Adding new elements to map in loop

Question

Ok i think its newbie question I have map with datetime-list, if i do something like this its work perfect:

    Map<DateTime, List> _activities;
   _activities = {
  _currentDay.add(Duration(days: 0)):activityNames.toList(),
  _currentDay.add(Duration(days: 7)):activityNames.toList(),
  _currentDay.add(Duration(days: 14)): activityNames.toList(),
  _currentDay.add(Duration(days: 21)): activityNames.toList(),
  _currentDay.add(Duration(days: 28)): activityNames.toList(),
  }

But i want it like in 'for loop' => wrong example:

  for( var i = 0 ; i <= 28; i=i+7 ) {
_activities[_currentDay.add(Duration(days:i))]=activityNames.toList();

 }

looks good to me but says NoSuchMethedError: The method '[]=' was called null.

Answer 1

In first code, you are initializing the _activites variable, because of that you don't get error;

_activities = {.....

But in the second one, you don't initialize, so this is what you need:

Map<DateTime, List> _activities = {};
for( var i = 0 ; i <= 28; i=i+7 ) {
_activities[_currentDay.add(Duration(days:i))]=activityNames.toList();

 }

In Dart every object is null by default, so you must initialize it.