Dart List consisting of Lists (i.e. 2d array) weirdly linked

I was just trying to create a 2D array in Dart as follows:

List<List<int>> data = List.filled(7, List.filled(15, 0));
  
data[1][0] = 5;
  
for (int i = 0; i< 7; i++) {
  print('Index $i: ${data[i]}');
}

Expected result would be:

Index 0: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 1: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 2: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 3: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 4: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 5: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 6: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Actual result is:

Index 0: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 1: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 2: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 3: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 4: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 5: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 6: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

It seems that the lists are somehow linked. Why is that and how can I fix it? Thanks!

>Solution :

Your issue is that you are using List.filled, which creates a list where all elements will point to the object you provide as the second argument.

Since your second element are a List object, then your first will will now have 15 references that points to that single List instance. This is why it seems like you have changed all sublists when doing a change to one of them.

What you instead what to do is create a new sublist for each position in your first List. To do this, you can use List.generate like showed in this example:

void main() {
  List<List<int>> data = List.generate(7, (_) => List.filled(15, 0));

  data[1][0] = 5;

  for (int i = 0; i < 7; i++) {
    print('Index $i: ${data[i]}');
  }
}

Output:

Index 0: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 1: [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 2: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 3: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 4: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 5: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Index 6: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Here we now call List.filled for each sublist we want in our first List, which means we now ends up with individual List objects so any change to one of them will not seem to happen in the other ones.