How to use operators like *= and += with Dart maps and null-safety

Question

If I have a map like:

Map<String, int> myMap = Map();

Add a key and value:

myMap.putIfAbsent("yolo", 1000);

Then add a number using an operator like +=

myMap["yolo"] += 100;

With null safety it throws an error saying

The method '+' can't be unconditionally invoked because the receiver can be 'null'.

In a map, what is the best way to set the value based on the key?

This is https://github.com/dart-lang/language/issues/1113. You currently will need to do `myMap['yolo'] = myMap['yolo']! + 100;`. — jamesdlin, Mar 13 '21 at 00:19

score 2 · Answer 1 · answered Mar 13 '21 at 00:18

2

This error is expected. If your map doesn't contain an entry with a key "yolo", then calling myMap["yolo"] += 100; would cause a runtime null error.

If 0 is a sensible "default value" for this context, you could write something like:

myMap["yolo"] = (myMap["yolo"] ?? 0) + 100;

answered Mar 13 '21 at 00:18

cameron1024

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Note that it doesn't matter whether `map` contains the entry or not; `Map.operator[]` always returns a nullable type, which prevents using `+=` with it. – jamesdlin Mar 13 '21 at 00:22
1

What you need is the `??+=` operator. :) – Randal Schwartz Mar 13 '21 at 03:03

How to use operators like *= and += with Dart maps and null-safety

1 Answers1