Kotlin
Search and filter
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.
Exercise 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) }.
Show example 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) }
}
Test yourself
-
What does books.filter { it.author == author } return?
- The list of all books
- The list of books whose author matches
- A single book
- An empty list
Show answer
The list of books whose author matches — filter builds a new list of elements that satisfy the condition.
-
Complete the filter call
books.____ { it.author == author }
Show answer
books.filter { it.author == author }
-
Match the filter expressions
- filter { it.author == a }
- Search by author
- filter { it.isbn !in b }
- Available books
- forEach { println(it) }
- Print
- mutableListOf<Book>()
- Create list
Next lesson: Putting it together
