scan

Emit its current value, which starts at zero, and then apply the current and next value to the given function, emitting the next current value.

Simple operators

Signature

Source.scanSource.scan Flow.scanFlow.scan

Description

Emit its current value, which starts at zero, and then apply the current and next value to the given function, emitting the next current value. This means that scan emits one element downstream before, and upstream elements will not be requested until, the second element is required from downstream.

Warning

Note that the zero value must be immutable, because otherwise the same mutable instance would be shared across different threads when running the stream more than once.

Examples

Below example demonstrates how scan is similar to fold, but it keeps value from every iteration.

Scala

sourceval source = Source(1 to 5)
source.scan(0)((acc, x) => acc + x).runForeach(println)
// 0  (= 0)
// 1  (= 0 + 1)
// 3  (= 0 + 1 + 2)
// 6  (= 0 + 1 + 2 + 3)
// 10 (= 0 + 1 + 2 + 3 + 4)
// 15 (= 0 + 1 + 2 + 3 + 4 + 5)

Java

sourceSource<Integer, NotUsed> source = Source.range(1, 5);
source.scan(0, (acc, x) -> acc + x).runForeach(System.out::println, system);
// 0  (= 0)
// 1  (= 0 + 1)
// 3  (= 0 + 1 + 2)
// 6  (= 0 + 1 + 2 + 3)
// 10 (= 0 + 1 + 2 + 3 + 4)
// 15 (= 0 + 1 + 2 + 3 + 4 + 5)

Reactive Streams semantics

emits when the function scanning the element returns a new element

backpressures when downstream backpressures

completes when upstream completes