CODEWITHSUNDEEP
javalistjava-8java-streamsublist
dragon7
dragon7

Reputation: 1133

Creating sublist from indices

Is it possible to create a sublist from another list, using only the element indices? I'm looking for nice solution e.g. lambdas, streams from Java 8.

For example (pseudocode):

a = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
b = a.getByIndices([0, 2, 4, 5, 3])
print(b) // [10, 8, 6, 5, 7]

Upvotes: 4

Views: 2071

Answers (4)

assylias
assylias

Reputation: 328735

For arrays of ints you could use this:

int[] b = IntStream.of(0, 2, 4, 5, 3)
      .map(i -> a[i])
      .toArray();

Upvotes: 6

Subito
Subito

Reputation: 36

I wrote this function that does what you ask. I can't figure out how to make it lambda function but I'll keep working on it.

public static <E> void createSublist(List<E> oldList, List<E> newList, int[] indicies){
        for(int i = 0; i < indicies.length; i++)
             newList.add(oldList.get(indicies[i]));} //Adds indicies to new list

Upvotes: 0

Boann
Boann

Reputation: 50041

Extending AbstractList is a quick way to get a list implementation up and running. Like the regular List.subList method, the following sublist is backed by the main list, so changes to the sublist will write through to the main list (which may or may not be a feature you wanted, but you can always create a new, separate ArrayList from the sublist if needed). Element removal through the sublist is not implemented; it would be doable but require a bit more work.

public static <E> List<E> sublistFromIndices(List<E> list, int... indices) {
    Objects.requireNonNull(list);
    Objects.requireNonNull(indices);
    return new AbstractList<E>() {
        @Override
        public int size() {
            return indices.length;
        }

        @Override
        public E get(int index) {
            return list.get(indices[index]);
        }

        @Override
        public E set(int index, E element) {
            return list.set(indices[index], element);
        }
    };
}

Upvotes: 2

Tunaki
Tunaki

Reputation: 137184

You could have the following method:

private static <T> List<T> getByIndices(List<T> list, List<Integer> indexes) {
    return indexes.stream().map(list::get).collect(toList());
}

This creates a Stream from the given indexes, maps to the element of the list at the index and collect the result to a list.

Sample usage:

List<Integer> list = Arrays.asList(10, 9, 8, 7, 6, 5, 4, 3, 2, 1);
List<Integer> indexes = Arrays.asList(0, 2, 4, 5, 3);
System.out.println(getByIndices(list, indexes)); // prints [10, 8, 7, 6, 5]

Upvotes: 10

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