Search and filter
- Dart course
- Intermediate
- Lesson 56/116
- 6 min
Write search and filtering with collection methods.
The search method
To search books we write a search method. It takes a query string and returns books whose title contains that text. It uses where and contains.
Example: List<Book> search(String query) { return books.where((b) => b.title.contains(query)).toList(); }. Here where filters each book; b.title.contains(query) checks whether the title has the query text.
The result is a List<Book>. toList() turns the lazy Iterable into a concrete list. This method lets a user find books by a keyword.
Search result
The search method returns a List<Book>. The length of the result tells how many books were found. For example if two of three books contain the query then results.length is 2.
Example: var results = lib.search('and');. If 'Crime and Punishment' and 'War and Peace' are added both contain 'and'; the result is 2 books. print(results.length) gives 2.
This method is the most useful feature of the library app for users. It gives results in seconds among thousands of books.
Practice Add the search method
Add a List<Book> search(String query) method to the Library class. The method should return books.where((b) => b.title.contains(query)).toList(). The main already adds three books and calls lib.search('and'); it prints the length of the result.
Starter code
class Book {
String title;
bool borrowed = false;
Book(this.title);
}
class Library {
List<Book> books = [];
void addBook(Book b) {
books.add(b);
}
void checkout(int index) {
books[index].borrowed = true;
}
void returnBook(int index) {
books[index].borrowed = false;
}
// add the search method
}
void main() {
var lib = Library();
lib.addBook(Book('Les Miserables'));
lib.addBook(Book('Crime and Punishment'));
lib.addBook(Book('War and Peace'));
var results = lib.search('and');
print(results.length);
}Sample solution
class Book {
String title;
bool borrowed = false;
Book(this.title);
}
class Library {
List<Book> books = [];
void addBook(Book b) {
books.add(b);
}
void checkout(int index) {
books[index].borrowed = true;
}
void returnBook(int index) {
books[index].borrowed = false;
}
List<Book> search(String query) {
return books.where((b) => b.title.contains(query)).toList();
}
}
void main() {
var lib = Library();
lib.addBook(Book('Les Miserables'));
lib.addBook(Book('Crime and Punishment'));
lib.addBook(Book('War and Peace'));
var results = lib.search('and');
print(results.length);
}Expected output
2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 The where call
List<Book> search(String query) {
return books.____((b) => b.title.contains(query)).toList();
}List<Book> search(String query) {
return books.where((b) => b.title.contains(query)).toList();
}Explanation: where filters by a condition.
2 What does b.title.contains(query) return?
var b = Book('Crime and Punishment');
var r = b.title.contains('Punishment');- A list
- void
- It throws
- bool (true or false) Correct answer
Explanation: contains returns a bool: true if found, false otherwise.
3 Match the search parts
| Term | Meaning |
|---|---|
String query |
Query parameter |
.where(...) |
Filter |
.contains(query) |
Search in title |
.toList() |
Convert to list |