从具有相同属性的多个列表创建列表答案

【问题标题】：Create list from multiple lists with a property is equal从具有相同属性的多个列表创建列表
【发布时间】：2021-12-27 06:31:42
【问题描述】：

我的行列表：

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)]

预期结果：

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),
               ]
             ]

我需要创建一个新的列表列表，其中每个列表由Student 添加，age 字段相等。结果的最佳方法是什么？。

【问题讨论】：

标签： list flutter dart

【解决方案1】：

如果您使用collection package。您可以使用groupListsBy 方法将所有学生按年龄分组。

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

然而，上面的数据以Map<int, List<Student> 的形式返回。您可以通过调用.values.toList() 将其转换为List<List<Student>>。

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

完整示例：

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)';
}

【讨论】：

【解决方案2】：

您也可以在没有任何包的情况下执行此操作。

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';
}

【讨论】：