Sort multiple sublists of list to create new list, Dart

Question

I'm trying to sort an entire list according to one property. Afterwards I'd like to sort this list according to a second property, but in groups of 4. So, after sorting the list once, I want to look at the first 4 positions and sort only these 4 according to the second property - then move on to the next 4 positions and sort these again, and so on...

This is what I have so far:

class myElements {
  int Position;
  String text;
  int Top;
  int Left;


  myElements(int Position, String text, int Top, int Left){
    this.Position = Position;
    this.text = text;
    this.Top = Top;
    this.Left = Left;
  }
}

var FirstList = new List<myElements>();
var newList = new List<myElements>();

Adding Elements to my first list:

myElements Test = myElements(ElementNumber, text, Top, Left);
              FirstList.add(Test);

Then sorting for the first time according to 'Top':

 Comparator<myElements> TextComparator = (a, b) => a.Top.compareTo(b.Top);
      FirstList.sort(TextComparator);

Here is where I'm stuck. I'm trying to sort the list again, but only in groups of 4 - this time according to 'Left':

   for (int i = 0; i < FirstList.length; i += 4) {
          Comparator<myElements> TextComparator2 = (a, b) =>
              a.Left.compareTo(b.Left);


          newList.addAll(FirstList.sublist(i, i + 3).sort(TextComparator2)); //this line does not work
        }

I think I am stuck trying to access my sorted sublist: (FirstList.sublist(i, i + 4).sort(TextComparator2) . If I could add these to a new list, it should work. However any other suggestions are more than welcome.

Thanks so much!

Answer 1

newList.addAll(FirstList.sublist(i, i + 3).sort(TextComparator2)); //this line does not work

Your code is almost correct. You have the right idea, but you ended up trying to do too much in one line of code.

Breaking it down a bit, your code is equivalent to:

var sublist = FirstList.sublist(i, i + 3);
newList.addAll(sublist.sort(...)); // Does not work

And that doesn't work because List.sort does not return a value. It mutates the list instead of returning a new list.

It would work if you instead did:

var sublist = FirstList.sublist(i, i + 3);
sublist.sort();
newList.addAll(sublist);

Also, List.sublist uses an exclusive end index. If you want to create sublists with 4 elements, you would need to use sublist(i, i + 4).