Combinations with repetitions

Write a program which generates the all k-combination with repetitions of n different objects. (Practically numerals!)

Java

Using the code of Michael Pryor. You can use this code not only for standard k-combination with repetitions, but for a object list too. E.g. list=(1, 2, 2, 3) or for other needed lists.

MultiCombinationData.java <lang java> public class MultiCombinationData {

   private int n = 0;
   private int k = 0;

   public MultiCombinationData(int n, int k) {
       this.n = Math.max(n, 0);
       this.k = Math.max(k, 0);
   }

   public int getN() {
       return n;
   }

   public int getK() {
       return k;
   }

} // class </lang>

MultiCombinationGenerator.java <lang java> import java.util.*;

public class MultiCombinationGenerator {

   private MultiCombinationData data = null;
   private Vector<Integer> list = new Vector<Integer>();
   private HashSet<Vector<Integer>> result = new HashSet<Vector<Integer>>();

   public MultiCombinationGenerator(MultiCombinationData data) {
       if (data != null) {
           this.data = data;
           if (data.getN() > 0 && data.getK() > 0) {
               for (int i = 1; i <= data.getN(); i++) {
                   for (int j = 0; j < data.getK(); j++) {
                       list.add(i);
                   }
               }
           }
       }
   }

   public MultiCombinationGenerator(int... integers) {
       for (int i = 0; i < integers.length; i++) {
           list.add(integers[i]);
       }
   }

   public void generate(int k) throws Exception {
       result.clear();
       if (k > list.size() || k < 0) {
           throw new Exception("Error: wrong value of k. (k = " + k + ")");
       } else {
           generate(new Vector<Integer>(), k, 0);
       }
   }

   public void generate() throws Exception {
       if (data != null) {
           generate(data.getK());
       }
   }

   private void generate(Vector<Integer> aList, int k, int currPos) {
       if (k == 0) {
           result.add(new Vector<Integer>(aList));
       } else if ((list.size() - currPos) >= k) {
           generate(aList, k, currPos + 1);
           aList.add(list.get(currPos));
           generate(aList, k - 1, currPos + 1);
           aList.removeElementAt(aList.size() - 1);
       }
   }

   void print() throws Exception {
       result = getResult();
       for (Vector<Integer> anIntegerVector : result) {
           System.out.println(anIntegerVector.toString());
       }
   }

   public HashSet<Vector<Integer>> getResult() throws Exception {
       if (result.size() == 0) {
           generate();
       }
       return result;
   }

   public static void main(String[] args) {
       int n = 3;
       int k = 2;

       System.out.println("Combination with repetitions. n=3 k=2");
       MultiCombinationGenerator rcg1 = new MultiCombinationGenerator(new MultiCombinationData(n, k));
       try {
           rcg1.print();
       } catch (Exception ex) {
           System.out.println(ex.getMessage());
       }

       System.out.println();

       System.out.println("Combination of a repetitions list. list=(1, 2, 2, 3) k=2");
       MultiCombinationGenerator rcg2 = new MultiCombinationGenerator(1, 2, 2, 3);
       try {
           rcg2.generate(k);
           rcg2.print();
       } catch (Exception ex) {
           System.out.println(ex.getMessage());
       }
   } // main()

} // class </lang>

I got this output:

Combination with repetitions. n=3 k=2
[1, 1]
[2, 2]
[1, 3]
[2, 3]
[1, 2]
[3, 3]

Combination of a repetitions list. list=(1, 2, 2, 3) k=2
[2, 2]
[1, 3]
[2, 3]
[1, 2]