Lambda syntax and declaration in Kotlin - Mini Project: Build & Apply

Lambda syntax and declaration

📖 Scenario: You are working on a simple Kotlin program that processes a list of numbers. You want to use a lambda function to double each number in the list.

🎯 Goal: Build a Kotlin program that uses a lambda expression to double each number in a list and then prints the doubled numbers.

📋 What You'll Learn

Create a list of integers with exact values

Declare a lambda expression that doubles a number

Use the lambda to transform the list

Print the resulting list

💡 Why This Matters

🌍 Real World

Lambdas are used in Kotlin to write short, reusable pieces of code for tasks like transforming lists, handling events, or processing data streams.

💼 Career

Understanding lambda syntax is essential for Kotlin developers working on Android apps or backend services, as it helps write clean and efficient code.

1

Create the initial list of numbers

Create a list of integers called numbers with these exact values: 1, 2, 3, 4, 5

Kotlin

# Create the list called numbers with values 1, 2, 3, 4, 5
// Your code here

Hint

Use val numbers = listOf(1, 2, 3, 4, 5) to create the list.

2

Declare a lambda expression to double a number

Declare a lambda expression called double that takes an integer n and returns n * 2

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
// Declare the lambda called double that doubles a number
// Your code here

Hint

Use val double: (Int) -> Int = { n -> n * 2 } to declare the lambda.

3

Use the lambda to transform the list

Create a new list called doubledNumbers by applying the double lambda to each element in numbers using map

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val double: (Int) -> Int = { n -> n * 2 }
// Create doubledNumbers by mapping numbers with double
// Your code here

Hint

Use val doubledNumbers = numbers.map(double) to apply the lambda.

4

Print the doubled numbers

Print the doubledNumbers list using println

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val double: (Int) -> Int = { n -> n * 2 }
val doubledNumbers = numbers.map(double)
// Print doubledNumbers
// Your code here

Hint

Use println(doubledNumbers) to display the doubled list.

Practice

1. What is a lambda expression in Kotlin?

easy

A. A small unnamed function stored in a variable

B. A class declaration

C. A type of loop

D. A way to declare variables

Solution

1. Step 1: Understand lambda basics

   Lambdas are small functions without a name that can be stored in variables.

2. Step 2: Compare with other options

   Classes, loops, and variable declarations are different concepts from lambdas.

3. Final Answer:

   A small unnamed function stored in a variable -> Option A

4. Quick Check:

   Lambda = unnamed function [OK]

Hint: Lambdas are unnamed functions in curly braces [OK]

Common Mistakes:

• Confusing lambdas with classes
• Thinking lambdas are loops
• Mixing lambdas with variable declarations

2. Which of the following is the correct syntax for a lambda that takes two Ints and returns their sum?

easy

A. { a, b : Int -> a + b }

B. (a, b) -> { a + b }

C. fun(a, b) { return a + b }

D. { a, b -> a + b }

Solution

1. Step 1: Recall lambda syntax

   Lambdas use curly braces with parameters before an arrow, then the body: { params -> body }

2. Step 2: Check each option

   { a, b -> a + b } matches correct syntax. (a, b) -> { a + b } wrongly uses parentheses outside braces. fun(a, b) { return a + b } is a function, not a lambda. { a, b : Int -> a + b } incorrectly places type annotation.

3. Final Answer:

   { a, b -> a + b } -> Option D

4. Quick Check:

   Correct lambda syntax = { params -> body } [OK]

Hint: Lambdas use braces with arrow separating params and body [OK]

Common Mistakes:

• Using parentheses instead of braces
• Trying to write full function syntax
• Incorrect type annotations inside lambda

3. What is the output of this Kotlin code?

val multiply = { x: Int, y: Int -> x * y }
println(multiply(3, 4))

medium

A. 12

B. 7

C. 34

D. Error

Solution

1. Step 1: Understand the lambda function

   The lambda takes two Ints x and y, and returns their product x * y.

2. Step 2: Calculate the output

   Calling multiply(3, 4) returns 3 * 4 = 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply parameters inside lambda to get output [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Syntax errors in lambda call

4. Identify the error in this Kotlin lambda declaration:

val greet = { name String -> "Hello, $name" }

medium

A. Lambda body must be outside braces

B. Arrow is missing after parameters

C. Missing colon between parameter and type

D. Cannot use string interpolation in lambda

Solution

1. Step 1: Check parameter syntax

   Parameters must have a colon between name and type: name: String

2. Step 2: Verify lambda structure

   The arrow -> is present after parameters, and string interpolation is allowed inside lambdas.

3. Final Answer:

   Missing colon between parameter and type -> Option C

4. Quick Check:

   Parameter syntax = name: Type [OK]

Hint: Use colon between param name and type in lambdas [OK]

Common Mistakes:

• Omitting colon in parameter declaration
• Forgetting arrow after parameters
• Thinking string interpolation is disallowed

5. How can you declare a lambda in Kotlin that filters a list of integers to keep only even numbers?

hard

A. val evenFilter = { list: List<Int> -> list.filter(it % 2 == 0) }

B. val evenFilter = { list: List<Int> -> list.filter { it % 2 == 0 } }

C. val evenFilter = { list: List<Int> -> list.filter { x -> x / 2 == 0 } }

D. val evenFilter = { list: List<Int> -> list.filter { x -> x % 2 = 0 } }

Solution

1. Step 1: Understand filter usage

   filter takes a lambda returning true for elements to keep. Correct condition is x % 2 == 0 for even numbers.

2. Step 2: Check each option

   val evenFilter = { list: List<Int> -> list.filter { it % 2 == 0 } } correctly uses filter with lambda and modulus check. val evenFilter = { list: List<Int> -> list.filter(it % 2 == 0) } misses braces for lambda. val evenFilter = { list: List<Int> -> list.filter { x -> x / 2 == 0 } } uses division instead of modulus. val evenFilter = { list: List<Int> -> list.filter { x -> x % 2 = 0 } } uses assignment (=) instead of comparison (==).

3. Final Answer:

   val evenFilter = { list: List<Int> -> list.filter { it % 2 == 0 } } -> Option B

4. Quick Check:

   Filter with { it % 2 == 0 } keeps evens [OK]

Hint: Use filter { it % 2 == 0 } to select even numbers [OK]

Common Mistakes:

• Using assignment (=) instead of comparison (==)
• Using division (/) instead of modulus (%)
• Omitting braces around lambda in filter