[java] Combine two integer arrays

Is there a way to create a single array out of two? E.g.

int[] array1 = {1,2,3};
int[] array2 = {4,5,6};
int[] array1and2 = array1 + array2;

I think you have to allocate a new array and put the values into the new array. For example:

int[] array1and2 = new int[array1.length + array2.length];
int currentPosition = 0;

for( int i = 0; i < array1.length; i++) {
    array1and2[currentPosition] = array1[i];
    currentPosition++;
}

for( int j = 0; j < array2.length; j++) {
    array1and2[currentPosition] = array2[j];
    currentPosition++;
}

As far as I can tell just looking at it, this code should work.

NOTE: didn't test it

int[] concatArray(int[] a, int[] b) {
  int[] c = new int[a.length + b.length];
  int i = 0;
  for (int x : a) { c[i] = x; i ++; }
  for (int x : b) { c[i] = x; i ++; }
  return c;
}

use ArrayUtils.addAll(T[], T...):

import org.apache.commons.lang3.ArrayUtils;

AnyObject[] array1 = ...;
AnyObject[] array2 = ...;
AnyObject[] mergedArray = ArrayUtils.addAll(array1, array2);

Yes but it is not quite that easy. Create a third array that is the size of the two arrays combined and loop through each original array and move the items over. Also look into System.arraycopy().

Find the total size of both array and set array1and2 to the total size of both array added. Then loop array1 and then array2 and add the values into array1and2.

Task: Given two int arrays array1 and array2 of the same length, zip should return an array that's twice as long, in which the elements of array1 and array2 are interleaved. That is, element #0 of the result array is array1[0], element #1 is array2[0], element #2 is array1[1], element #3 is array2[1], and so on.

public static int [] zip(int [ ] array1, int [] array2) {
//make sure both arrays have same length
if (array1.length != array2.length) {
    throw new IllegalArgumentException("Unequal array lengths - zip not possible");
}

int [] zippedArray = new int [array1.length+ array2.length]; 
int idx_1 = 0;
int idx_2 = 0;

//for each element of first array, add to new array if index of new array is even

for (int i=0; i < zippedArray.length; i+=2){
    zippedArray[i]= array1[idx_1++];
}
for (int i=1; i < zippedArray.length; i+=2){
    zippedArray[i]= array2[idx_2++];
}

//check contents of new array       
for (int item: zippedArray){
    System.out.print(item + " ");
}

return zippedArray;

}

int [] newArray = new int[old1.length+old2.length];
System.arraycopy( old1, 0, newArray, 0, old1.length);
System.arraycopy( old2, 0, newArray, old1.length, old2.length );

Don't use element-by-element copying, it's very slow compared to System.arraycopy()

Here a simple function that use variable arguments:

final static
public int[] merge(final int[] ...arrays ) {
    int size = 0;
    for ( int[] a: arrays )
        size += a.length;

        int[] res = new int[size];

        int destPos = 0;
        for ( int i = 0; i < arrays.length; i++ ) {
            if ( i > 0 ) destPos += arrays[i-1].length;
            int length = arrays[i].length;
            System.arraycopy(arrays[i], 0, res, destPos, length);
        }

        return res;
}

To use:

int[] array1 = {1,2,3};
int[] array2 = {4,5,6};
int[] array3 = {7,8,9};
int[] array1and2and3 = merge(array1, array2, array3);
for ( int x: array1and2and3 ) 
    System.out.print( String.format("%3d", x) );