Insertion Sort Algorithm in java

Question

This is my algorithm for Insertion Sort. It always skips the first element in the array.

Example:

Input: 5 8 4 9
Output: 5 4 8 9

Here is the code.

import java.util.*;

class InsertionSort {
    public void InsertAsc(int A [],int n)
    {
        for (int j=1; j<n; ++j){
            int key=A[j];
            int i=j-1;
            while(i>0 &&A[i]>key)
            {
                A[i+1]=A[i];            
                i=i-1;
            }
            A[i+1]=key;
        }
        System.out.println("The sorted numbers are"+Arrays.toString(A));
    }

    public static void main(String args[])
    {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter the numbers of number that you want to sort");
        int n=scan.nextInt();
        int A[]=new int[n];
        System.out.println("Enter the numbers that you want to sort");
        for(int i=0;i<n;i++)
        {
            A[i]=scan.nextInt();
        }
        
        System.out.println("The numbers are"+Arrays.toString(A));
        InsertionSort ob = new InsertionSort();
        ob.InsertAsc(A,n);
    }
}

Answer 1

This is my algorithm for Insertion Sort. But it always skips the first element in the array.

Example:

Input: 5 8 4 9
Output: 5 4 8 9

Here is the code.

        import java.util.*;
        class InsertionSort {
        public void InsertAsc(int A [],int n)
        {
            for (int j=1; j<n; ++j){
                int key=A[j];
                int i=j-1;
                while(i>0 &&A[i]>key)
                {
                    A[i+1]=A[i];            
                    i=i-1;
                }
                A[i+1]=key;
            }
            System.out.println("The sorted numbers are"+Arrays.toString(A));
        }
    public static void main(String args[])
    {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter the numbers of number that you want to sort");
        int n=scan.nextInt();
        int A[]=new int[n];
        System.out.println("Enter the numbers that you want to sort");
        for(int i=0;i<n;i++)
        {
            A[i]=scan.nextInt();
        }

        System.out.println("The numbers are"+Arrays.toString(A));
        InsertionSort ob = new InsertionSort();
        ob.InsertAsc(A,n);
    }
}

Thanks!

Answer 2

Pay close attention to this line while(i>0 &&A[i]>key) your variable i needs to go to index 0 as well and in your case it isn't so i>=0 fixes it.

Also I hope you can visualize how the algorithm is working-

1. You go from index 1 to n-1 in your array with the variable j
   • You store the element at index j in a key variable so that you don't lose the value
   • You check the indexes from 0 to j-1 with your variable i and keep moving the elements greater than key one step ahead
   • You thus stop at an index i containing an element less than or equal to the key or you stop at -1 if all elements from 0 to i-1 are greater than key
   • Finally you place your key element at index i+1