Reverse array index of ints

Question

I am attempting to reverse an Array of int values. It does not seem to be doing it correctly. You will note that I am printing in different ways, like Array.toString() and also using println.

The original array prints:

1 4 9 16 9

I need it to print:

9 16 9 4 1

Here is my code:

import java.util.Arrays;

public class ReverseArray {
    public static void main(String[] args) {
        int[] data = { 1, 4, 9, 16, 9 };
        reverseArray(data);
        int[] dataR = reverseArray(data);
        System.out.println("Reveresed data " + Arrays.toString(dataR));
    }

    public static int[] reverseArray(int[] data) { // returns the reverse of the
                                                    // array data
        // int [] data = new int[5];
        int[] reversedData = new int[data.length];

        data[0] = 1;
        data[1] = 4;
        data[2] = 9;
        data[3] = 16;
        data[4] = 9;

        System.out.println(Arrays.toString(data));
        for (int i = 0; i < data.length - 1; i++) {
            System.out.println("Original Array: " + data[i]);
        }

        int reveresedData[] = data;

        for (int i = 0; i < reveresedData.length - 1; i++) {
            reversedData[(data.length - 1) - i] = data[i];
            System.out.println(reveresedData[i]);
        }
        return reveresedData;
    }

}

Why are you having so much of duplicate code, and re-assignment of same thing again and again? — Rohit Jain
– Rohit Jain, Commented Feb 13, 2013 at 18:33
Its a class session. Going through loops, arrays, different ways to print arrays and more. — PrimalScientist
– PrimalScientist, Commented Feb 13, 2013 at 18:40

NPE · Accepted Answer · 2013-02-13 18:38:41Z

2

The problem is that the following:

int reveresedData[] = data;

makes reveresedData point to the same array as data. Instead, you need to create a new array (or change the algorithm so that it reverses the array in place).

To create a new array, you could use either of:

int reveresedData[] = new int[data.length];
int reveresedData[] = data.clone();

edited Feb 13, 2013 at 18:38

answered Feb 13, 2013 at 18:32

NPE

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1 Comment

Woot4Moo Over a year ago

i would have recommended Arrays.copyOf(...)

Shridutt Kothari · Accepted Answer · 2013-02-14 13:17:05Z

1

(Using For each loop : Smarter way) Create a method as follows:

 private int[] reverseArray(int [] array1){
        int index=array1.length;
        int [] array2 = new int[index];
        for(int i:array1){
            array2[index-1]= i;
            index--;
        }
        return array2;    
}

And call it like:

int [] array1 = {1,2,3};
int reversedArray[] = reverseArray(array1);

Thats it.

edited Feb 14, 2013 at 13:17

answered Feb 13, 2013 at 18:43

Shridutt Kothari

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8 Comments

Woot4Moo Over a year ago

how is a for each a smarter way than a regular for loop?

PrimalScientist Over a year ago

I seem to be getting an out of bounds exception? array2[index]= i;

Shridutt Kothari Over a year ago

I have Edited Answer. There was a small counter mistake. now test it, It should be running now as a charm. i dint runt it, i have only written the logic here so Also please let me know after running, if it helps you.

Shridutt Kothari Over a year ago

it is called "for each loop", and not the short form, it is new loop introduced i java 6, but does the same work as for(int i = 0; i < array1.length; i++) but reduces human efforts. just search on google about java for each loop. it's syntex is like- for(ContentTypeOfArray x : ArrayName){Here for each iteration you will get content in x}. I always prefer to use for each loop over normal for loop because it minimizes human errors of maintaining start index, loop limit, increment etc.

PrimalScientist Over a year ago

For that algorithm to work, did you write it out first, or has this come with great experience? I only ask this as I am just starting out with regards to programming.

|

Woot4Moo · Accepted Answer · 2013-02-13 18:35:07Z

1

should be something like this:

int reversed[] = new int[data.length];

This will allocate space for a new array that you can fill with the values of data in reverse order.

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

answered Feb 13, 2013 at 18:35

Woot4Moo

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1 Comment

Woot4Moo Over a year ago

My syntax may be off in the for loop, as i didnt type this out in a web ide.

Prince · Accepted Answer · 2013-02-14 22:49:15Z

1

Here is a basic approach to reverse array of any size. It reverses the original array.

public class ReverseMyArray {
public static int[] reverseArray(int[] array) {
    for(int i=0; i<array.length/2; i++) {
        int temp = array[i];                    // temp is a temporary variable of type int
        array[i] = array[array.length-1-i];
        array[array.length-1-i] = temp;
    }
    return array;
}

public static void main(String[] args) {
    int[] array = {1,4,9,16,9};
    int[] newArray = reverseArray(array);
    for(int i=0; i<newArray.length; i++) {          
        System.out.print(newArray[i]+" ");      // Print the reversed array values
    }
}
}

answered Feb 14, 2013 at 22:49

Prince

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