从Java列表中获取具有满足流条件的第n个元素的子列表

Question

I have a very simple use case

Given a list of letters with A and Bs, I want to get the sublist that contains the first N Bs, for example:

• f(3, [AAABABABAA]) = [AAABABAB]
• f(2, [AAABABABAA]) = [AAABAB]
• f(1, [AABABA]) = [AAB]
• f(0, [AABAB]) = []

By following an imperative approach, this is relatively easy, count until we find N Bs, and then get the sublist until that position.

However, I couldn't find any functional solution with lambdas, since the operation on every node seems to be independent from the others (which I guess make sense for parallelization).

Answer 1

If your input is a List with fast random access, you can solve your problem using the stream of indices:

public static List<String> f(int n, List<String> input) {
    int fence = IntStream.range(0, input.size())
                         .filter(idx -> input.get(idx).equals("B")) // leave only B's
                         .skip(n-1)
                         .findFirst() // an index of n-th B
                         .getAsInt(); // with throw NoSuchElementException if not enough B's
    return input.subList(0, fence+1);
}

Usage example:

System.out.println(f(3, Arrays.asList("A", "A", "A", "B", "A", "B", "A", "B", "A", "A")));
System.out.println(f(2, Arrays.asList("A", "A", "A", "B", "A", "B", "A", "B", "A", "A")));
System.out.println(f(1, Arrays.asList("A", "A", "A", "B", "A", "B", "A", "B", "A", "A")));

However despite I love Stream API I would solve this problem imperatively.

Answer 2

This code snippet do the job.

public static void main(String[] args) {
    final String input = "A A A B A B A B A A";
    final String[] array = input.split(" ");
    final List<String> list = Arrays.asList(array);
    final List<String> result = getSubList(list, 3);
    System.out.println("Result=" + result);
}

public static List<String> getSubList(final List<String> list, final int limit) {
    final AtomicInteger counter = new AtomicInteger();
    return list.stream()
            .filter(ch -> {
                if (ch.equals("B") && counter.incrementAndGet() == limit) return true;
                return counter.get() < limit;
            })
            .collect(Collectors.toList());
}

Answer 3

Here is code by AbacusUtil

List<String> input = N.asList("A", "A", "A", "B", "A", "B", "A", "B", "A", "A");
List<String> list = Stream.of(input).takeWhile(MutableInt.of(3), (e, cnt) -> cnt.getAndAdd(e.equals("B") ? -1 : 0) > 0).toList();
N.println(list);

Declaration： I'm the developer of AbacusUtil.