Dart group list of items into a different data structure? - list

Question

I have these classes:
class Car {
String carId;
int yearOfManufacture;
//constructor, fromJson, toJson ...
}
class CarsByYears {
int yearOfManufacture;
List<Car> carList;
//constructor, fromJson, toJson ...
}
And I need to generate a data structure like this based on the given car list only:
List<CarsByYears>
So I get a List<Car> list, and I need to create a List<CarsByYears> list from that. So I need to group the cars by years.
For example I have a List<Car> list which contains these:
Car(1, 1990)
Car(2, 2001)
Car(3, 1990)
I need to create a List<CarsByYears>, which contains 2 CarsByYears object:
The first one is like: yearOfManufacture = 1990 and the carList contains the 1st and the 3rd car object, and the other CarsByYears is contains the 2nd Car object in its carList, and the yearOfManufacture = 2001
How can I do that?

Answer 1

You can use the groupBy function provided by the collection package (which creates a map from a List groping it using a common value as key) and then map the Map entries.
class Car {
final String carId;
final int yearOfManufacture;
Car(this.carId, this.yearOfManufacture);
#override
String toString() => 'Car($carId)';
// Use your constructor and methods
}
class CarsByYears {
final int yearOfManufacture;
final List<Car> carList;
CarsByYears(this.yearOfManufacture, this.carList);
#override
String toString() => 'Year: $yearOfManufacture, Cars: $carList';
// Use your constructor and methods
}
void main() {
var cars = [
Car('1', 2000),
Car('2', 2002),
Car('3', 2000),
Car('4', 2002),
Car('1', 2005)
];
var carsByYear = groupBy<Car, int>(cars, (e) => e.yearOfManufacture)
.entries
.map((e) => CarsByYears(e.key, e.value))
.toList();
print(carsByYear); //Output: [Year: 2000, Cars: [Car(1), Car(3)], Year: 2002, Cars: [Car(2), Car(4)], Year: 2005, Cars: [Car(1)]]
}