foldWhile

Start with current value zero and then apply the current and next value to the given function. When upstream completes or the predicate p returns false, the current value is emitted downstream.

Simple operators

Signature

Source.foldWhileSource.foldWhile Flow.foldWhileFlow.foldWhile

Description

Start with current value zero and then apply the current and next value to the given function. When upstream completes, the current value is emitted downstream.

Example

foldWhile is typically used to ‘fold up’ the incoming values into an aggregate with a predicate. For example, you can use foldWhile to calculate the sum while some predicate is true.

Scala

copysourceimport org.apache.pekko
import pekko.actor.ActorSystem
import pekko.stream.scaladsl.Source

Source(1 to 10)
  .foldWhile(0)(_ < 10)(_ + _)
  .runForeach(println)
// Expect prints:
// 10

Java

copysourceSource.range(1, 10)
    .foldWhile(0, acc -> acc < 10, Integer::sum)
    .runForeach(System.out::println, system);
// Expect prints:
// 100

Reactive Streams semantics