Insertion Sort from Right to Left

Question

I am trying to learn Insertion Sort in Java but I have a problem. I learned the insertion sort in ascending order. Now, I need to do it in descending order but I have to start sorting the array from right to left.

public class Sorting {

public static void sort(Comparable[] a) {
    int N = a.length;
    for (int i = 1; i < N; i++) {                            
        for (int j = i; j > 0 && less(a[j], a[j - 1]); j--)
            swap(a, j, j - 1);
    }
}

private static void swap(Comparable[] a, int i, int j) { 
    Comparable t = a[i];
    a[i] = a[j];
    a[j] = t;
}

private static boolean less(Comparable v, Comparable w) {                                                   
    return v.compareTo(w) < 0;
}

}

The code which is above sorts the array from left to right in ascending order(1,2,3,4,..).

I need to change it in descending order but sorts from right to left(5,4,3,2..). I change the code and tried this:

    public static void sort(Comparable[] a) { 
    int N = a.length;
    for (int i = N-1; i>=0; i--) { 
        for (int j = i; j >= 1 && less(a[j-1], a[j]); j++)
            swap(a, j, j-1);
    }
}

I changed the sort method several times but I got errors..

Answer 1

Use the same algorithm, just in the check, which is smaller, exchange the arguments for the method less such that: less(a[j - 1], a[j]) . Then the smaller elemenets will be shifted to the right.

Answer 2

here, I also tested it, try out:

    public class InsertionSort {
        public static void main(String[] args) {
            Comparable[] a = new Comparable[]{5,6,12,1,3,15,4,0,9};
            sort(a);
            for (int i = 1 ; i < a.length; i++)
                assert a[i].compareTo(a[i-1]) >= 0;
            System.out.println("sort is correct");
        }

        public static void sort(Comparable[] a) { // Sort a[] into         increasing order.
            int N = a.length;
            for (int i = N - 1; i > 0; i--) { // Insert a[i] among         a[i-1], a[i-2],a[i-3]... ..                               
                for (int j = i; j < N && less(a[j - 1], a[j]); j++)
                    exch(a, j, j - 1);
            }
        }

        private static void exch(Comparable[] a, int i, int j) { 
            Comparable t = a[i];
            a[i] = a[j];
            a[j] = t;
        }

                private static boolean less(Comparable v, Comparable w)                 {                                                   
            return v.compareTo(w) < 0;
        }

    }