.sum(): Double

Returns the sum of all elements in the sequence.

sumBy

fun <T> Sequence<T>.sumBy(selector: (T) -> Int): Int

Returns the sum of all values produced by selector function applied to each element in the sequence.

sumByDouble

fun <T> Sequence<T>.sumByDouble(
    selector: (T) -> Double
): Double

Returns the sum of all values produced by selector function applied to each element in the sequence.

take

fun <T> Sequence<T>.take(n: Int): Sequence<T>

Returns a sequence containing first n elements.

takeWhile

fun <T> Sequence<T>.takeWhile(
    predicate: (T) -> Boolean
): Sequence<T>

Returns a sequence containing first elements satisfying the given predicate.

toCollection

fun <T, C : MutableCollection<in T>> Sequence<T>.toCollection(
    destination: C
): C

Appends all elements to the given destination collection.

toHashSet

fun <T> Sequence<T>.toHashSet(): HashSet<T>

Returns a HashSet of all elements.

toList

fun <T> Sequence<T>.toList(): List<T>

Returns a List containing all elements.

toMap

fun <K, V> Sequence<Pair<K, V>>.toMap(): Map<K, V>

Returns a new map containing all key-value pairs from the given sequence of pairs.

fun <K, V, M : MutableMap<in K, in V>> Sequence<Pair<K, V>>.toMap(
    destination: M
): M

Populates and returns the destination mutable map with key-value pairs from the given sequence of pairs.

toMutableList

fun <T> Sequence<T>.toMutableList(): MutableList<T>

Returns a MutableList filled with all elements of this sequence.

toMutableSet

fun <T> Sequence<T>.toMutableSet(): MutableSet<T>

Returns a mutable set containing all distinct elements from the given sequence.

toSet

fun <T> Sequence<T>.toSet(): Set<T>

Returns a Set of all elements.

toSortedSet

fun <T : Comparable<T>> Sequence<T>.toSortedSet(): SortedSet<T>
fun <T> Sequence<T>.toSortedSet(
    comparator: Comparator<in T>
): SortedSet<T>

Returns a SortedSet of all elements.

unzip

fun <T, R> Sequence<Pair<T, R>>.unzip(): Pair<List<T>, List<R>>

Returns a pair of lists, where first list is built from the first values of each pair from this sequence, second list is built from the second values of each pair from this sequence.

withIndex

fun <T> Sequence<T>.withIndex(): Sequence<IndexedValue<T>>

Returns a sequence of IndexedValue for each element of the original sequence.

zip

infix fun <T, R> Sequence<T>.zip(
    other: Sequence<R>
): Sequence<Pair<T, R>>

Returns a sequence of pairs built from elements of both sequences with same indexes. Resulting sequence has length of shortest input sequence.

fun <T, R, V> Sequence<T>.zip(
    other: Sequence<R>,
    transform: (a: T, b: R) -> V
): Sequence<V>

Returns a sequence of values built from elements of both collections with same indexes using provided transform. Resulting sequence has length of shortest input sequences.

Inheritors

FileTreeWalk

class FileTreeWalk : Sequence<File>

This class is intended to implement different file traversal methods. It allows to iterate through all files inside a given directory.