Lambda syntax and declaration in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin lambda expression?

Consider the following Kotlin code snippet. What will be printed when it runs?

Kotlin

val multiplyByTwo = { x: Int -> x * 2 }
println(multiplyByTwo(5))

A 10

B 25

C 5

D Compilation error

💡 Hint

Remember the lambda takes an integer and multiplies it by 2.

The lambda takes an integer x and returns x multiplied by 2. So multiplyByTwo(5) returns 10.

❓ Predict Output

intermediate

What does this Kotlin lambda return?

What is the output of this Kotlin code?

Kotlin

val greet: (String) -> String = { name -> "Hello, $name!" }
println(greet("Anna"))

A Hello, name!

B Hello, Anna!

C greet

D Compilation error

💡 Hint

The lambda uses string interpolation with the parameter name.

The lambda takes a string and returns "Hello, " plus the name plus "!". So it prints "Hello, Anna!".

❓ Predict Output

advanced

What is the output of this Kotlin lambda with implicit it?

What will this Kotlin code print?

Kotlin

val square: (Int) -> Int = { it * it }
println(square(4))

A 16

B 4

C Itself (function reference)

D Compilation error

💡 Hint

When a lambda has one parameter, you can use 'it' to refer to it.

The lambda squares the input number. 4 squared is 16, so it prints 16.

❓ Predict Output

advanced

What is the output of this Kotlin lambda with multiple parameters?

What will this Kotlin code print?

Kotlin

val sum: (Int, Int) -> Int = { a, b -> a + b }
println(sum(3, 7))

A 37

B 3

C Compilation error

D 10

💡 Hint

The lambda adds two integers a and b.

The lambda adds 3 and 7, resulting in 10, which is printed.

🧠 Conceptual

expert

Which Kotlin lambda declaration correctly defines a lambda that returns the length of a string?

Choose the correct Kotlin lambda declaration that takes a String and returns its length as an Int.

A val lengthLambda: (String) -> String = { s -> s.length }

B val lengthLambda = { s: String -> s.length }

C val lengthLambda: (String) -> Int = { s -> s.length }

D val lengthLambda: (Int) -> Int = { s -> s.length }

💡 Hint

Check the lambda parameter type and return type carefully.

Option C correctly declares a lambda taking a String and returning an Int (the length). Option C misses explicit type but is valid Kotlin, but since the question asks for declaration with type, A is best. Option C returns String but length is Int, so wrong return type. Option C takes Int but tries to get length, which is invalid.

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