Question

This is what I have so far.

package testproject;

public class Combinations{

      public Combinations(){

        String str_arr[]={"a","b","c"};
        GenCombinations(str_arr);
      }




      public void GenCombinations(String[] str_arr){

        System.out.println("Generating All possible Combinations for  the following "+str_arr.length+" strings.");        

        for(int i=0;i<str_arr.length;i++)
         {
            System.out.print(str_arr[i]+" ");
         }
        System.out.println("\n------------------------------------------");

        /*COMBINATIONS OF LENGTH ONE*/
        for(int i=0;i<str_arr.length;i++)
            System.out.println(str_arr[i]);        

        /*COMBINATIONS OF LENGTH TWO*/
        for(int i=0;i<str_arr.length;i++)
         {
           for(int j=0;j<str_arr.length;j++)
             {
                System.out.println(str_arr[i]+""+str_arr[j]);                   
             }
         } 

        /*COMBINATIONS OF LENGTH THREE*/
        for(int i=0;i<str_arr.length;i++)
         {
          for(int j=0;j<str_arr.length;j++)
           {
            for(int k=0;k<str_arr.length;k++)
             {  
                System.out.println(str_arr[i]+""+str_arr[j]+""+str_arr[k]);                      
             }
           }
         }
      } 


      public static void main(String[] args){

        new Combinations();
      }
    }

Any suggestions will be appreciated..

Answer 1

If you want to generate all possible substrings of length k from a given alphabet, then what you’re doing is pretty much optimal.

If you want to generate all possible combinations of length k from a given set (or alphabet) with n elements (C(n, k)), then a classical solution is to use Rosen’s algorithm.

For example, given a 5-letter alphabet, if you want to take 3 combinations at a time, the gist of it is to count like so:

012
013
014
023
024
034
123
.
.
.

Here’s my implementation of a RosenIterator in Java.