Does Dart have something like `defaultdict` in Python?
473
Solution 1
You can use the putIfAbsent
method on Map
to create and insert e.g. an empty list to the map if it does not already exists: https://api.dart.dev/stable/2.7.2/dart-core/Map/putIfAbsent.html
void main() {
Map<String, List<int>> m = {};
m.putIfAbsent('001', () => []).add(1);
m.putIfAbsent('001', () => []).add(2);
m.putIfAbsent('002', () => []).add(3);
print(m); // {001: [1, 2], 002: [3]}
}
Alternative solution
If you are going to use this pattern a lot it can be easier to just implement your own Defaultdict class which is not that complicated since we just want it to behave like a Map:
import 'dart:collection';
class Defaultdict<K, V> extends MapBase<K, V> {
final Map<K, V> _map = {};
final V Function() _ifAbsent;
Defaultdict(this._ifAbsent);
@override
V operator [](Object? key) => _map.putIfAbsent(key as K, _ifAbsent);
@override
void operator []=(K key, V value) => _map[key] = value;
@override
void clear() => _map.clear();
@override
Iterable<K> get keys => _map.keys;
@override
V? remove(Object? key) => _map.remove(key);
}
void main() {
// Create a Defaultdict where we return a empty new list for unknown keys
final m = Defaultdict<String, List<int>>(() => []);
m['001'].add(1);
m['001'].add(2);
m['002'].add(3);
print(m); // {001: [1, 2], 002: [3]}
}
Solution 2
I usually use ??=
when possible:
var m = <String, List<DataObject>>{};
(m['001'] ??= []).add(DataObject(something: something));
(m['002'] ??= []).add(DataObject(something: something));
Note that this isn't quite the same as using putIfAbsent
since it will overwrite entries where the key exists but the value is null
. If you never store null
in your Map
, then use whichever you think is cleaner.
Comments
-
dmjy over 1 year
What I want to do is this below. Could you teach me how to do this if possible?
import 'package:my_package/data_object.dart'; Map<String, List<DataObject>> m = Map<int, List<DataObject>>(); // Create an empty map. m['001'].add(DataObject(something: something)); // Can add DataObject without checking if '001' exists. m['002'].add(DataObject(something: something));
-
julemand101 about 4 yearsUpdated with alternative solution with defaultdict class example.
-
julemand101 over 2 yearsUpdated alternative solution to null-safety.