Display list of map without duplication/repetition - Dart - dart

Question

I'm trying to display/print list of map without duplicate element. For example:
List userList = [
{'name': 'john', 'user_id': '251'},
{'name': 'will', 'user_id': '255'},
{'name': 'jack', 'user_id': '251'} // duplicate
];
this is what I'm trying to output/print
List userList = [
{'name': 'john', 'user_id': '251'},
{'name': 'will', 'user_id': '255'},
];
some of you may suggest to use toSet method but that is valid only for list (not list of map)

Answer 1

This is one of the reasons why we should not use maps in Dart as data structures since it makes it rather complicated to express what it means for elements to be equal to each other.
So I would suggest solving this issue by creating a User class like this:
class User {
String name;
String user_id;
User({required this.name, required this.user_id});
#override
int get hashCode => user_id.hashCode;
#override
bool operator ==(Object other) => other is User && user_id == other.user_id;
#override
String toString() => '{Name: $name, User ID: $user_id}';
}
class User {
String name;
String user_id;
User({required this.name, required this.user_id});
#override
int get hashCode => Object.hash(name, user_id);
#override
bool operator ==(Object other) =>
other is User && name == other.name && user_id == other.user_id;
#override
String toString() => '{Name: $name, User ID: $user_id}';
}
By doing so, we can do:
void main() {
List<User> userList = [
User(name: 'john', user_id: '251'),
User(name: 'will', user_id: '255'),
User(name: 'jack', user_id: '251'),
];
userList.forEach(print);
// {Name: john, User ID: 251}
// {Name: will, User ID: 255}
// {Name: jack, User ID: 251}
}
And we can then do the toSet() trick like this:
void main() {
List<User> userList = [
User(name: 'john', user_id: '251'),
User(name: 'will', user_id: '255'),
User(name: 'jack', user_id: '251'),
];
List<User> uniqueUserList = userList.toSet().toList();
uniqueUserList.forEach(print);
// {Name: john, User ID: 251}
// {Name: will, User ID: 255}
}
I notice that your example is made so you are only looking at the user_id to determine if you have a duplicate element. I don't know if your example is just wrong but if you want to compare both name and user_id your class would look like this:
class User {
String name;
String user_id;
User({required this.name, required this.user_id});
#override
int get hashCode => Object.hash(name, user_id);
#override
bool operator ==(Object other) =>
other is User && name == other.name && user_id == other.user_id;
#override
String toString() => '{Name: $name, User ID: $user_id}';
}
And finally, if you really want to solve this using Map objects, you could solve it using the following where you create a Set with your own definition of what it means to be equal:
void main() {
List<Map<String, String>> userList = [
{'name': 'john', 'user_id': '251'},
{'name': 'will', 'user_id': '255'},
{'name': 'jack', 'user_id': '251'} // duplicate
];
final set = LinkedHashSet<Map<String, String>>(
equals: (map1, map2) {
final id1 = map1['user_id'];
final id2 = map2['user_id'];
return id1 != null && id2 != null && id1 == id2;
},
hashCode: (map) => map['user_id'].hashCode,
);
set.addAll(userList);
List<Map<String, String>> uniqueUserList = set.toList();
uniqueUserList.forEach(print);
// {name: john, user_id: 251}
// {name: will, user_id: 255}
}
There are other ways to solve this but it will always get kinda ugly since Map is not intended for data structures as previous described.