Passing lambdas to functions in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a lambda passed to a function

What is the output of this Kotlin code when the lambda is passed to the function?

Kotlin

fun operateOnNumber(x: Int, operation: (Int) -> Int): Int {
    return operation(x)
}

fun main() {
    val result = operateOnNumber(5) { it * 3 }
    println(result)
}

A 8

B 15

C Compilation error

D 5

💡 Hint

Remember that the lambda multiplies the input by 3.

The function operateOnNumber takes an integer and a lambda that multiplies the integer by 3. Passing 5 results in 5 * 3 = 15.

❓ Predict Output

intermediate

Lambda with multiple parameters

What will be printed by this Kotlin program?

Kotlin

fun combineStrings(a: String, b: String, combiner: (String, String) -> String): String {
    return combiner(a, b)
}

fun main() {
    val result = combineStrings("Hello", "World") { x, y -> "$x $y!" }
    println(result)
}

A Hello World!

B HelloWorld!

C Hello World

D Compilation error

💡 Hint

Look at how the lambda formats the strings with a space and exclamation mark.

The lambda combines the two strings with a space and adds an exclamation mark, resulting in "Hello World!".

🔧 Debug

advanced

Identify the error when passing a lambda

This Kotlin code does not compile. What is the cause of the error?

Kotlin

fun applyOperation(x: Int, operation: (Int) -> Int): Int {
    return operation(x)
}

fun main() {
    val result = applyOperation(10) { it + 5 }
    println(result)
}

A Type mismatch: lambda parameter type should be (Int) -> Int

B Syntax error: missing parentheses around lambda parameter type

C Runtime exception: null pointer

D No error, code compiles and runs

💡 Hint

Check the function parameter type declaration syntax for lambdas.

The parameter type for the lambda must be declared as (Int) -> Int, not Int -> Int. The missing parentheses cause a type mismatch error.

❓ Predict Output

advanced

Lambda capturing external variable

What is the output of this Kotlin code?

Kotlin

fun main() {
    var counter = 0
    val increment: () -> Int = { counter += 1; counter }
    println(increment())
    println(increment())
    println(counter)
}

A Compilation error

B

0
1
2

C

1
1
1

D

1
2
2

💡 Hint

The lambda modifies the external variable counter each time it is called.

Each call to increment increases counter by 1 and returns the new value. So first call returns 1, second returns 2, and counter is 2 at the end.

🧠 Conceptual

expert

Effect of inline lambdas on performance

Which statement about inline lambdas in Kotlin is true?

A Inline lambdas cannot capture variables from the outer scope.

B Inlining lambdas always increases the size of the compiled code without any performance benefit.

C Inlining lambdas reduces runtime overhead by avoiding object allocation and call overhead.

D Inlining lambdas disables the use of return statements inside the lambda.

💡 Hint

Think about how inlining affects function calls and memory allocation.

Inlining lambdas replaces the lambda call with the lambda body, avoiding creating function objects and call overhead, improving performance.