unable to sort java array in descending order in two steps : first from index 0 to 12, second from index 13 to 24

Question

Sorry, it may be a dumb question, but I'm a total a beginner : When I try to execute the following program, it compiles, but I get the following error:

"Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1 at Numbers.main(Numbers.java:6)"

Can someone help?

 public class Numbers {

    public static void main (String[] args) {

            int [] harry = new int [25];
            for (int i = 0, n = 12; i < harry.length; i++, n--) {
                if (i % 2 > 0) {
                    harry[n] = i;
                }
            } 
            for (int j = 0, m = 25; j < harry.length; j++, m--) {
                if (j % 2 == 0) {
                    harry[m] = j;
                }
            }

            for (int k = 0; k < harry.length; k++) {
                System.out.println(harry[k]);
            }
     }
  }

Answer 1

You have this kind of runtime exception because you try to get harry[-1], but it's impossible. You will also see this exception in the second for, because you set m = 25, but indexces of your array are from 0 to 24. Set n=harry.length-1 and m = harry.length-1, then your program will work.

Answer 2

On the following line,

 for (int j = 0, m = 25; j < harry.length; j++, m--) {

m = 25, it should be 25-1, otherwise it will throw index out of bound exception

Answer 3

Besides having to initialize m to harry.length - 1 (since 25 is out of bounds), you should only decrement n and m when you use them (ie when you assign a value to the harry array):

int [] harry = new int [25];
for (int i = 0, n = 12; i < harry.length; i++) {
    if (i % 2 > 0) {
        harry[n] = i;
        n--;
    }
} 
for (int j = 0, m = harry.length - 1; j < harry.length; j++) {
    if (j % 2 == 0) {
        harry[m] = j;
        m--;
    }
}

Otherwise, n will become negative before the first loop is done, and m will become too small.

Answer 4

I am not sure if I understand everything but I believe if you do this your code should work perfectly:

Use if (i % 2 > 0 && n >= 0) instead of if (i % 2 > 0)

and use if (j % 2 == 0 && m<=24) instead of if (j % 2 == 0)

Hope this helps.