How to copy list values to another list in flutter

Question

I am trying to copy values of one list to another, I use three buttons 1st one to append a value to mylist, second one to clear the mylist, 3rd button to copy values from mynewlist to mylist.

i tried this

List<String> mylist = [
    'Albania',
    'Andorra',
    'Armenia',
    'Austria',
    'Azerbaijan',
    'Belarus',
    'Belgium',
    'Albania',
    'Andorra',
    'Armenia',
    'Austria',
    'Azerbaijan',
    'Belarus',
    'Belgium',
  ];

  List<String> mynewlist = [
    'Albania',
    'Andorra',
    'Armenia',
    'Austria',
    'Azerbaijan',
    'Belarus',
    'Belgium',
    'Albania',
    'Andorra',
    'Armenia',
    'Austria',
    'Azerbaijan',
    'Belarus',
    'Belgium',
  ];



Padding(
                padding: const EdgeInsets.all(5.0),
                child: Row(
                  children: <Widget>[
                    Expanded(
                      child: FlatButton(
                        onPressed: () {
                          setState(() {
                            print('clicked 1st');
                            print(mylist.length);
                            print(mynewlist.length);
                            mylist.add('sdsds');
                          });
                        },
                        child: Container(
                          child: Column(
                            children: <Widget>[
                              Image.asset(
                                'images/bulb.png',
                                width: 100,
                                height: 100,
                              ),
                              Text('bulb')
                            ],
                          ),
                        ),
                      ),
                    ),
                    Expanded(
                      child: FlatButton(
                        onPressed: () {
                          setState(() {
                            print('clicked 2nd');
                            print(mylist.length);
                            print(mynewlist.length);
//after i set mylist = mynewlist; when i click this button it clears the old and new list.
                            mylist.removeRange(0, mylist.length);
                          });
                        },
                        child: Container(
                          child: Column(
                            children: <Widget>[
                              Image.asset(
                                'images/bulb.png',
                                width: 100,
                                height: 100,
                              ),
                              Text('bulb')
                            ],
                          ),
                        ),
                      ),
                    ),
                    Expanded(
                      child: FlatButton(
                        onPressed: () {
                          setState(() {
                            print('clicked 3rd');
                            print(mylist.length);
                            print(mynewlist.length);
                         mylist = mynewlist;
                          });
                        },
                        child: Container(
                          child: Column(
                            children: <Widget>[
                              Image.asset(
                                'images/bulb.png',
                                width: 100,
                                height: 100,
                              ),
                              Text('bulb')
                            ],
                          ),
                        ),
                      ),
                    ),
                  ],
                ),
              )

On the initial time it works perfectly the second time i click the second button it clears the mylist and mynewlist.

How can i copy the values of second list without clearing the new new list

score 51 · Accepted Answer · edited May 02 '21 at 18:47

51

Use myList = List.from(mynewlist); instead of mylist = mynewlist;

edited May 02 '21 at 18:47

dKen

3,029
1
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answered Oct 15 '19 at 07:54

Ibrahim Karahan

2,206
1
6
18

3

mylist = List.from(mynewlist); – YYY Oct 10 '20 at 04:40
Thank you :) and sorry for late reply :P – Alvin John Niravukalayil Dec 08 '20 at 16:42
Its weird but `streamDataListExtra = streamDataList;` works for me. – Kamlesh Jun 17 '21 at 20:00
3

@Kamlesh Yes, it might work for you, but you will not actually copy the value of the list. You only copied the pointer from the initial list. – Paul Sep 30 '21 at 10:51
This doesn't work with me because my list has sub-list, the main list it is cloned properly, but if i filter the sub-list the original list affected even when i use *myList = List.from(mynewlist);* any idea? – Osama Remlawi Oct 09 '21 at 21:40
@OsamaRemlawi for me, its affected just cloned list. https://jdoodle.com/ia/hVs – Ibrahim Karahan Oct 11 '21 at 10:56
I've got the answer, and i liked to post it here as a reference for deep copy https://stackoverflow.com/questions/69514366/how-to-copy-complex-list-to-another-list-without-affecting-the-original-list-in – Osama Remlawi Oct 11 '21 at 12:29

Logemann · Answer 2 · 2019-10-15T08:01:10.010

9

Thats because you copied the object references (mylist = mynewlist) and not the content of the list. So after the first click, mylist has a reference to the same object in memory as mynewlist. So any operation on one of them, affect both.

To solve your problem you need to keep the object references intact and just copy around the contents of these lists.

edited Oct 15 '19 at 08:01

answered Oct 15 '19 at 07:51

Logemann

2,457
29
48

Thank you for your answer. – Alvin John Niravukalayil Oct 15 '19 at 15:14
sorry, didnt elaborated on this one. But Ibrahim Karahan was faster than me ;-) – Logemann Oct 15 '19 at 16:32

score 6 · Answer 3 · edited May 02 '21 at 18:48

6

var YOURCOPY = YOURLIST.map((v) => v).toList();

edited May 02 '21 at 18:48

dKen

3,029
1
25
35

answered Jan 26 '21 at 20:54

Johan Witters

1,463
9
21

score 1 · Answer 4 · answered Jul 01 '21 at 09:43

1

You can also call method:toList() on any iterables (in this case List) which you want to create a copy of and not a reference.

answered Jul 01 '21 at 09:43

Atul Singh

21
3

Paul · Answer 5 · 2022-05-11T09:12:34.247

Deep copy of a custom class List

If you have a list of classes, make sure to clone/copy each class. Otherwise, if you change objects in the original list, it will also change in the new one. Here is one way to prevent it:

Add a clone() function to your class

class RandomObject {

RandomObject(this.x, this.y);

//add clone function to your class:
RandomObject.clone(RandomObject randomObject): this(randomObject.x, randomObject.y);

int x;
int y;
}

To copy, map through each element and clone it

final List<RandomObject> original = [RandomObject(1, 2), RandomObject(3, 4)];

final List<RandomObject> copy = original.map((v) => RandomObject.clone(v)).toList();

Source inspired: [deep copy object](https://stackoverflow.com/a/54136662/11003497) — Paul, Sep 30 '21 at 16:18

How to copy list values to another list in flutter

5 Answers5

Deep copy of a custom class List

Linked