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, Mar 16 '14 at 01:52

score 3 · Accepted Answer · edited May 23 '17 at 11:57

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.

score 0 · Answer 2 · answered Mar 16 '14 at 02:16

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.

Add values to an array in loop

2 Answers2