Create list from multiple lists with a property is equal-CodePudding

My row lists:

List rowlist = [Student(name: 'John',age: 12),
             Student(name: 'Kity',age: 13),
             Student(name: 'Micle',age: 14),
             Student(name: 'Jack',age: 12),
             Student(name: 'Cha',age: 13),
             Student(name: 'Duc',age: 13),
             Student(name: 'Ran',age: 12)]

Expected result:

List result = [
               [
                Student(name: 'John',age: 12),
                Student(name: 'Jack',age: 12),
                Student(name: 'Ran',age: 12),
               ],
               [
                Student(name: 'Kity',age: 13),
                Student(name: 'Cha',age: 13),
                Student(name: 'Duc',age: 13),
               ],
               [
                Student(name: 'Micle',age: 14),
               ]
             ]

I need to create a new list of list wherein each list is added by Student with age field is equal. What is the best way for the result?.

CodePudding user response：

If you use the collection package. You can use the groupListsBy method to group all of the students by age.

Map<int, List<Student>> result = rowlist.groupListsBy((student) => student.age);

The above however, returns the data as a Map<int, List<Student>. You can convert this to a List<List<Student>> by calling .values.toList().

List<List<Student>> result = rowlist.groupListsBy((student) => student.age).values.toList();

Complete example:

import 'package:collection/collection.dart';

void main() {
  List<Student> rowlist = [
    Student(name: 'John', age: 12),
    Student(name: 'Kity', age: 13),
    Student(name: 'Micle', age: 14),
    Student(name: 'Jack', age: 12),
    Student(name: 'Cha', age: 13),
    Student(name: 'Duc', age: 13),
    Student(name: 'Ran', age: 12),
  ];

  List<List<Student>> result =
      rowlist.groupListsBy((student) => student.age).values.toList();

  print(result);
}

class Student {
  final String name;
  final int age;
  const Student({required this.name, required this.age});
  @override
  String toString() => 'Student(name: $name, age: $age)';
}

CodePudding user response：

You can do this without any package also.

List rowlist = [
  Student(name: 'John', age: 12),
  Student(name: 'Kity', age: 13),
  Student(name: 'Micle', age: 14),
  Student(name: 'Jack', age: 12),
  Student(name: 'Cha', age: 13),
  Student(name: 'Duc', age: 13),
  Student(name: 'Ran', age: 12)
];

void main() {
  Map<int, List<Student>> groupMap = {};

  for (final s in rowlist) {
    if (groupMap.containsKey(s.age)) {
      groupMap[s.age]!.add(s);
    } else {
      groupMap[s.age] = [s];
    }
  }

  final result = groupMap.entries.map((e) => e.value).toList();
  
  print(result);
}

class Student {
  Student({required this.name, required this.age});
  final String name;
  final int age;

  @override
  String toString() => '$name - $age';
}