Manual reverse order in Array (Java)

Question

Hello people of StackOverflow! I have used the searrch option, I've found some related answers but none of them explained why this particular method of reverse ordering of an array in Java doesn't work:

class ReverseOrder
{
    public static void main(String[] args)
    {
        int x[] = {1,2,3,4,5};
        int y[] = x;

        int i, j;
        for(i = 0, j = x.length - 1; i < x.length; i++, j--)
        {
            y[i] = x[j];
        }

        for(int b = 0; b < x.length; b++)
        {
            System.out.println("Inverse order is: " + y[b]);
        }
    }
}

Why is the result 5,4,3,4,5 instead of 5,4,3,2,1??? It drives me absolutely insane and makes no sense to me. Any help would be greatly appreciated!

mihaisimi · Accepted Answer · 2012-06-26 17:31:49Z

7

Because of this: int y[] = x

y and x are now references to the same array. You should make sure you initialize y as a new array.

answered Jun 26, 2012 at 17:31

mihaisimi

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3 Comments

aglassman Over a year ago

Yeah, you need to do int y[] = new int[x.length];

Vegeta_DTX Over a year ago

Thank you so much! That definitely solved the problem. However I have an extra question - should I do it this way now? int x[] = {1,2,3,4,5}; int y[] = new int[5]; y = x.clone(); Or perhaps there is faster and more efficient way of doing it? Thanks a lot anyway!

mihaisimi Over a year ago

it is enough to replace int y[] = x with int y[] = new int [x.lengt], this assures y is initialized and has enough capacity to hold the x content. Y is filled with values in the loop you are filling y in the loop.

Jason Sturges · Accepted Answer · 2012-06-26 21:47:01Z

2

class ReverseOrder {
    public static void main(String[] args)
    {

        int x[] = {1,2,3,4,5};
        int y[] = new int[5]; // or you could use [x.length]
        int i, j;
        for(i = 0, j = x.length - 1; i < x.length; i++, j--)
        {
            y[i] = x[j];
        }

        for(int b = 0; b < x.length; b++)
        {
            System.out.println("Inverse order is: " + y[b]);
        }
    }
}

edited Jun 26, 2012 at 21:47

Jason Sturges

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answered Jun 26, 2012 at 17:47

J Noel

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2 Comments

J Noel Over a year ago

well, basically what was explained above, with the x.length ,I did not see the answer before i posted

Vegeta_DTX Over a year ago

Thanks a lot man! You people are so quick to answer and help! I wish I could give you all up and mark everyone's answer as accepted! But I just registered :)

user949300 · Accepted Answer · 2012-06-26 17:33:18Z

1

int y[] = x;

Makes y refer to the same data as x.

y[i] = x[j];

you are also modifying the input array x.

You want y to be totally independent:

int y[] = new int[x.length];

instead.

answered Jun 26, 2012 at 17:33

user949300

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1 Comment

Vegeta_DTX Over a year ago

Oh thank you too! So is that more efficient way than this one: int x[] = {1,2,3,4,5}; int y[] = new int[x.length]; y = x.clone();

Ted Hopp · Accepted Answer · 2012-06-26 17:41:33Z

1

If you want to reverse the order of array elements in place (that is, modify the array x directly), you can iterate halfway through the array, swapping elements:

final int last = x.length - 1;
final int n_2 = x.length / 2; // round down for odd lengths
for (int i = 0; i < n_2; ++i) {
    int tmp = x[i];
    x[i] = x[last - i];
    x[last - i] = tmp;
}

answered Jun 26, 2012 at 17:41

Ted Hopp

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1 Comment

Vegeta_DTX Over a year ago

Thanks a lot!! I've heard of that method, but I was just plain confused and wanted to first clarify the whole case before continuing with studying Java :)

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