Array methods

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, 4

map 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.

Exercise 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).

Show example solution

fun main() {
    val nums = intArrayOf(1, 2, 3, 4)
    val doubled = nums.map { it * 2 }
    println(doubled)
}

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.

Test yourself

What does arr.map { it * 2 } do?
- Reverses the array
- Selects even numbers
- Prints each element
- Multiplies each element by 2
Show answer

Multiplies each element by 2 — map transforms each element; here it multiplies by 2.

Complete the map call

val doubled = nums.____ { it * 2 }

Show answer

val doubled = nums.map { it * 2 }

Match the methods

map { it * 2 }

Transforms
filter { it > 0 }

Selects
forEach { println(it) }

Acts on each
it

Lambda parameter

Next lesson: Summary - Arrays

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