Array methods
- Kotlin course
- Beginner
- Lesson 57/116
- 5 min
Learn array processing with map, filter and forEach.
map, filter, forEach
map transforms each element, filter selects those matching a condition, forEach runs an action for each element. Lambdas use { }; it represents the element.
val nums = intArrayOf(1, 2, 3, 4)
val doubled = nums.map { it * 2 } // [2, 4, 6, 8]
val even = nums.filter { it % 2 == 0 } // [2, 4]
nums.forEach { println(it) } // 1, 2, 3, 4map returns a new list (each element transformed). filter makes a new list of elements where the condition is true. forEach runs an action, returns nothing.
These methods use lambdas (anonymous functions); it is the default parameter name.
Practice Double each element
Write a fun main(). Create an array with val nums = intArrayOf(1, 2, 3, 4). Double each element with val doubled = nums.map { it * 2 }. Print the result with println(doubled).
Starter code
fun main() {
// Double each element with map
}Sample solution
fun main() {
val nums = intArrayOf(1, 2, 3, 4)
val doubled = nums.map { it * 2 }
println(doubled)
}Expected output
[2, 4, 6, 8]
Method chaining
These methods can be chained: nums.filter { ... }.map { ... } first filters then transforms.
val nums = intArrayOf(1, 2, 3, 4, 5, 6)
val result = nums.filter { it % 2 == 0 }.map { it * 10 }
println(result) // [20, 40, 60]First even numbers (2, 4, 6) are filtered, then each is multiplied by 10 (20, 40, 60). The chain runs left to right.
Chaining keeps code short and readable; no intermediate variables needed.
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does arr.map { it * 2 } do?
- Reverses the array
- Selects even numbers
- Prints each element
- Multiplies each element by 2 Correct answer
Explanation: map transforms each element; here it multiplies by 2.
2 Complete the map call
val doubled = nums.____ { it * 2 }val doubled = nums.map { it * 2 }Explanation: nums.map { it * 2 } doubles each element.
3 Match the methods
| Term | Meaning |
|---|---|
map { it * 2 } |
Transforms |
filter { it > 0 } |
Selects |
forEach { println(it) } |
Acts on each |
it |
Lambda parameter |