Map transformation in Kotlin

Introduction

Map transformation helps you change each item in a list or collection into something new. It is like turning raw ingredients into a finished dish.

You want to change a list of numbers into their squares.

You have a list of names and want to make them all uppercase.

You want to convert a list of strings into their lengths.

You need to add a fixed value to every number in a list.

Syntax

Kotlin

val newList = oldList.map { item -> transformation }

The map function creates a new list with transformed items.

The item inside the curly braces is each element from the original list.

Examples

This example squares each number in the list.

Kotlin

val numbers = listOf(1, 2, 3)
val squares = numbers.map { it * it }

This example changes all names to uppercase letters.

Kotlin

val names = listOf("alice", "bob")
val upperNames = names.map { it.uppercase() }

This example finds the length of each word.

Kotlin

val words = listOf("cat", "dog")
val lengths = words.map { it.length }

Sample Program

This program adds 5 to each price in the list and prints both the original and new lists.

Kotlin

fun main() {
    val prices = listOf(10, 20, 30)
    val increasedPrices = prices.map { it + 5 }
    println("Original prices: $prices")
    println("Increased prices: $increasedPrices")
}

Important Notes

The map function does not change the original list; it returns a new list.

You can use it as a shortcut for the single parameter inside the lambda.

Summary

Map transformation changes each item in a list to a new form.

It returns a new list without changing the original.

Use it when you want to apply the same change to every item.

Practice

1. What does the map function do when applied to a list in Kotlin?

easy

A. It removes elements from the list based on a condition.

B. It creates a new list by applying a transformation to each element.

C. It sorts the list in ascending order.

D. It changes the original list by modifying its elements.

Solution

1. Step 1: Understand the purpose of map

   The map function applies a given transformation to each element of the list.

2. Step 2: Check the effect on the list

   It returns a new list with transformed elements without changing the original list.

3. Final Answer:

   It creates a new list by applying a transformation to each element. -> Option B

4. Quick Check:

   map transforms each item and returns new list [OK]

Hint: Remember: map returns a new list, original stays same [OK]

Common Mistakes:

• Thinking map modifies the original list
• Confusing map with filter or sort
• Assuming map removes elements

2. Which of the following is the correct syntax to transform a list of integers by doubling each value using map in Kotlin?

easy

A. val result = list.map { it * 2 }

B. val result = list.map(it * 2)

C. val result = list.map -> it * 2

D. val result = list.map { it + 2 }

Solution

1. Step 1: Identify correct lambda syntax

   The map function requires a lambda inside curly braces: { it * 2 }.

2. Step 2: Check the transformation logic

   Doubling means multiplying by 2, so it * 2 is correct.

3. Final Answer:

   val result = list.map { it * 2 } -> Option A

4. Quick Check:

   Correct lambda syntax with doubling [OK]

Hint: Use curly braces and 'it' for lambda in map [OK]

Common Mistakes:

• Missing curly braces around lambda
• Using parentheses instead of braces
• Wrong operation inside lambda

3. What is the output of the following Kotlin code?

val numbers = listOf(1, 2, 3)
val result = numbers.map { it + 5 }
println(result)

medium

A. [6, 7, 8]

B. [1, 2, 3, 5]

C. [5, 6, 7]

D. [1, 2, 3]

Solution

1. Step 1: Apply the map transformation

   Each element in numbers is increased by 5: 1+5=6, 2+5=7, 3+5=8.

2. Step 2: Form the new list

   The new list is [6, 7, 8], which is printed.

3. Final Answer:

   [6, 7, 8] -> Option A

4. Quick Check:

   Each number +5 = [6, 7, 8] [OK]

Hint: Add 5 to each element inside map to get output [OK]

Common Mistakes:

• Confusing original list with transformed list
• Adding 5 only once instead of to each element
• Misreading the output format

4. Identify the error in this Kotlin code snippet:

val list = listOf(1, 2, 3)
val newList = list.map(it * 3)
println(newList)

medium

A. The multiplication operator is incorrect.

B. Using map on a list is not allowed.

C. The list should be mutable to use map.

D. Missing curly braces around the lambda expression.

Solution

1. Step 1: Check lambda syntax in map

   The map function requires a lambda inside curly braces, but here parentheses are used without braces.

2. Step 2: Confirm correct usage

   Correct syntax is list.map { it * 3 } to apply multiplication.

3. Final Answer:

   Missing curly braces around the lambda expression. -> Option D

4. Quick Check:

   Lambda needs braces in map [OK]

Hint: Always use curly braces for lambda in map [OK]

Common Mistakes:

• Using parentheses instead of braces for lambda
• Thinking map needs mutable list
• Misunderstanding operator usage

5. Given a list of strings val words = listOf("apple", "banana", "cherry"), how can you create a map where each word is a key and its length is the value using Kotlin's map transformation?

hard

A. val wordLengths = words.associateWith { it.length }

B. val wordLengths = words.map { it.length to it }

C. val wordLengths = words.map { it to it.length } .toMap()

D. val wordLengths = words.map { it.length }

Solution

1. Step 1: Understand the goal

   We want a map where keys are words and values are their lengths.

2. Step 2: Use map to create pairs and convert to map

   words.map { it to it.length } creates pairs (word, length). Then toMap() converts the list of pairs to a map.

3. Final Answer:

   val wordLengths = words.map { it to it.length } .toMap() -> Option C

4. Quick Check:

   Map pairs then convert to map [OK]

Hint: Use map to pairs and then toMap() for map creation [OK]

Common Mistakes:

• Using associateWith is correct but not the asked map transformation
• Mapping only lengths returns list, not map
• Swapping key and value in pairs