Removing item from ArrayList and Resorting with Array

Programming Questions

X Y X

Removing item from ArrayList and Resorting with Array

in Programming Questions • 2 years ago

Hi guys

I'm trying to remove enemies from an array list when they pass off the screen by using the objects.index value, removing that from the arrayList, then looping through copying al the neccesary ones back into an Array (to modify their index values appropriately) and then back into the arrayList but im having some problems

The speed seems to increase and while it does seem to work (keeping the arraylist size down) im not sure, any help would be great thanks!

void update() {
    ypos += speed;
    if(ypos > height){
      println(index + " - " + waveIndex + " - " + waves.size());
      //if(index>1){
       // waves.remove(int(index-2));
      //}else{
       // waves.remove(int(index-1));
     // }
      Wave[] wavesTemp = new Wave[waves.size()-1];
      wavesTemp = waves.toArray(wavesTemp);
      for(int i = int(index); i < waves.size()-1; i++){
       wavesTemp[i] = wavesTemp[i+1];
       wavesTemp[i].index = i;
      }
      waves.remove(index);
      for(int i = 0; i < waves.size(); i++){
        waves.set(i,wavesTemp[i]);
      }
}

Replies(5)

PhiLho

Re: Removing item from ArrayList and Resorting with Array

2 years ago

I don't understand what is your problem, exactly.
And I don't understand either why you do such complicated gymnastics... If you use remove() on an ArrayList, the other elements are shifted automatically. But if you need to delete items of an array list while iterating on it, you should start at the end of the list and go backward.

X Y X

Re: Removing item from ArrayList and Resorting with Array

2 years ago

Sorry for not making myself too clear

Basically the Arraylist 'waves; contains enemy class
The class has a variable 'index' which corresponds to the index value it has (in the arraylist) when it spawns
Enemies move at different rates so, when one passes off the bottom of the screen, its removed using its 'index' value
This means that all of the enemies after the removed one need to have their index value updated (to -1) which i guess means i have to copy them all into a standard array in order to modifiy those values, and then update the arraylist

Is that right? Seemed like i had to have a value in the class refering to its index in order to use arraylist.remove(index) within the actual class

PhiLho

Re: Removing item from ArrayList and Resorting with Array

2 years ago

If you really need to maintain the index inside the class, you can update it too. For example:

void setup()
{
ArrayList<Wave> waves = new ArrayList<Wave>();
for (int i = 0; i < 10; i++)
{
waves.add(new Wave(i));
}
println(waves);
// We want to remove the item 5
int itemToRemove = 5;
waves.remove(itemToRemove);
println(waves);
for (int i = itemToRemove; i < waves.size(); i++)
{
Wave w = waves.get(i);
w.index--;
}
println(waves);
}
class Wave
{
int index;
Wave(int i) { index = i; }
String toString() { return "Wave " + index; }
}

X Y X

Re: Re: Removing item from ArrayList and Resorting with Array

2 years ago

Thanks! Sorry for the delayed response, this bit confused me:

Wave w = waves.get(i);
w.index--;
}

Because it doesnt look like your updating the Arraylist but it works! Thanks!

PhiLho

Re: Removing item from ArrayList and Resorting with Array

2 years ago

get() returns a reference to the object stored in the array list, it doesn't remove it from there. So you can update the object via the variable holding a reference to it.