Initializing Lists to Avoid Potential Errors - list

Question

This function
List<int> _calculateTrips() {
List<int> trips = [];
trips = List.generate(
30,
(index) {
var counter = 0;
var aDay = DateTime.now().subtract(Duration(days: index));
for (var aWalk in walks) {
if ((aDay.month == aWalk.month) && (aDay.day == aWalk.day)) {
counter++;
}
}
trips.add(counter);
},
);
return trips;
}
creates the error The body might complete normally, causing null to be returned, but the return type is a potentially non-nullable type.Try adding either a return or a throw statement at the end. I'm struggling a bit to understand the message because (a) I thought I initialized the list at the beginning of the function and (b) I thought I had a return statement at the end.

Answer 1

The issue is with the function you pass to List.generate(). It expects a E Function(int), where E is the type of the element, for example:
final evenNumbers = List.generate(10, (index) {
return index * 2;
});
Your issue comes from trips.add(counter):
List<int> trips = [];
trips = List.generate(30, (index) {
final trip = calculateTrip(index);
trips.add(trip);
})
The inner function needs to be an int Function(int) (i.e. a function that takes an int, and returns an int), because your list is a List<int>.
However, your inner function never returns anything.
Simply replace trips.add(counter); with return counter; and it should solve this error. You may also want to refactor your function a little:
List<int> _calculateTrips() => List.generate(30, (index {
var counter = 0;
var aDay = DateTime.now().subtract(Duration(days: index));
for (var aWalk in walks) {
if ((aDay.month == aWalk.month) && (aDay.day == aWalk.day)) {
counter++;
}
}
return counter;
});