How to copy elements from `2d array` to `Arraylist`?

Question

folks, what is the technique of copying elements from array to Arraylist?

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          for(int j = 0; j < x[0].length; j++){
              myBoard.get(i).add(j); //<<------ getting error!
          }
      }
  }

score 1 · Answer 1 · edited Jun 20 '20 at 09:12

You'll need to initialize each member of myBoard in the outer loop:

Untested Code Ahead

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          myBoard.add(new ArrayList<T>); //Gotta add something to stick stuff in
          for(int j = 0; j < x[0].length; j++){
              myBoard.get(i).add(j); //<<------ no more error?
          }
      }
  }

Since myBoard is an ArrayList of ArrayLists of Ts, we need to give it somewhere to put the T's. Initially, myBoard looks like this:

[] <-- empty ArrayList

So we give it somewhere to put data for each row, like this

myBoard.add(new ArrayList<T>);

Now it looks like

[ [] ] <--- ArrayList with an empty ArrayList in it, ready to accept T's

We add some T's, and end up with this

[ [T1, T2, T3] ].

And on the next iteration, we'll end up with something like this

[ [T1, T2, T3], [T4, T5, T6] ]

Hope that cleared things up.

could you please explain this line? WHat does `myBoard.add(new ArrayList);` do?, @CollinDriscoll — Michael, Sep 05 '15 at 04:45
It adds a new ArrayList to myBoard. myBoard is an ArrayList of ArrayLists of T's. Initially it's empty. So we need to give it somewhere to put T's. — CollinD, Sep 05 '15 at 04:49

afzalex · Answer 2 · 2015-09-05T05:29:24.810

1

You are getting error because your are trying to get() a value from ArrayList but not had inserted any value first. To do it your way, here is the correct code :

ArrayList<ArrayList<T>> myBoard = new ArrayList<>();
for(int i = 0; i < x.length; i++){
    ArrayList<T> values = new ArrayList<>();
    for(int j = 0; j < x[0].length; j++){
        values.add(x[i][j]);
    }
    myBoard.add(values);
}

Just use the following function :

Arrays.asList(T...a)

In your case it will be done as :

ArrayList<T> myBoard = new ArrayList<>();
for(T[] arr : x){
    myBoard.add(Arrays.asList(arr));
}

Try to use diamond operator <> to make code more readable
Don't reinvent the wheel, use utilities provided at least by core libraries
Make use of for-each statement where you could

edited Sep 05 '15 at 05:29

answered Sep 05 '15 at 04:46

afzalex

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but I have 2d array in this case which is `T[][] x`. Would it cause any problems ? @afzalex – Michael Sep 05 '15 at 04:58
Obviously not. Just use `T` in the place of `Integer`, well I edit my post. – afzalex Sep 05 '15 at 05:29

score 0 · Answer 3 · edited May 23 '17 at 11:58

Inspired from this post.

new ArrayList<Item>(Arrays.asList(array))

creates a new ArrayList of Item elements from an input array. For two-dimensionals. In this case, you tried this:

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          for(int j = 0; j < x[0].length; j++){
              myBoard.get(i).add(j); //<<------ getting error!
          }
      }
  }

The problem is that the line throwing the error assumes you have an ArrayList of ArrayList, but the inner element you try to refer to is not initialized. This should be a fix:

public DenseBoard(T[][] x, T fillElem){
      ArrayList<ArrayList<T>> myBoard = new ArrayList<ArrayList<T>>();
      for(int i = 0; i < x.length; i++){
          myBoard.add(new ArrayList<T>(Arrays.asList(x[i])));
      }
      //Do something with myBoard
}

How to copy elements from `2d array` to `Arraylist`?

3 Answers3

Untested Code Ahead