Question

In Java, is there a way to add a value not at a specific index, but to the next undeclared index? Say:

int[] negativeArray = new int[21];
int[] positiveArray = new int[21];

There are two arrays for two different types of ints, for example negative and positive. I’m looping through this, and I want it to work something like a stack (I don’t know too much about stacks, but to my knowledge you don’t go through its index, you just pop/push items onto it) to where if its a negative number, put in the number in the next undeclared index spot in the negative array.

I thought of a way to do this with some extra code. I’d set all values in the array to 0. When checking if the variable is negative or positive, I would loop through the array to the next value that is 0. Once I find it I know what index I’m on. This would require a bit of effort though, are there any simpler ways of doing this?

---

Edit: Some comments are stating different ways to do this without using a basic array. I was assigned this, and I am required to use an array to get credit for it…

Answer 1

If you are actually using the array as a stack (and thus, will only add or remove items at the top of the stack) then you could keep in another variable the next free index in the array.

int[] array = new int[21];
int nextIndex = 0;

public void push(int e) {
    array[nextIndex] = e;
    ++nextIndex;
}

public int pop() {
    --nextIndex;
    return array[nextIndex];
}

If removals can occur anywhere, then I don’t see a better solution than iterating over the array to find a free spot.