Limit IntStream.iterate to a specific value

Question

I'm generating, let's say, the following range:

IntStream.iterate(1, i -> 3*i)

How do I limit the stream to a specific element value e.g. 100 (not elements count with limit())?

Thank you!

UPDATE the function can be arbitrary

Look into this if it helps https://stackoverflow.com/questions/40358827/intstream-iterate-in-steps?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa — surendrapanday, Jun 05 '18 at 13:14

score 11 · Accepted Answer · answered Jun 05 '18 at 17:29

If you can’t use Java 9 yet, you can use the following reimplementation of the three-arg IntStream.iterate:

public static IntStream iterate(int seed, IntPredicate hasNext, IntUnaryOperator next) {
    Objects.requireNonNull(next); Objects.requireNonNull(hasNext);
    return StreamSupport.intStream(
        new Spliterators.AbstractIntSpliterator(
            Long.MAX_VALUE, Spliterator.ORDERED|Spliterator.NONNULL) {
        private IntUnaryOperator op = i -> { op = next; return i; };
        private int value = seed;

        @Override
        public boolean tryAdvance(IntConsumer action) {
            Objects.requireNonNull(action);
            if(op == null) return false;
            int t = op.applyAsInt(value);
            if(!hasNext.test(t)) { op = null; return false; }
            action.accept(value = t);
            return true;
        }

        @Override
        public void forEachRemaining(IntConsumer action) {
            Objects.requireNonNull(action);
            IntUnaryOperator first = op;
            if(first == null) return;
            op = null;
            for(int t = first.applyAsInt(value); hasNext.test(t); t = next.applyAsInt(t))
                action.accept(t);
        }
    }, false);
}

It works similar to Java 9’s IntStream.iterate, except that you have to change the class you’re invoking the static method on (or adapt the import static statement):

iterate(1, i -> i < 100, i -> i*3).forEach(System.out::println);

that is cool but woow.. so much code.. ok, I guess I will workaround it at least in my case at least unless I have Java 8 — Dmitry Senkovich, Jun 06 '18 at 10:38

Anton Balaniuc · Answer 2 · 2018-06-05T13:30:56.187

5

A new method

Stream<T> iterate(T seed,Predicate<? super T> hasNext,UnaryOperator<T> next)

was introduced in Java-9. So starting with that version it is possible to do something like this:

IntStream.iterate(1, i -> i < 100, i -> 3*i)

Which will produce 1 3 9 27 81

edited Jun 05 '18 at 13:30

answered Jun 05 '18 at 13:15

Anton Balaniuc

10,889
1
35
53

ok, seems to be the thing I'm looking for. But what about Java 8? – Dmitry Senkovich Jun 05 '18 at 13:20
@DmitrySenkovich: Java Stream API has been introduced in Java 8. – Nikolas Charalambidis Jun 05 '18 at 13:21
@Nikolas I mean this predicate `hasNext` – Dmitry Senkovich Jun 05 '18 at 13:21
@DmitrySenkovich, the overloaded iterate method was added in Java-9 – Anton Balaniuc Jun 05 '18 at 13:22
@Aominè, this will produce only `1 3 9 27 81` – Anton Balaniuc Jun 05 '18 at 13:30
@AntonBalaniuc , Dmitry Senkovich Right I get the question now. – Ousmane D. Jun 05 '18 at 13:31
@Aominè might be just me, but whenever I end-up choosing `Stream#iterate` from java-9, probably time to write that as a loop, unless there are other operations that I would need like `filter, map` etc... – Eugene Jun 05 '18 at 13:57
@Eugene, I agree. Developers often use `Stream#iterate` to iterate over an array or/and do some index manipulation. But `steam` isn't designed for this purpose. It can be done easily with a loop – Anton Balaniuc Jun 05 '18 at 16:15
@Eugene Agreed! indeed. – Ousmane D. Jun 05 '18 at 22:10
@AntonBalaniuc there is nothing wrong with using for example `IntStream.iterate(0, i -> i < 100, i -> i + 1)` to index into an array or a list and do something with it, but in this case it would be _better_ to use `IntStream.range(0, 100)`. Nevertheless, I also agree that using an imperative loop for the aforementioned example would probably be better. – Ousmane D. Jun 05 '18 at 22:12
@Aominè depends on the operation. Even an operation as simple as `toArray()` may justify using a stream, as it eliminates the need for an allocation statement with a predicted size, as well as dealing with array indices. – Holger Jun 06 '18 at 07:31

Yassin Hajaj · Answer 3 · 2019-07-31T10:43:48.717

5

As addition to other answers, if you can use java-9 already, there is another possibility using Stream#takeWhile taking a Predicate as parameter.

Tested in jshell

jshell> IntStream.iterate(1, i -> 3 * i).takeWhile(i -> i < 100).toArray();
$3 ==> int[5] { 1, 3, 9, 27, 81 }

edited Jul 31 '19 at 10:43

answered Jun 05 '18 at 17:37

Yassin Hajaj

21,337
9
51
89

surendrapanday · Answer 4 · 2018-06-05T13:21:07.920

0

IntStream.range(0, N).forEach(this::doSomething);


int[] arr = IntStream.range(start, end).toArray();

edited Jun 05 '18 at 13:21

answered Jun 05 '18 at 13:16

surendrapanday

770
2
9
24

the example was not about incrementing with 3 itself, there could be a random function. I need to limit the value – Dmitry Senkovich Jun 05 '18 at 13:19
maybe I'm being too stupid, sorry:) But I don't understand you: the problem is that I don't know a limit (N in your case) but I have a function and an initial value – Dmitry Senkovich Jun 05 '18 at 13:26

Limit IntStream.iterate to a specific value

4 Answers4