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.

Examples

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

Scala

copysourceval 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

copysourceSource<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