Reversing array order in Java

Question

I am trying to reverse the array order..but to no avail. I understand that there are existing methods, but i do not understand why mine does not work? When i try to reverse Array of [1,2] i get [2,2], when trying [1,2,3] i get a reverse array of [3,2,3] instead. I cannot see what is wrong with my method..please help thanks!

      public static void reverseArray(int[] arr) {
        int j = arr.length-1;
        int [] temp = arr;

        for (int i=0; i<arr.length; i++){
          arr [i] = temp [j];
          j--;
        }
}

Possible duplicate of stackoverflow.com/questions/2137755/… — jrook
– jrook, Commented Sep 26, 2019 at 16:13
Possible duplicate of How do I reverse an int array in Java? — Michał Krzywański
– Michał Krzywański, Commented Sep 26, 2019 at 16:33

Malt · Accepted Answer · 2019-09-26 16:13:47Z

3

temp points to the same array as arr, so the array you're copying into is the same array you're copying out of. Your loop is equivalent to this:

int j = arr.length-1;

for (int i=0; i<arr.length; i++){
  arr [i] = arr[j];
  j--;
}

(note arr in both the source and the destination)

Either swap the array elements on every iteration, or have temp be a copy of arr:

for (int i = 0; i < arr.length / 2; i++) {
    int temp = arr[i];
    arr[i] = arr[arr.length - 1 - i];
    arr[arr.length - 1 - i] = temp;
}

edited Sep 26, 2019 at 16:13

answered Sep 26, 2019 at 16:04

Malt

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2 Comments

Jey.L Over a year ago

how do i modify in such a way that it then can be reversed?

Malt Over a year ago

@Jey.L I've added example code with a swap operation.

Malt · Accepted Answer · 2019-09-26 16:14:50Z

1

That is because Java works with references, so when you assign temp = arr you are not duplicating the array, you are just creating another reference to the same array. That way, when you edit the array arr you are also editing the array temp.

Try this instead:

public static void reverseArray(int[] arr) {
    int[] temp = new int[arr.length];

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

edited Sep 26, 2019 at 16:14

Malt

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answered Sep 26, 2019 at 16:10

donkikote

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