How to create a int[] or a Collection with the value 0,1,2,...,N in one line in Java using lambda expression?

Question

I would like to do something like this:

int[] seq = new int[N];
for (int i = 0 ; i < N ; i++) {
    seq[i] = i;
}

...in one line, and i am wondering if it is possible with lambda expression.

If it works with ArrayList<Integer>, it is okay for me.

I'm just wondering how useful is this, no offense. Every `seq[k]`, can be replaced with `k` in other parts of code and you need no additional memory... — Betlista, Feb 08 '18 at 11:36
[XY Problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)? It was asked here - https://stackoverflow.com/questions/20058366/shuffle-a-list-of-integers-with-java-8-streams-api — Betlista, Feb 08 '18 at 13:47
Possible duplicate of [How can I generate a list or array of sequential integers in Java?](https://stackoverflow.com/questions/10242380/how-can-i-generate-a-list-or-array-of-sequential-integers-in-java) — Bernhard Barker, Feb 08 '18 at 21:05

score 28 · Accepted Answer · edited Jun 08 '18 at 10:32

28

Use IntStream.range():

int[] seq = IntStream.range(0, N).toArray();

or IntStream.rangeClosed() if you want to include N:

int[] seq = IntStream.rangeClosed(0, N).toArray();

edited Jun 08 '18 at 10:32

Lii

answered Feb 08 '18 at 11:21

Eran

Oleksandr Pyrohov · Answer 2 · 2018-10-06T10:36:18.470

Starting from Java 9 you can use the three-argument IntStream.iterate:

int[] seq = IntStream.iterate(0, x -> x < N, x -> x + 1).toArray();

Where:

IntStream.iterate(int seed, IntPredicate hasNext, IntUnaryOperator next):

seed - the initial element;
hasNext - a predicate to apply to elements to determine when the stream must terminate;
next - a function to be applied to the previous element to produce a new element.

score 2 · Answer 3 · answered Feb 08 '18 at 11:28

2

A BitSet would do too. Though then without lambda.

BitSet bset = new BitSet(N); // Initial capacity for N bits.
bset.set(0, N); // Set a range to true for 0 upto N-1.

answered Feb 08 '18 at 11:28

Joop Eggen

Interesting suggestion - but this is neither one line, nor produces an `int[]` or `Collection` as requested by the OP. – Hulk Feb 08 '18 at 12:15
1

@Hulk and it does not use a lambda stylish syntax. However (1) BitSet is equivalent to a `Set`, (2) BitSet can set ranges, (3) the use case seems to indicate that BitSet might be appropriate, (4) BitSet might be more efficient, (5) the code is actually more compact as a foreach. – Joop Eggen Feb 08 '18 at 13:16
Agreed - and it can be converted into an `IntStream` over the indices of the set bits via `bset.stream()`, if needed. I'd still prefer `IntStream.range(0,N)` without further information, but +1 for creativity – Hulk Feb 08 '18 at 13:49
2

`BitSet` is a highly underestimated class in Java, +1 for helping make this class more known to the world. – fps Feb 08 '18 at 14:12
1

@JoopEggen Re (3) the use case seems to be applying `Collections.shuffle()` as per OPs comment. Seems a `BitSet` doesn't make much sense then. – BlackJack Feb 08 '18 at 16:18
@BlackJack you are right, I overlooked that. Maybe a permutation for something like blackjack. I leave the answer for other similar questions as there is an already accepted answer. – Joop Eggen Feb 08 '18 at 16:33

3 Answers3