sort array using recursive method

Question

I am working on trying to write a program where a user will enter 6 strings and then it will sort the array in reverse alphabetical order using a recursive method. This is one concept I do not understand despite multiple videos, readings and attempts. Any support and insight is greatly appreciated. Thank you.

import java.util.Arrays;
import java.util.Scanner;

public class SRecusion {


    public static void sort2 (String[] sort2) {
        int i;
        int min = 0;
        int max;

        for (i = 0; i <sort2.length -1; i++) {
            if (sort2[i].charAt(0)> sort2[i=1].charAt(0)) {
                sort2[i] = sort2[min];
            }
            else {
                min = (sort2(sort2[i-1]));
            }
        }
    }



    public static void main(String[] args) {
        // TODO Auto-generated method stub

        String [] test = new String[6];
        Scanner scnr = new Scanner(System.in);
        String userEntry = "";

        for(int i = 0; i <= test.length - 1; i++) {
            System.out.println("Please enter a word:");
            test[i] = scnr.nextLine();
        }

        sort2(test);




            System.out.println("your list is" + Arrays.asList(test));
            System.out.println();

        }

}

Answer 1

Sort Array using recursion in kotlin

    fun main() {
        print(sortArray(arrayListOf(1,3,2,6,8,3)))
    }
    
    fun sortArray(arr: MutableList<Int>): MutableList<Int>{
        if(arr.size==1) {
            return arr
        }
        val lastValue = arr.last()
        arr.removeLast()
        sortArray(arr)
        insert(arr, lastValue)
        return arr
    }
    
    fun insert (arr: MutableList<Int>, value: Int): MutableList<Int> {
        if(arr.size == 0 || arr.last() < value) {
            arr.add(value)
            return arr
        }
        val lastValue = arr.last()
        arr.removeLast()
        insert(arr, value)
        arr.add(lastValue)
        return arr
    }

Answer 2

So from what I was asking above as to why you need a recursive sorting algorithm Here it goes I will try to explain how recursive sorting works. It took my some time to figure it out as I am sure it does for most people who first come in contact with it.

public static void Qsort(int[] array, int start, int end)
{
    //find the current center of the whole or parital array part I am working on.
    int center = (start+end)/2;
    ///System.out.println("\n This is the center : " + center);
    int pivot, i, pivotplace;
    i = 0;
    pivot = 0;
    pivotplace = 0;
    //if start = end then we are at a single element.  just return to the previous iterative call.
    if(start == end)
    {
       // System.out.println("\n Inside base case return :");
        return;
    }
    //find the pivot value we are using.  using a 3 prong selection we are assured to at least get some type of median value and avoid the N^2 worst case.
    pivot = getpivot(array[start], array[center], array[end]); //gets median value of start, center and end values in the array.
   // System.out.println("\n pivotvalue is  : " + pivot);
    //find where the current pivot is located and swap it with the last element in the current portion of the array.
    if(array[start] == pivot)
    {
        //System.out.print("\n Inside pivot at start");
        swap(array, start, end);
    }
    else
    {
        if(array[center] == pivot)
        {
            //System.out.print("\n Inside pivot at center");
            swap(array, center, end);
        }
    }
    //due to iteration the pivot place needs to start at the passed in value of 'start' and not 0.
    pivotplace = start;
    //due to iteration the loop needs to go from the passed in value of start and not 0 and needs to go 
    //until it reaches the end value passed in.
    for(i = start; i < end; i++)
    {
        //if the current slot of the array is less than then pivot swap it with the current pivotplace holder
        //since the pivotplace keeps getting iterated up be each swap the final place of pivot place
        //is where the pivot will actually be swapped back to after the loop cpompletes.
        if(array[i] < pivot)
        {
            //System.out.print("\n Swapping");
            swap(array, i, pivotplace);
            pivotplace++;
        }
    }
    //loop is finished, swap the pivot into the spot it belongs in.
    swap(array, pivotplace, end);
//there are 2 cases for recursive iteration.
//The first is from the start to the slot before the pivot
if(start < pivotplace){Qsort(array, start, pivotplace-1);}
//the second is from the slot after the pivot to the end.
if(pivotplace+1 < end){Qsort(array, pivotplace+1, end);}

}

public static int getpivot(int a, int b, int c)
{
    if((a > b)  && (a < c))
    {
        return a;
    }
    if((b > a)  && (b < c))
    {
        return b;
    }
    return c;
}
public static void swap(int[] array, int posa, int posb)
{
    int temp;
    temp = array[posa];
    array[posa] = array[posb];
    array[posb] = temp;
}

This is a basic Quick Sort or recursive sort I wrote this while in programming classes. You will probably not need to use the getpivot code as you are dealing with a small set of strings, but if you do some research you will see using a possible sample of 3 drastically speeds up the recursion due to balanced work load of the recursion tree.

Answer 3

Sorting is a pretty broad topic as there are many different sorting methods (quicksort, merge sort, etc.) However, a pretty basic and simple sorting method is bubble sort. Although it isn't the fastest one, it's pretty easy to understand and code using recursion.

Essentially, bubble sort with iterate through the elements in pairs of 2 and swap the two elements if they're in the wrong order.

For example, let's sort (3, 2, 5, 4, 1) using bubble sort.

(2, 3, 5, 4, 1) First, it'll look at the first two elements swap them if needed. Since 3 is greater than 2, it'll swap them.

(2, 3, 5, 4, 1) Next, it'll look at 3 and 5. Since 3 is less than 5, there is no need to swap

(2, 3, 4, 5, 1) It now looks at 5 and 4 and swaps them.

(2, 3, 4, 1, 5) Finally, it looks at 5 and 1 and swaps them.

Now start from the beginning and repeat the whole process. The sorting ends if exactly 0 swaps are made during an iteration.

If you're still a bit confused, try watching a tutorial on bubble sort or visit this link.