Get all 1-k tuples in a n-tuple

Question

With n=5 and k=3 the following loop will do it

List l=new ArrayList();
l.add(\"A\");l.add(\"B\");l.add(\"C\");l.add(\"D\");l.add(\"E\         


        

Answer 1

Apache commons has iterators for subsets of size k, and for permutations. Here is an iterator that iterates through 1-k tuples of an n-tuple, that combines the two:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

import org.apache.commons.collections4.iterators.PermutationIterator;
import org.apache.commons.math3.util.Combinations;

public class AllTuplesUpToKIterator implements Iterator> {
    private Iterator combinationIterator;
    private PermutationIterator permutationIterator;
    int i;
    int k;
    int n;

    public AllTuplesUpToKIterator(int n, int k) {
        this.i = 1;
        this.k = k;
        this.n = n;
        combinationIterator = new Combinations(n, 1).iterator();
        permutationIterator = new PermutationIterator(intArrayToIntegerList(combinationIterator.next()));
    }

    @Override
    public boolean hasNext() {
        if (permutationIterator.hasNext()) {
            return true;
        } else if (combinationIterator.hasNext()) {
            return true;
        } else if (i next() {
        if (!permutationIterator.hasNext()) {
            if (!combinationIterator.hasNext()) {
                i++;
                combinationIterator = new Combinations(n, i).iterator();
            }
            permutationIterator = new PermutationIterator(intArrayToIntegerList(combinationIterator.next()));          
        }
        return permutationIterator.next();
    }

    @Override
    public void remove() {
        // TODO Auto-generated method stub

    }

    public static List intArrayToIntegerList(int[] arr) {
        List result = new ArrayList();
        for (int i=0; i< arr.length; i++) {
            result.add(arr[i]);
        }
        return result;
    }


    public static void main(String[] args) {
        int n = 4;
        int k = 2;
        for (AllTuplesUpToKIterator iter= new AllTuplesUpToKIterator(n, k); iter.hasNext();) {
            System.out.println(iter.next());
        }

    }
}