How to properly sort list by another list in Dart

122

The way I would fix this is to find the index of the description in the order list and if it cannot be found, I would use a number that is out of index inside the order list to indicate that this item should be at the bottom of the list.

This would be my solution:

void testIt() {
  final outOfBounds = order.length + 1;
  const description = 'description';
  items.sort(
    (lhs, rhs) {
      final lhsDesc = (lhs[description] as String).toLowerCase();
      final rhsDesc = (rhs[description] as String).toLowerCase();
      final lhsIndex =
          order.contains(lhsDesc) ? order.indexOf(lhsDesc) : outOfBounds;
      final rhsIndex =
          order.contains(rhsDesc) ? order.indexOf(rhsDesc) : outOfBounds;
      return lhsIndex.compareTo(rhsIndex);
    },
  );
}

And the result is:

[{id: 1, id2: 3, description: Top European}, {id: 0, id2: 5, description: Top USA}, {id: 6, id2: 6, description: Top Rest of the world}, {id: 2, id2: 2, description: Top A}, {id: 3, id2: 0, description: Top Z}, {id: 4, id2: 4, description: Top C}, {id: 5, id2: 1, description: Top D}]
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Philipos D.
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Philipos D.

“The best programs are written so that computing machines can perform them quickly and so that human beings can understand them clearly. A programmer is ideally an essayist who works with traditional aesthetic and literary forms as well as mathematical concepts, to communicate the way that an algorithm works and to convince a reader that the results will be correct.” ― Donald E. Knuth, Selected Papers on Computer Science

Updated on December 19, 2022

Comments

  • Philipos D.
    Philipos D. 11 months

    I have two lists, 1 is a list of Map items, and another list which is the order.
    I would like to sort the items based on their description attribute and compare them with the order list and have them inserted at the top.

    import 'package:collection/collection.dart';
    
    void main() {
      List<String> order = [
        'top european',
        'top usa',
        'top rest of the world'
      ];
    
      List<Map> items = [
        {'id': 0, 'id2': 5, 'description': 'Top USA'},
        {'id': 2, 'id2': 2, 'description': 'Top A'},
        {'id': 3, 'id2': 0, 'description': 'Top Z'},
        {'id': 6, 'id2': 6, 'description': 'Top Rest of the world'},
        {'id': 4, 'id2': 4, 'description': 'Top C'},
        {'id': 5, 'id2': 1, 'description': 'Top D'},
        {'id': 1, 'id2': 3, 'description': 'Top European'},
      ];
      
      //this works but adds the items at the end
      items.sort((a,b)  {
        return order.indexOf(a['description'].toLowerCase()) - 
          order.indexOf(b['description'].toLowerCase());
      });
    
      ///Results: print(items);
      // List<Map> items = [
      //   {'id': 2, 'id2': 2, 'description': 'Top A'},
      //   {'id': 3, 'id2': 0, 'description': 'Top Z'},
      //   {'id': 4, 'id2': 4, 'description': 'Top C'},
      //   {'id': 5, 'id2': 1, 'description': 'Top D'},
      //   {'id': 1, 'id2': 3, 'description': 'Top European'},
      //   {'id': 0, 'id2': 5, 'description': 'Top USA'},
      //   {'id': 6, 'id2': 6, 'description': 'Top Rest of the world'},
      // ];
    }
    

    SOLUTION: I also tried this approach which is not ideal, but it works.

    List <Map> itemsOrder = items
      .where(
        (ele) => order.contains(ele['description'].toString().toLowerCase()))
      .toList();
    
    itemsOrder.sort((a, b) {
      return order.indexOf(a['description'].toLowerCase()) -
        order.indexOf(b['description'].toLowerCase());
    });
    
    items.removeWhere(
      (ele) => order.contains(ele['description'].toString().toLowerCase()));
    
    itemsOrder = itemsOrder.reversed.toList();
    
    for (int i = 0; i < itemsOrder.length; i++) {
      items.insert(0, itemsOrder[i]);
    }
    
    ///Results: print(items);
    // List<Map> items = [
    //   {'id': 1, 'id2': 3, 'description': 'Top European'},
    //   {'id': 0, 'id2': 5, 'description': 'Top USA'},
    //   {'id': 6, 'id2': 6, 'description': 'Top Rest of the world'},
    //   {'id': 2, 'id2': 2, 'description': 'Top A'},
    //   {'id': 3, 'id2': 0, 'description': 'Top Z'},
    //   {'id': 4, 'id2': 4, 'description': 'Top C'},
    //   {'id': 5, 'id2': 1, 'description': 'Top D'},
    // ];
    

    Ideally, I would like to use sortBy or sortByCompare but unfortunately, I cannot find a proper example or get a grasp of how to use it.