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

def scan[T](zero: T)(f: (T, Out) => T): Repr[T]

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.

Note that the zero value must be immutable.

Examples

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, materializer);
// 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)