Thread: Need some help with grouping an arraylist

I need to make every possible version of a list of groups by a text file full of names.

I have the part where it makes unique groups (it all gets stored in a List<List<String>> called rootList)
And that holds this:
[a, b]
[a, c]
[a, d]
[b, c]
[b, d]
[c, d]

And now I want to make another List<List<String>> called resultList that saves the information from rootList as:
List 1:
[a, b]
[c, d]

List 2:
[a, c]
[b, d]

List 3:
[a, d]
[b, c]

How would I go about doing this?

Source:

package com.company;
 
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.regex.Pattern;
 
public class createGroups
{
    public static List<List<String>> rootList = new ArrayList<List<String>>();
    public static List<List<String>> resultList = new ArrayList<List<String>>();
    public static String comb = "";
 
    public static void main(String[] args) {
        String[] names = new String[] {"Rik", "Erwin", "Ian", "Mustafa"};
 
 
 
        int r = 2;
        int n = names.length;
        String[] data = new String[r];
        String[] groups = createGroups.combinationUtil(names, data, 0, n-1, 0, r);
    }
 
    static void printCombination(String arr[], int n, int r)
    {
        // A temporary array to store all combination one by one
        String[] data = new String[r];
 
        // Print all combination using temprary array 'data[]'
        combinationUtil(arr, data, 0, n-1, 0, r);
    }
 
    /* arr[]  ---> Input Array
       data[] ---> Temporary array to store current combination
       start & end ---> Staring and Ending indexes in arr[]
       index  ---> Current index in data[]
       r ---> Size of a combination to be printed */
    static String[] combinationUtil(String[] arr, String[] data, int start, int end, int index, int r)
    {
        if (index == r)
        {
 
            for (int j=0; j<r; j++)
            {
                if (j == 1)
                {
                    comb += data[j];
                }else if(j == 0){
                    comb += data[j] + ", ";
                }
 
            }
 
            List<String> node = new ArrayList<String>();
            node.add(comb);
            rootList.add(node);
            comb = "";
 
            return data;
        }
        for (int i=start; i<=end && end-i+1 >= r-index; i++)
        {
            data[index] = arr[i];
            combinationUtil(arr, data, i+1, end, index+1, r);
        }
 
 
        if(start == 0){
            for (List<String> str: rootList){
                System.out.println(str);
            }
            System.out.println("Done");
        }
        return data;
    }
}

The output right now is:
[Rik, Erwin]
[Rik, Ian]
[Rik, Mustafa]
[Erwin, Ian]
[Erwin, Mustafa]
[Ian, Mustafa]
Done

But what I want is:
List 1:
[Rik, Erwin]
[Ian, Mustafa]

List 2:
[Rik, Ian]
[Erwin, Mustafa]

List 3:
[Rik, Mustafa]
[Erwin, Ian]

How do you execute the program for testing? I do not see a main() method.

Originally Posted by Norm

How do you execute the program for testing? I do not see a main() method.

Oh sorry I forgot, it gets called from a JPanel clickhandler.
A file gets converted to an array and eventually it runs this:
createGroups.combinationUtil(names, data, 0, n-1, 0, r);
In which "names" is the array that holds all the names and data looks like this: String[] data = new String[r];
And r is the amount of people in one group, in this case 2 and n is the length of the names array
All the other parameters can be found in the constructor (combinationUtil)

Do you have a small program with a main that could be used for testing your code?

Be sure to post the program's output with some comments saying what is wrong and show the desired output.

Ok, now where is the documentation/ design specifications on how the code is going to solve the problem?
Do you have an algorithm? Please post the steps in the algorithm that the program needs to take to solve the problem.

You need to make a design before writing any code.

Note: For ease of debugging I suggest using digits vs names:

       String[] names = new String[]   {"1","2", "3", "4"};