Dart 2D List - add() adds temp element to List but initialises the previous element to null when the temp element is cleared

Issue

I have this function that works with List:

List<List<String>> experience() async {

  List<List<String>> totalExperience = [];
  List<String> temp = []; //a temporary List

  temp = ["a", "b"];
  totalExperience.add(temp); //adding the temp List to the 2D List
  print(totalExperience);
  //here I intend to remove the 2 elements ("a", "b") from the temp List,
  //so that it can be used again.
  temp.clear(); 

  temp = ["c", "d"]; //initialising the temp List with 2 new elements
  totalExperience.add(temp);
  print(totalExperience);
  temp.clear();

  temp = ["e", "f"];
  totalExperience.add(temp);
  print(totalExperience);

  return totalExperience;
}

This is actual the output in the console:

[[a, b]]
[[], [c, d]]
[[], [], [e, f]]

As clear from the output, the previous 2 elements in the totalExperience List got removed.

However, I expected the output of this code to be something like this:

[[a, b], [c, d], [e, f]]

I can not figure out what could be the possible error in this code.

Also I might have framed the question title wrong. Will update it as soon as I figure out the error. Thanks.

Solution

Every time clear your temp array which reference actually you added to your totalExperience array. That's why whenever you clear temp array and then clear also in reference which add in totalExperience array. You can just copy your temp when adding to the totalExperience array.

List<List<String>> experience()  {
    
      List<List<String>> totalExperience = [];
      List<String> temp = []; //a temporary List
      List<String> tempOrgi;
      temp = ["a", "b"];
     
      totalExperience.add([...temp]); //adding the temp List to the 2D List
      print(totalExperience);
      //here I intend to remove the 2 elements ("a", "b") from the temp List,
      //so that it can be used again.
      temp.clear(); 
    
      temp = ["c", "d"]; //initialising the temp List with 2 new elements
      totalExperience.add([...temp]);
      print(totalExperience);
      temp.clear();
    
      temp = ["e", "f"];
      totalExperience.add([...temp]);
      print(totalExperience);
    
      return totalExperience;
    }

Answered By - Poran