Add values to an array in loop

Question

In my program, I am trying to add values to an array while it is looping. Before it is put in the array, it has to fulfill the if statement. I need to be able to add as many values as needed based on the input. I am not sure how to do this. Any help would be appreciated.

for(int a=0; a<= subset1white.length-1;a++){
  String w = Integer.toString(Integer.parseInt(subset1white[a]) + 2);
  String x = Integer.toString(Integer.parseInt(subset1white[a]) - 2);
  String y = Integer.toString(Integer.parseInt(subset1white[a]) + 10);
  String z = Integer.toString(Integer.parseInt(subset1white[a]) - 10);
  String[] arithmetic = {w, x, y, z};
     for(int b=0; b<= arithmetic.length-1; b++){
        if(arithmetic[b] == subset1black[a]){

        }
     }

  }

If the if loop returns true, I need the subset1black[a] value to be put in an array called result. I know how to declare arrays but I do not know how to declare an array where the length can be changed(new values added).

Why don't you use the ArrayList? It's like an array that can grow dynamically when needed. — Bhesh Gurung
– Bhesh Gurung, Commented Mar 16, 2014 at 1:52

Community · Accepted Answer · 2017-05-23 11:57:37Z

3

Use ArrayList<Integer> which can grow dynamically as needed (as suggested by Bhesh Gurung):

// Declare
List<Integer> result = new ArrayList<Integer>();

// Add to end of the list
result.add(subset1black[a]);

See the Javadoc for more information or if you want to add the elements to the list in a different way.

If you really need a primitive array, you can convert the List. See this related Q.

edited May 23, 2017 at 11:57

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answered Mar 16, 2014 at 1:56

xlm

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Comments

PNS · Accepted Answer · 2014-03-16 02:16:39Z

There are no variable size primitive arrays in Java, so the closest to such a data structure is the ArrayList class, which internally is an array of objects, resized when necessary. It helps to initialize the ArrayList to the anticipated size (or a bit more), to avoid resizing, but this optimization is not necessary.

For the code in question, an implementation of the above could be:

List<Integer> result = new ArrayList<Integer>(subset1white.length);

for (int a=0; a<subset1white.length; a++) {

  String w = Integer.toString(Integer.parseInt(subset1white[a]) + 2);
  String x = Integer.toString(Integer.parseInt(subset1white[a]) - 2);
  String y = Integer.toString(Integer.parseInt(subset1white[a]) + 10);
  String z = Integer.toString(Integer.parseInt(subset1white[a]) - 10);

  String[] arithmetic = {w, x, y, z};

  for (int b=0; b<= arithmetic.length-1; b++) {
    if (arithmetic[b] == subset1black[a]) {
      result.add(new Integer(subset1black[a]));
    }
  }    
}

It is not necessary to initialize an Integer object to add to the ArrayList, since Java does that automatically (via "autoboxing"), but doing so makes the code a bit cleaner.

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