ArrayIndexoutofbound exception merging two sorted array

Question

I have seen other solutions like this and wondering if there is anything wrong with my approach of using for loop. I don't want to use while loop as others have used in their solution.

package com.my.practice;

public class MedianOfTwoArrays {

    public static void main(String[] args) {
        // Given two sorted arrays of same size

        int[] a1 = {1,2,3,4,5,6};
        int[] a2 = {7,8,9,10,11,12};        

        int[] mergedArray = new int[a1.length + a2.length];

        for(int i=0 ; i < a1.length; i++){
            mergedArray[i] = a1[i];
        }

        //System.out.println("Length:"+2*(a1.length));

        for(int i= a1.length; i < 2 * (a1.length); i++) {
            mergedArray[i] = a2[i];
        }

        for(int i=0 ; i < 2*(a1.length); i++){          
            System.out.println("Part of Array: "+mergedArray[i]+ " Length is: "+mergedArray.length);
        }
    }
}

I am getting following error :

Length:12
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 6
    at com.my.practice.MedianOfTwoArrays.main(MedianOfTwoArrays.java:30)

Eran · Answer 1 · 2015-09-19T19:43:05.680

1

It's not a good idea to rely on the two arrays having the same length, so I wouldn't use a1.length * 2. In addition, you can't use the same index for the original arrays and the merged array, since the merged array is longer.

A suggested fix:

int[] mergedArray = new int[a1.length + a2.length];

for(int i=0 ; i < a1.length; i++){
    mergedArray[i] = a1[i];
}

for(int i= 0 ; i < a2.length; i++){
    mergedArray[a1.length + i] = a2[i];
}

for(int i=0 ; i < mergedArray.length; i++){
    System.out.println("Part of Array: "+mergedArray[i]+ " Length is: "+mergedArray.length);
}

edited Sep 19 '15 at 19:43

answered Sep 18 '15 at 05:00

Eran

387,369
54
702
768

You missed the main problem: `mergedArray[i] = a2[i];` (which is also in your first code sample) will always result in a ArrayIndexOutOfBoundException. – Thomas Stets Sep 18 '15 at 05:12
@Thomas Stets Thanks. I removed the first snippet, since the second snippet is correct. – Eran Sep 18 '15 at 06:13

mynawaz · Answer 2 · 2015-09-18T09:06:57.913

The issue in your code is due to using i as an index for a2 in second loop. Its valid for mergedArray but not for a2

Either use i-a1.length as index in your second loop

for(int i= a1.length; i < 2 * (a1.length); i++) {
    mergedArray[i] = a2[i-a1.length];
}

Or use three indices: i, j & k. Just to give you an idea

int i, j, k = 0;

for (i = 0; i<a1.length; i++){
    mergedArray[k] = a1[i];
    k++;
}

for (j=0; j<a2.length; j++){
    mergedArray[k] = a2[j];
    k++;
}

score 1 · Accepted Answer · answered Sep 18 '15 at 05:16

In this line:

for (int i= a1.length; i < 2 * (a1.length); i++) {
    mergedArray[i] = a2[i];
}

you are trying to access a2[i] for i from 6 to 11. Since a2 is an array of size 6, a2[6], a2[7] ... a2[11] do not exist.

In your case, you want to insert values a2[1] into mergedArray[6], a2[2] into mergedArray[7] etc.

You either need to substract a1.length on the right side:

for (int i = a1.length; i < 2 * (a1.length); i++) {
    mergedArray[i] = a2[i - a1.length];
}

or add a1.length on the left side:

for (int i = 0; i < a1.length; i++) {
    mergedArray[i + a1.length] = a2[i];
}

Choose the more convenient one.

score 1 · Answer 4 · answered Sep 18 '15 at 05:37

Mistake (for ArrayOutOfBoundException):

    for(int i= a1.length; i < 2 * (a1.length); i++) {
        mergedArray[i] = a2[i];  //Using 'i' incorrectly to access a2[i]
    }

Correcting above:

    for(int i= 0; i < a2.length; i++) {
        mergedArray[i+a1.length] = a2[i];  //Using 'i' incorrectly to access a2[i]
    }

Reasons:

'i' here will ensure that a2 is not exceeded beyond its size because i's max limit is a2.length.
Do not assume that a2 and a1's size would be equal. While traversing a1 using a1.length and while traversing a2, use a2.length.
A safety (redundant) check can be added to ensure that (i+a1.length) does not exceed mergedArray size. Redundant in this case because the mergedArray's length = a1.size + a2.size.

score 1 · Answer 5 · 2015-09-18T08:30:51.813

Problem is your are using same index for arrays of different length:

for(int i= a1.length; i < 2 * (a1.length); i++)
    mergedArray[i] = a2[i];

Here at initial, i = 6, array bounds for a2 is 0-5 however a2[i] is a2[6], out of bounds, which gives you the exception.

You can skip those loops, by using System.arraycopy()

 System.arraycopy(a1, 0, mergedArray,0, a1.length);
 System.arraycopy(a2, 0, mergedArray,a1.length, a2.length);

If you still wana use loop, simply use another index variable:

for(int i = 0, j = a1.length; i < (a2.length); i++, j++)
    mergedArray[j] = a2[i];

ArrayIndexoutofbound exception merging two sorted array

5 Answers5