Collections
- Dart course
- Intermediate
- Lesson 114/116
- 6 min
Write data filtering with List, Map and Stream.
Collections
You manage tasks with a List<Task>. It holds many tasks in one structure; a Map can group them by category.
You build a list with var tasks = <Task>[Task('Code write'), Task('Test write', done: true)];. tasks.length gives the count.
tasks.where((t) => t.isDone) filters the completed ones, and .toList() turns them into a list.
Filtering with where
Typical usage: var done = tasks.where((t) => t.isDone).toList(); takes the completed ones into a new list.
Then print(done.length) prints how many are completed. where returns an iterable; toList() turns it into a concrete list.
This is the basis of functional collection handling; you filter without writing a loop.
Practice Tasks with List<Task>
In main define var tasks = <Task>[Task('Code write'), Task('Test write', done: true)];. Filter the completed ones with var done = tasks.where((t) => t.isDone).toList();. Print print(done.length).
Starter code
abstract class Trackable {
bool get isDone;
}
class Task implements Trackable {
final String title;
bool done;
Task(this.title, {this.done = false});
@override
bool get isDone => done;
}
void main() {
// create a tasks list, filter with where, print length
}Sample solution
abstract class Trackable {
bool get isDone;
}
class Task implements Trackable {
final String title;
bool done;
Task(this.title, {this.done = false});
@override
bool get isDone => done;
}
void main() {
var tasks = <Task>[Task('Code write'), Task('Test write', done: true)];
var done = tasks.where((t) => t.isDone).toList();
print(done.length);
}Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Collection operations
| Term | Meaning |
|---|---|
List<Task> |
Task list |
tasks.length |
Count |
where(...) |
Filtering |
toList() |
To a list |
2 How do you filter the completed tasks?
- tasks.where((t) => t.isDone) Correct answer
- tasks.find(...)
- tasks.filter(...)
- tasks.select(...)
Explanation: where filters elements by a condition.
3 List<Task> declaration
var tasks = ____[Task('Kod yaz'), Task('Test yaz', done: true)];var tasks = <Task>[Task('Kod yaz'), Task('Test yaz', done: true)];Explanation: <Task>[...] is a typed list literal.