Dart fold with null safety

Question

I have the following code that sums cash for people with the same name using list fold.

void main() { 
 List<Map<String,dynamic>> people = [{'name': 'Jim', 'cash':44.86},{'name': 'Jim', 'cash':40.55},{'name': 'Bob', 'cash':10.99},{'name': 'Bob', 'cash':10.99}];
  Map resultAmount = people.fold<Map<String, num>>({}, (totalMap, element) {
    final String key = element['name'];
      if (totalMap[key] == null) totalMap[key] = 0;
      totalMap[key] += element['cash'].toDouble();
      return totalMap;
    });
  print(resultAmount);
}

prints:

{Jim: 85.41, Bob: 21.98}

How can I get it to work with null-safety?

score 0 · Accepted Answer · answered Jul 11 '21 at 20:42

You can simplify the line

if (totalMap[key] == null) totalMap[key] = 0;

by just using the ??= operator.

Then you'll need to rework the totalMap[key] increment to handle null-safety a bit better as Dart's static analysis isn't quite that smart.

void main() { 
 List<Map<String,dynamic>> people = [{'name': 'Jim', 'cash':44.86},{'name': 'Jim', 'cash':40.55},{'name': 'Bob', 'cash':10.99},{'name': 'Bob', 'cash':10.99}];
  Map<String, num> resultAmount = people.fold<Map<String, num>>({}, (totalMap, element) {
    final String key = element['name'];
    totalMap[key] ??= 0;
    totalMap[key] = element['cash'].toDouble() + totalMap[key];
    return totalMap;
  });
  print(resultAmount);
}

Or, perhaps a more elegant solution, would be to use a temp variable:

void main() { 
 List<Map<String,dynamic>> people = [{'name': 'Jim', 'cash':44.86},{'name': 'Jim', 'cash':40.55},{'name': 'Bob', 'cash':10.99},{'name': 'Bob', 'cash':10.99}];
  Map<String, num> resultAmount = people.fold<Map<String, num>>({}, (totalMap, element) {
    final String key = element['name'];
    
    double tmp = totalMap[key]?.toDouble() ?? 0.0;
    tmp += element['cash'].toDouble();
    
    totalMap[key] = tmp;
    return totalMap;
  });
  print(resultAmount);
}

Dart fold with null safety

1 Answers1