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?
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);
}