how to change copied list member value without affecting original list in dart flutter?

IMemon
December 30, 2023
104 views
0 votes
2 Answers

Its been long time i am suffering between byVal and byRef in dart…

To make myself more clear i have created this demo to explain what i am suffering

I want simply copy of a list and than want to make changes in new list but keeping in mind that original list should be unchanged…

in following program i need to get output like


users =['A','B','C','D']
friends=['AA','BB'];
templist=['AAA','BBB']


but i am getting 


users    =['AAA','BBB','C','D']
friends  =['AAA','BBB'];
templist =['AAA','BBB']


void main() {
  List<User> users = [
    User(name: 'A', age: '43'),
    User(name: 'B', age: '12'),
    User(name: 'C', age: '53'),
    User(name: 'D', age: '22'),
  ];

  List<User> friends = [];
  List<User> templist = [];

  final u1 = users[0];
  friends.add(u1);
  friends[0].name = 'AA';
  final p2 = users[1];
  friends.add(p2);
  friends[1].name = "BB";

  templist = List.from(friends);

  templist[0].name = "AAA";
  templist[1].name = "BBB";

  for (var u in users) {
    print(u.name);
  }

  for (var f in friends) {
    print(f.name);
  }

  for (var t in templist) {
    print(t.name);
  }
}

Tags: flutter

Answers

You need to make copies of the users when filling the other lists.
Assuming that your User class is something like this

class User {
  String? name;
  String? age;

  User({this.name, this.age});

}

you could add a copy function to it like

class User {
  String? name;
  String? age;

  User({this.name, this.age});

  User copy() => User(name: name, age: age);
}

And then your final code could be something like this

void main() {
  List<User> users = [
    User(name: 'A', age: '43'),
    User(name: 'B', age: '12'),
    User(name: 'C', age: '53'),
    User(name: 'D', age: '22'),
  ];

  List<User> friends = [];
  List<User> templist = [];

  final u1 = users[0].copy();
  friends.add(u1);
  friends[0].name = 'AA';
  final p2 = users[1].copy();
  friends.add(p2);
  friends[1].name = "BB";

  templist = friends.map((e) => e.copy()).toList();

  templist[0].name = "AAA";
  templist[1].name = "BBB";

  for (var u in users) {
    print(u.name);
  }

  for (var f in friends) {
    print(f.name);
  }

  for (var t in templist) {
    print(t.name);
  }
}

By the way. It’s very common that your data classes are more complicated but have some kind of toJson and fromJson functions. In that case an easy way to make a copy is to call them back to back. So like

User copy() => User.fromJson(toJson());

in your code…

u are not creating copy but making reference to a list…

like templist=friends

that statement do not mean that you are creating new duplicate list but both templist and friends are refering to same list…

for single object..

User x=users[0];
User y=x;

here both x and y refering to same object

so to make copy

User y=x.makeCopy();

and for a list of user
to make copy u need to code as below

for(var x in friends)
  templist.add(x.makeCopy());


void main() {
  List<User> users = [
    User(name: 'A', age: '43'),
    User(name: 'B', age: '12'),
    User(name: 'C', age: '53'),
    User(name: 'D', age: '22'),
  ];

  List<User> friends = [];
  List<User> templist = [];

  User u1 = users[0];
  friends.add(u1.makeCopy());
  friends[0].name = 'AA';
  User p2 = users[1];
  friends.add(p2.makeCopy());
  friends[1].name = "BB";

  for(var x in friends)
    {
      templist.add(x.makeCopy());
    }

  templist[0].name = "AAA";
  templist[1].name = "BBB";

  for (var u in users) {
    print(u.name);
  }

  for (var f in friends) {
    print(f.name);
  }

  for (var t in templist) {
    print(t.name);
  }
}

add method to class

class User
{
  String name;
  String age;
  User({required this.name,required this.age});
  User makeCopy() => User(name: name, age: age);
}

try this code..it will give u same output that u want

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