Query about decrementing a variable.

Question

Hi, all:
I'm writing an agent-based model about battlefield strategies. There's a previous model I'm working off of, which contains a bit of code that I don't understand. It's a hack to remove an element from an arraylist, and I don't think I understand it very well. My model works just fine, but I don't like not understanding WHY this bit of code works the way that it does. I've read tutorials about incrementing/decrementing, but I still don't quite get what's going on here. The code is to remove instances of the Soldier class that have "died" and whose unique identifiers have been added to an arraylist called "killEmAll" based on probabilities located elsewhere in the model. Here's the relevant loop:
          for (int j = 0; j < soldierList.size(); j++) {
               Soldier soldier = soldierList.get(j);
               solID = soldier.getID();
               if (killEmAll.contains(solID)) {
                    soldierList.remove(soldier);
                    --j;//this is kludgy. Why does it work?
                    world.putObjectAt( soldier.getX(), soldier.getY(), null );
          }What exactly is going on with that "--j" thing? Why does having it/not having it in the code result in different behavior in my model?

Answer 1

soldierList.remove(soldier);
This line removes an element from your list. So, now the size of the list is decremented by 1 and hence you decrement the value of 'j' also...
Otherwise, you will miss an element.
e.g. You have 10 soldiers in the list.
Now j=0; you take soldierList(0) and remove it. Now the element in soldierList(1) is moved up to 0th place. If you do not decrement 'j', in the next loop, j will become 1 and it will miss the 0th element in the list.... hope it helps...:)