Collections
- Kotlin course
- Beginner
- Lesson 114/116
- 6 min
Write data management with List and Map.
List operations: filter
On Kotlin lists filter keeps the elements that match a condition. It is ideal for separating done and pending tasks.
val done = tasks.filter { it.done }
val pending = tasks.filter { !it.done }it stands for each element in the list. Elements whose condition returns true stay in the result. This builds a sublist in one line.
forEach and count
forEach runs a block for each element in the list. count returns the number of elements. Both are handy list tools.
tasks.forEach { println(it) }
val n = tasks.count()forEach acts like a loop but is more concise. count gives the size; it can also take a condition: tasks.count { it.done }. This lets you walk and summarise the list.
Practice Filter done tasks and print them
Add a fun listDone() = tasks.filter { it.done } method to the TaskList class. In main add two tasks, complete the first with list.tasks[0].done = true, then print list.listDone() with println.
Starter code
class Task(val title: String) {
var done: Boolean = false
override fun toString(): String {
return if (done) "[x] $title" else "[ ] $title"
}
}
class TaskList {
val tasks = mutableListOf<Task>()
fun addTask(title: String) {
tasks.add(Task(title))
}
// listDone metodunu buraya yaz
}
fun main() {
val list = TaskList()
// Add two tasks; mark one done; print the done ones
}Sample solution
class Task(val title: String) {
var done: Boolean = false
override fun toString(): String {
return if (done) "[x] $title" else "[ ] $title"
}
}
class TaskList {
val tasks = mutableListOf<Task>()
fun addTask(title: String) {
tasks.add(Task(title))
}
fun listDone() = tasks.filter { it.done }
}
fun main() {
val list = TaskList()
list.addTask("Study Kotlin")
list.addTask("Shopping")
list.tasks[0].done = true
println(list.listDone())
}Expected output
[[x] Kotlin çalış]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does filter do?
- Gives a subset matching a condition Correct answer
- Returns all elements
- Sorts the list
- Returns the first element
Explanation: filter keeps the elements matching a condition.
2 Fill in the blank: filter done tasks
val done = tasks.____ { it.done }val done = tasks.filter { it.done }Explanation: filter { it.done } returns completed tasks.
3 Match operation to use
| Term | Meaning |
|---|---|
filter |
Filter by condition |
forEach |
Do for each |
count |
Count |
find |
Find first |