Concatenating two int arrays

Question

I am looking to take two int[] arrays, and concatenate them into a single one. I realize that there are quite a few posts already on here, yet all of them are using more advanced scripts (System.arraycopy, etc.) that I am not familiar with. I could easily use them, but I lose understanding for why I am using these methods.

My plan looking forward is to set both my arrays to Strings (using the toString() method), and then trace them with a for loop. Each time an integer is found, it prints it onto the end of a new String that is originally initialized to "".

Example (line breaks for spacing):

array1 = [1, 2, 3, 4];

array2 = [5, 6, 7];

array3 = [1, 2, 3, 4, 5, 6, 7]; //this is what array should look like.

Can anyone offer me any advice on my proposed method to fulfilling this part of my work?

Removed (Student) from question header/title. It's not needed. — Rahul
– Rahul, Commented Mar 28, 2014 at 16:36
Why would you use toString()? Do you want to concatenate the two arrays or to print their elements? — Juggernaut93
– Juggernaut93, Commented Mar 28, 2014 at 16:37
Excuse my poor explanation. Yes, I would like to print the array after combining them. — Rivers31334
– Rivers31334, Commented Mar 28, 2014 at 16:38
if you don't want to use built in methods arraycopy then do it manually. Create a 3rd array with size arr1+arr2 and then loop through both array and store their contents to arr3. — Rahul
– Rahul, Commented Mar 28, 2014 at 16:39
Ended up using this approach (creating a 3rd array and using loops to fill it in). Much thanks. — Rivers31334
– Rivers31334, Commented Mar 28, 2014 at 17:11

r.v · Accepted Answer · 2014-03-28 16:43:36Z

You do not need to use strings here. Just get a new array like:

int[] array3 = new int[array1.length + array2.length];

Now you can use arraycopy to copy the two arrays or, if you do not like that, just use loops. I guess you already know from elsewhere how to use arraycopy. As for loops, you can two loops, one that runs from 0 to array1.length and copies array1 into array3 and another that runs from 0 to array2.length and copies array2 into array3 beginning beginning from the index array1.length in array3.

If you want to subsequently print the arrays, use Arrays.toString()

Mureinik · Accepted Answer · 2014-03-28 16:40:49Z

There's no need to use a String. You can just allocate a new array the size of the two others combined, and copy all the values into it:

int[] array1 = ...; int[] array2 = ...; int[] result = new int[array1.length + array2.length]; for (int i = 0; i < array1.length; ++i) { result[i] = array1[i]; } for (int i = 0; i < array2.length; ++i) { result[array1.length + i] = array2[i]; }

Of course, you could use System.arrayCopy to do the copying more efficiently, but the principal still stands.

Rahul · Accepted Answer · 2014-03-28 16:46:43Z

if you don't want to use built in methods arraycopy then do it manually. Create a 3rd array with size arr1+arr2 and then loop through both array and store their contents to arr3. Like

int size = arr1.length + arr2.length; int indexval=0; int[] arr3 = new int[size]; for(int i=0;i<arr1.length;i++) { arr3[i] = arr1[i]; indexval++; } for(int i=0,i<arr2.length;i++) { arr3[indexval] = arr2[i]; indexval++; }

Jakub H · Accepted Answer · 2014-03-28 16:40:58Z

You can use ArrayUtils from Apache Commons

int [] array1 = {1,2,3, 4}; int [] array2 = {5,6,7}; int [] array3 = ArrayUtils.addAll(array1, array2);

Chamil · Accepted Answer · 2014-03-28 16:41:58Z

using Apache Commons Lang library

int[] array1 = { 1, 2, 3, 4, 5 }; int[] array2 = { 6, 7, 8, 9, 10 }; int[] array3 = ArrayUtils.addAll(array1, array2);

http://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/ArrayUtils.html#addAll(int[], int[])

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