Kotlin list map

In this guide, you will learn how to use the map function in Kotlin with examples.

What is List Mapping in Kotlin? 

Mapping is the process of transforming each element in a list based on a given function. The result is a new list where each item is the outcome of applying this function to the original item. 

Basic Syntax:

fun <T, R> List<T>.map(transform: (T) -> R): List<R>

Examples with Output

Basic Mapping

Converting a list of numbers to their string representations:
val numbers = listOf(1, 2, 3)
val asStrings = numbers.map { it.toString() }
println(asStrings)  // Output: [1, 2, 3]

Mapping with Index 

Sometimes, you might need the index of an element during the transformation:

val words = listOf("apple", "banana", "cherry")
val indexedWords = words.mapIndexed { index, word -> "$index: $word" }
println(indexedWords)  // Output: [0: apple, 1: banana, 2: cherry]

Mapping Custom Objects 

Consider mapping a list of Person objects to retrieve their names:

data class Person(val name: String, val age: Int)
val people = listOf(Person("Alice", 30), Person("Bob", 25))
val names = people.map { it.name }
println(names)  // Output: [Alice, Bob]

Nested Mapping 

When dealing with nested lists or objects, you can use nested mappings:

data class Classroom(val name: String, val students: List<Student>)
data class Student(val name: String, val grade: Int)

val classrooms = listOf(
    Classroom("A1", listOf(Student("John", 90), Student("Doe", 85))),
    Classroom("B2", listOf(Student("Jane", 92), Student("Smith", 88)))

val averageGrades = classrooms.map { classroom ->
    val average = classroom.students.map { it.grade }.average()
    "$${classroom.name}: $average"

println(averageGrades)  // Output: [A1: 87.5, B2: 90.0]

Combining map with Other List Operations 

Chain map with other transformations for more complex manipulations:

val numbers = listOf(1, 2, 3, 4, 5)
val squaredEvens = numbers.filter { it % 2 == 0 }.map { it * it }
println(squaredEvens)  // Output: [4, 16]

Why and When to Use map

Data Transformation: Whenever you need to transform every element in a collection based on a certain rule or function, the map function is your go-to utility. 

Data Extraction: For extracting specific properties from a list of objects. 

Composing Operations: map can be combined with other list functions for more intricate manipulations.


Remember that the map function, like other transformation functions in Kotlin's standard library, returns a new list and doesn't modify the original list. 

For large data sets or intensive operations, consider utilizing Kotlin sequences (asSequence()) to achieve lazy evaluations, enhancing efficiency.


Kotlin's map function provides a succinct and powerful mechanism to transform collections. With the provided examples as a foundation, you're well-equipped to wield map function in your Kotlin projects, enhancing both your code's clarity and efficiency. Happy mapping!

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