can someone check my code plz

Hello,
I have to make a method that rotates the array to the right
ie if i have an array of int which has the following content: {3, 8, 19, 7}
if i invoke this method it should return {7, 3, 8, 19}

heres my code

Code:

---

import java.util.*;

public class TestRotateRight
{
    public static void main (String[] args)
    {
        int [] list = {3, 8, 19, 7};
       
        rotateRight(list);
       
        System.out.println(Arrays.toString(list));  // [7, 3, 8, 19]
       
        rotateRight(list);
        rotateRight(list);
       
        System.out.println(Arrays.toString(list));  // [8, 19, 7, 3]
       
        rotateRight(list);
       
        System.out.println(Arrays.toString(list));  // [3, 8, 18, 7]
       
        rotateRight(list);
        rotateRight(list);
        rotateRight(list);
       
        System.out.println(Arrays.toString(list));  // [8, 19, 7, 3]
       
        rotateRight(list);
        rotateRight(list);
        rotateRight(list);
        rotateRight(list);
       
        System.out.println(Arrays.toString(list));  // [8, 19, 7, 3]
    }
   
    public static void rotateRight (int[] list)
    {
        int length = list.length;
       
        int [] temp = new int [length];
        for ( int i = 0; i < list.length; i++)
        {
            temp[i] = list[i];
        }
       
        list = new int [length];
       
        list[0] = temp[length - 1];
       
        int index = 1;
       
        for ( int i = 0; i < length - 1; i++)
        {
            list[index] = temp[i];

            index++;
        }
    }
}

---

i used the debugger on the method that rotates, and i got the correct formation of the new array, but in the main all its printing out is the same list we created it for all the printlns

can someone tell me where my code is wrong at?
thanks in advance

Code:

---

public static void rotateRight (int[] list)
    {
        int length = list.length;
       
        int [] temp = new int [length];
        for ( int i = 0; i < list.length; i++)
        {
            temp[i] = list[i];
        }
       
        list = new int [length];

        // code continues...

---

The rotateRight() method is passed a reference (or pointer) to an array. That's OK because you can use this reference to read and write array elements.

But then in the last line of the part I quoted you make the variable list point to a new and completely different array. You do things to that array, but what you do will have no effect on the array that list pointed to at the start of the method (and that's the one that counts, because that's the one the calling method can see.)

but i want it to do the changes to the same array since its being printed in the main, and the question doesnt allow me to print out a new array, in the question the main method was given as above and i had to do the method

Quote:

---

but i want it to do the changes to the same array

---

Yes I know that - so why create a new and completely different array and proceed to set its values?

(You do see that you are creating a new array, don't you? Say if not because its key to what's going on here.)

Quote:

---

Originally Posted by aizen92

but i want it to do the changes to the same array since its being printed in the main, and the question doesnt allow me to print out a new array, in the question the main method was given as above and i had to do the method

---

Maybe you want to have a look at the System.arraycopy( ... ) method; it can copy (move) elements in the same array. All you have to do is save the last element of the arry and stick it back in as the first element after the arraycopy( ... ) method has finished.

kind regards,

Jos

Okay
ill read the arraycopy method and do it, and ill post back my result

EDIT: but where in the java doc can i find it?

Quote:

---

Originally Posted by pbrockway2

Yes I know that - so why create a new and completely different array and proceed to set its values?

(You do see that you are creating a new array, don't you? Say if not because its key to what's going on here.)

---

im creating a new array to hold the content of the original array since i will be changing the original array, but i dont want to lose its content, so i put them in a temporary array by which they will hold the value of the list array and then i arrange the original array as i want while still keeping my original values

Quote:

---

Originally Posted by aizen92

im creating a new array to hold the content of the original array since i will be changing the original array, but i dont want to lose its content, so i put them in a temporary array by which they will hold the value of the list array and then i arrange the original array as i want while still keeping my original values

---

Have you given up on the arraycopy( ... ) method already?

kind regards,

Jos

no, but i just noticed pbrockway2's post, and i didnt reply to him
and for the arraycopy method where do i find it, in the java doc?

Quote:

---

Originally Posted by aizen92

no, but i just noticed pbrockway2's post, and i didnt reply to him
and for the arraycopy method where do i find it, in the java doc?

---

Yep, it's in the System class (it's a public static method, ready for use).

kind regards,

Jos

okay
ill read it and try and see how it goes
thnx Jos for the help

okay, i used the arraycopy method, and tried the method alone, and gave me the correct results but still when i run the main method, its not working, it only gives me the orginal array
heres my modified rotate method with the arraycopy method

Code:

---

public static void rotateRight (int[] list)
    {
        int [] temp = new int [list.length];
       
        System.arraycopy(list, 0, temp, 0, list.length);
       
        list = new int [temp.length];
       
        list[0] = temp[temp.length - 1];
       
        System.arraycopy(temp, 0, list, 1, temp.length - 1);
    }

---

EDIT: I removed the "list = new int [temp.length];" and it worked
im glad it worked but how come it didnt when it was there?

Quote:

---

Originally Posted by aizen92

okay, i used the arraycopy method, and tried the method alone, and gave me the correct results but still when i run the main method, its not working, it only gives me the orginal array
heres my modified rotate method with the arraycopy method

Code:

---

public static void rotateRight (int[] list)
    {
        int [] temp = new int [list.length];
       
        System.arraycopy(list, 0, temp, 0, list.length);
       
        list = new int [temp.length];
       
        list[0] = temp[temp.length - 1];
       
        System.arraycopy(temp, 0, list, 1, temp.length - 1);
    }

---

EDIT: I removed the "list = new int [temp.length];" and it worked
im glad it worked but how come it didnt when it was there?

---

For the same reason as before: you are changing the 'list' variable which is an entirely local and new array. This is how I'd do it:

Code:

---

public static void rotateRight(int[] a) {
  int n= a.length-1;
  if (n > 0) { // something to do?
      int last= a[n]; // keep the previous last element
      System.arraycopy(a, 0, a, 1, n); // shift everything to the right;
      a[0]= last; // stick back in the previous last element
  }
}

---

kind regards,

Jos

ohh i see
well thanks very much Jos for the help

Quote:

---

Originally Posted by aizen92

ohh i see
well thanks very much Jos for the help

---

You're welcome of course; here's a sketch for another approach: suppose you can reverse part of an array a, from position l up to (but not including) position h. Reversing a sequence of things can be implemented with a sequence of elementary swap operations. You right rotate operation can be implemented with a couple of reverse operations like this:

Code:

---

public static void rotateRight(int[] a) {
  reverse(a, 0, a.length);
  reverse(a, 1, a.length);
}

---

kind regards,

Jos

This approach seems simpler and more straightforward to me:

Code:

---

        public void rotateRight(int[] a) {
            if (a.length == 0) return; // safety first!
            int tmp = a[a.length - 1];
            for (int i = a.length - 1; i > 0; i--) {
                // this part should be simple -- don't want to spoon-feed everything
            }
            a[0] = tmp;
        }

---

-Gary-

Quote:

---

Originally Posted by gcalvin

Code:

---

            if (a.length == 0) return; // safety first!

---

---

Allow me to change that to:

Code:

---

if (a == null || a.length <= 1) return; // safety and laziness first!

---

kind regards,

Jos

Quote:

---

Originally Posted by aizen92

im creating a new array to hold the content of the original array since i will be changing the original array, but i dont want to lose its content, so i put them in a temporary array by which they will hold the value of the list array and then i arrange the original array as i want while still keeping my original values

---

To return to your original code for a moment...

Code:

---

public static void rotateRight (int[] list)
{
    int length = list.length;
       
    int [] temp = new int [length];
    for ( int i = 0; i < list.length; i++)
    {
        temp[i] = list[i];
    }
       
    list = new int [length];  // --- why? ---
       
    list[0] = temp[length - 1];
       
    int index = 1;
       
    for ( int i = 0; i < length - 1; i++)
    {
        list[index] = temp[i];

        index++;
    }
}

---

I wasn't talking about the temp array, but the new array that you assign to list. Giving list a new value causes the oddity you reported: the debugger shows list with the correct values in the method, but the caller prints the old values.

To answer my own question: there is no reason for doing that. What would happen if you removed that line? As you say, you have a copy of its values and you are about to assign new values to each position of the array.

well i though id empty the original array to fill it from the start with what i want it to be filled with, thats y i re-declare it (sorry if wrong term, i forgot the exact term for it), so that the original array would be emptyed, and fill it from the start

Code:

---

list = new int [length];

---

I hope you can see now that the effect of this line is *not* to empty the array. Rather it makes list point to an entirely new array. The old one (which remains the one seen by the caller) will not be affected by anything you subsequently do.

And I hope you can see for yourself the effect of removing this line from your original code.