Java: How to change an Integer array parameter from inside a method

Question

I have a programming assignment where I have to make a method that takes in an array that is split up into subarrays, and merge them. My method works fine but the Integer array that is passed in 'a' is only changed from inside the method, and remains unchanged from the outside.

After looking around on google, I have concluded that it is impossible to change an array parameter, but then I thought about Arrays.sort which does end up changing the array parameter from outside of the method.

How do I change the array that is passed in as a parameter so that it stays changed outside of the method?

    /**
        * Merge three sorted arrays with these ranges [lo..mid1], [mid1+1..mid2], [mid2+1..hi] into one sorted array.
        * Array a has the original input and final sorted input. 
        * Array aux is the auxiliary array.
        */

   public static void Merge(Integer[] a, int lo, int mid1, int mid2, int hi, Integer[] aux) {

      int[] arr1 = copyArray(a, lo, mid1);
      int[] arr2 = copyArray(a, mid1+1, mid2);
      int[] arr3 = copyArray(a, mid2+1, hi);

      int i = 0, j = 0, k = 0, idx = 0;

      while(i < arr1.length-1 || j < arr2.length-1 || k < arr3.length-1) {

         // arr1[i] is the smallest
         if(arr1[i] <= arr2[j] && arr1[i] <= arr3[k]) {
            System.out.println("i is bigger at " + arr1[i]);
            aux[idx] = arr1[i];
            i++;
         }

         // arr2[j] is the smallest
         else if(arr2[j] <= arr1[i] && arr2[j] <= arr3[k]) {
         System.out.println("j is bigger at " + arr2[j]);
            aux[idx] = arr2[j];
            j++;
         }

         // arr3[k] is the smallest
         else if(arr3[k] <= arr2[j] && arr3[k] <= arr1[i]) {
            System.out.println("k is bigger at " + arr3[k]);
            aux[idx] = arr3[k];
            k++;
         }

         idx++;
      }


      System.out.println(Arrays.toString(arr1));
      System.out.println(Arrays.toString(arr2));
      System.out.println(Arrays.toString(arr3));


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

      a = aux;
      a[0] = 900;
      System.out.println("a from Merge method: " + Arrays.toString(a));


   }

You have to change each of the elements, like you did for aux: aux[idx] = arr3[k]; Assignment like this won't work a = aux; because Java is pass by value. (Look that up on Google.) — markspace
– markspace, Commented Mar 3, 2019 at 21:00

derUser · Accepted Answer · 2019-03-03 21:44:26Z

1

You have to iterate over your aux array and assign the value to the 'a' array:

a[i] = aux[i]

Or return your merged array as result.

public static int[] Merge(int[] a, int mid1, ...){
   // do something
  return a;
}

edited Mar 3, 2019 at 21:44

answered Mar 3, 2019 at 21:16

derUser

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Comments

user9643348 · Accepted Answer · 2019-03-03 21:06:53Z

0

You can change void to int[][] and then return the arrays

public static int[][] Merge(arguments) {
    //Do math
    int[][] result = new int[][] {arr1, arr2, arr3};
    return result;
}

Then, outside of the method you call It like below:

int[][] integerArray = Merge(arguments);
arr1 = integerArray[0];
arr2 = integerArray[1];
arr3 = integerArray[2];

answered Mar 3, 2019 at 21:06

user9643348

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