Search and filter
- Kotlin course
- Intermediate
- Lesson 62/116
- 6 min
Write search and filtering with collection methods.
Search and filter
A library may hold thousands of books; to find books by a given author use filter. The line fun searchByAuthor(author: String): List<Book> = books.filter { it.author == author } produces a new list of matching books.
To list available books you filter out the checked-out ones: fun availableBooks(): List<Book> = books.filter { it.isbn !in borrowed }. Here the !in operator checks that the key is absent from the map. This lets you filter non-borrowed books in a single line.
availableBooks and !in
To find non-borrowed books use the !in operator. The expression it.isbn !in borrowed checks that the book number is not a key in the borrowed map.
The method definition: fun availableBooks(): List<Book> = books.filter { it.isbn !in borrowed }. This yields the list of books that are on the shelf. searchByAuthor and availableBooks can be combined: first filter by author, then pick the available ones. Both return new lists, so the original books list is unchanged.
Practice Add searchByAuthor and availableBooks
Add fun searchByAuthor(author: String): List<Book> = books.filter { it.author == author } to the Library class. Also add fun availableBooks(): List<Book> = books.filter { it.isbn !in borrowed }. In main add two Tolstoy books and print them with lib.searchByAuthor("Tolstoy").forEach { println(it) }.
Starter code
data class Book(val title: String, val author: String, val isbn: String)
data class Member(val name: String, val id: String)
class Library {
val books = mutableListOf<Book>()
val members = mutableListOf<Member>()
val borrowed = mutableMapOf<String, String>()
fun addBook(book: Book) { books.add(book) }
fun addMember(member: Member) { members.add(member) }
fun checkout(memberId: String, isbn: String) { borrowed[isbn] = memberId }
fun returnBook(isbn: String) { borrowed.remove(isbn) }
// Add searchByAuthor and availableBooks methods
}
fun main() {
val lib = Library()
lib.addBook(Book("War and Peace", "Tolstoy", "978-1"))
lib.addBook(Book("Anna Karenina", "Tolstoy", "978-2"))
// searchByAuthor call: author Tolstoy
}Sample solution
data class Book(val title: String, val author: String, val isbn: String)
data class Member(val name: String, val id: String)
class Library {
val books = mutableListOf<Book>()
val members = mutableListOf<Member>()
val borrowed = mutableMapOf<String, String>()
fun addBook(book: Book) { books.add(book) }
fun addMember(member: Member) { members.add(member) }
fun checkout(memberId: String, isbn: String) { borrowed[isbn] = memberId }
fun returnBook(isbn: String) { borrowed.remove(isbn) }
fun searchByAuthor(author: String): List<Book> = books.filter { it.author == author }
fun availableBooks(): List<Book> = books.filter { it.isbn !in borrowed }
}
fun main() {
val lib = Library()
lib.addBook(Book("War and Peace", "Tolstoy", "978-1"))
lib.addBook(Book("Anna Karenina", "Tolstoy", "978-2"))
lib.searchByAuthor("Tolstoy").forEach { println(it) }
}Expected output
Book(title=War and Peace, author=Tolstoy, isbn=978-1) Book(title=Anna Karenina, author=Tolstoy, isbn=978-2)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does books.filter { it.author == author } return?
- The list of all books
- The list of books whose author matches Correct answer
- A single book
- An empty list
Explanation: filter builds a new list of elements that satisfy the condition.
2 Complete the filter call
books.____ { it.author == author }books.filter { it.author == author }Explanation: filter selects elements matching the condition.
3 Match the filter expressions
| Term | Meaning |
|---|---|
filter { it.author == a } |
Search by author |
filter { it.isbn !in b } |
Available books |
forEach { println(it) } |
|
mutableListOf<Book>() |
Create list |