Write a function which return a list of all of the n element

Write a function which return a list of all of the n element subset of a given set

Implement in JAVA

Question 3 Write a function that returns a list of all of the n-element subsets of a given set. For example, if the given set is [1,2,3] and n is 2, then the returned list should contain [1,2], [1,3], and [2,3]. The order of the elements is not important.

Solution

The program for this problem is as follows:

Comments are included in the program for explaining the steps.

A sample output is attached below the program.

package com.temp;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
/**
* Program to find sublists of a list.
* @author Rohit Phatak
*
*/
public class subset {
   /**
   * This list will contain the resultant sublists.
   */
   static List<String[]> list = new ArrayList<>();
  
   public static void main(String arg[]){
       /**
       * Input a list comma separated.
       */
       System.out.println(\"Enter a list : \");
       Scanner scanner = new Scanner(System.in);
       String s = scanner.nextLine();
       String[] sArr = s.split(\",\");
       /**
       * Input a value of n.
       */
       System.out.println(\"Enter value of n : \");
       Integer n = scanner.nextInt();
       /**
       * Check if value of n is greater than entered list.
       */
       if(sArr.length < n){
           System.out.println(\"Length of list is less than value of n\");
       } else if(sArr.length > 0){
           getSubsetList(sArr,n);
           System.out.println(\"List is as :\");
           /**
           * Printing the sublists.
           */
           for(String[] str : list){
               String printVar = \"\";
               for(String temps : str)
                   printVar+=temps+\",\";
               System.out.println(\"[\"+printVar.substring(0, printVar.length()-1)+\"]\");
           }
       } else {
           System.out.println(\"Please enter atleast one element:\");
       }
       scanner.close();
   }
   /**
   * This method calls the recursive function which is used to obtain the sublists.
   * @param sArr
   * @param n
   * @return
   */
   public static List<String[]> getSubsetList(String[] sArr,Integer n){
       List<String[]> lista = new ArrayList<>();
       sublist(sArr, 0, n, new String[n]);
       return lista;
   }
   /**
   * This is a recursive function.
   * This function will iterate over itself for finding the sublist.
   *
   * Parameters are as follows:
   *
   * @param arr -> List entered by the user.
   * @param n -> Value of sublist entered by user.
   * @param startPosition -> index to start creating the sublist.
   * @param result -> resultant sublist
   */
   public static void sublist(String[] arr, int startPosition, int n, String[] result){
       /**
       * Check for the value of n ==0.
       * If n==0 break from the recursion.
       */
        if (n == 0){
           /**
           * Add the sublist \'result\' in the list.
           */
           list.add(result.clone());
            return;
        }
        /**
         * Iterate through for loop starting from startPosition till length of list minus entered value n.
         */
        for (int i = startPosition; i <= arr.length-n; i++){
           /**
           * Here result is a temporary sublist.
           * This for loop adds an element to the temporary sublist in each iteration.
           * The element is added from the complete list entered by user.
           * Location of element is iTh index.
           */
            result[result.length - n] = arr[i];
            /**
             * After adding the element to the temporary sublist, the sublist and the updated start and end positions are recurssed to itself.
             */
            sublist(arr, i+1, n-1, result);
        }
    }
}

Sample output of the above program :

Enter a list :
1,2,3,5
Enter value of n :
2
List is as :
[1,2]
[1,3]
[1,5]
[2,3]
[2,5]
[3,5]