修改IntList的ArrayList中的Int

Question

In processing, I am attempting to create an ArrayList of IntLists such that the initial number of Elements in the ArrayList is variable to begin with (but I will not change it in the code) and the number of elements in the IntList is initially 0 but will be added to and edited within the code. My code is a Japanese Ladder game where the ArrayList is of Ladders, each of which has an IntList containing "Rungs" which are int components that correspond to a Y location on the ladder.

ArrayList <IntList> Ladders = new ArrayList <IntList>();
IntList temp = new IntList();

void setup()
{
  for(int i=0;i<numRails-1;i++)
  {
    Ladders.add(new IntList());
    temp.clear();
    temp.append(0);
    Ladders.set(i,temp);
  }
}
void addRung (int spot)
{
  temp.clear();
  temp = Ladders.get(spot);
  temp.append(50);
  //note that 50 is an arbitrary number, it would be given by MouseY
  Ladders.set(spot,temp);
  print(Ladders);

  //I have also tried...
  Ladders.get(spot).append(50);
}

So my problem is that addRung() seems to add 50 to every single element ofthe ArrayList and "print(Ladders);" outputs...

[IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ], IntList size=1 [ 50 ]]

Is there a better way to go about this entirely or am I doing something wrong?

I included an image of what my code is outputting... It should be a single rung at location 50 but it is 9 rungs at 50.

Answer 1

Please think carefully about exactly what this code is doing:

for(int i=0;i<numRails-1;i++)
  {
    Ladders.add(new IntList());
    temp.clear();
    temp.append(0);
    Ladders.set(i,temp);
  }

Here you're adding a new IntList to your Ladders ArrayList . Then you're clearing temp and adding 0 to it, and then you're setting the index (which you just added a new IntList to) to temp , which replaces the new IntList you just added.

In other words, you're adding temp to every index of the ArrayList . That's just a bunch of references to the same IntList , which is why when you add a value to one index, it adds the value to every index.

To fix this, you need to get rid of your temp variable completely. Just add a new IntList to each index, and then use the get() function to get the IntList from the index in the ArrayList . It would look like this:

ladders.get(index).append(value);

As a side note, you should really get into the habit of debugging your code before asking a question. A few print statements would have gone a long way to helping you figure out what's going on. Then if you get stuck, you can post a MCVE instead of disconnected snippets.

You should also try to use standard naming conventions: variables should start with a lower case letter.