Reversing an Array

Question

I wrote code to reverse an integer array. Code is as shown below:

public class ReverseArray {

    public static void main(String[] args) {
        try {
            int[] arr = new int[5];
            arr[0] = 1;
            arr[1] = 2;
            arr[2] = 3;
            arr[3] = 4;
            arr[4] = 5;
            for (int i = 0; i <= arr.length/2; i++)
                int temp = arr[0];
                arr[0] = arr[arr.length - i - 1];
                arr[arr.length - i - 1] = temp;
            }
            System.out.println(arr);

        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

But it's not reversing the array.. I'm getting following output.

 [I@3bad086a

I don't see anything wrong with my logic.

Answer 1

That is printing out the reference to the array.
If you print out the array one element at a time, you will see the reversed array.

EDIT: Two more points.

1. You're using arr[0] when you intend to use arr[i] .
2. More insidious: you're iterating too much . You only need to iterate until arr.length / 2 . Since this is a homework question, I leave it to you to find out why; try printing out the intermediate results and explaining them.

Answer 2

好吃的：

System.out.println(Arrays.toString(arr));

Answer 3

u need to loop through the array to print it

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

Firstly , i will let the above code remain (it is wrong ( ArrayIndexOutOfBoundsException ), and now i believe it was good that you down-voted me )

public class ReverseArray {    
    public static void main(String[] args) {
        try {
            int[] arr = new int[5];
            arr[0] = 1;
            arr[1] = 2;
            arr[2] = 3;
            arr[3] = 4;
            arr[4] = 5;
            for (int i = 0; i <= arr.length/2; i++){            
                int temp = arr[i];
                arr[i] = arr[arr.length - i - 1];
                arr[arr.length - i - 1] = temp;
                }
                for (int i = 0; i <= arr.length -1 ; i++){ 
                    System.out.println(arr[i]);
                    }                
            }          
         catch (Exception e) {
            System.out.println(e);
        }
    }
}

For now , i will make a few simple suggestions

1. the {} were missing around the for loop.
2. instead of traversing the whole array , you only need need to go through half of it, so the arr.length/2

Answer 4

You should use Arrays.toString(arr) for that. Arrays don't have specific implementation of toString() , so calling it results in the default toString implementation of Object. In Java5 the toString functionality was added for arrays, as a static method in Arrays class. I guess that it wasn't added to arrays directly due to backward compatibility considerations.

And by the way, you have a small bug in the code. I believe you want to swap values in indexes length-i-1 and i (not 0 ).

Answer 5

you need to print the contents of the array not the array itself. try using a loop to iterate over the array and print each int one at a time.

Answer 6

use following to print your array

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

instead of

System.out.println(arr);

Answer 7

Just do

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

Answer 8

You are printing

System.out.println(arr);

This will going to print memory location of variable arr.

You should try

for(int i=0;i<arr.length;i++)
System.out.println(arr[i]);

Answer 9

Regarding printing the array, several of the previous answers are correct. However, your code still doesn't work. You need to replace arr[0] with arr[i] in your loop.

Answer 10

Some of the easiest approaches to reverse an array in Java.

package reversearray;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

final public class Main
{
    private Object reverseArray(Object[]arr)[]
    {
        List<Object>list=Arrays.asList(arr);
        Collections.reverse(list);
        return(list.toArray());
    }

    public static void main(String... args)
    {
        Object[] arr ={1, 2, 3, 4, 5};
        arr=new Main().reverseArray(arr);        

        int newArray[]=new int[arr.length];  //Array of your desired type.

        for(int i=0;i<arr.length;i++)
        {
            newArray[i]=Integer.valueOf(arr[i].toString());
        }

        for(int i=0;i<arr.length;i++)
        {
            System.out.println(newArray[i]);
        }
    }
}

---

You can simply use with Commons.Lang as follows.

ArrayUtils.reverse(int[] arr)

---

Using guava :

Collections.reverse(Ints.asList(arr));

Answer 11

from my limited recall of C

for i = (length( array ) -1 ) to 0
  { print( array( i ) ; i-- }

NOTE: you are counting ELEMENTS not BYTES ALSO: you subtract 1 from length because you are counting down to ZERO not 1