Returning functions from functions in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code returning a function?

Look at this Kotlin code that returns a function. What will it print?

Kotlin

fun makeMultiplier(x: Int): (Int) -> Int {
    return { y -> x * y }
}

fun main() {
    val multiplyBy3 = makeMultiplier(3)
    println(multiplyBy3(5))
}

A 15

B 8

C 35

D Compilation error

💡 Hint

Remember the returned function multiplies its input by the captured value.

The function makeMultiplier returns a function that multiplies its input by x. Here, x is 3, so calling multiplyBy3(5) returns 3 * 5 = 15.

🧠 Conceptual

intermediate

Which Kotlin function type correctly represents a function returning a function?

Which of these Kotlin function types correctly describes a function that returns another function taking an Int and returning a String?

A (Int) -> Int -> String

B (Int) -> (String) -> Int

C (Int) -> (Int, String) -> String

D (Int) -> (Int) -> String

💡 Hint

Remember to use parentheses to group function types properly.

In Kotlin, a function returning a function is written as (InputType) -> (ReturnedFunctionType). Here, the returned function takes an Int and returns a String, so the full type is (Int) -> (Int) -> String.

🔧 Debug

advanced

What error does this Kotlin code produce?

Consider this Kotlin code that tries to return a function. What error will it cause?

Kotlin

fun getAdder(x: Int): (Int) -> Int {
    return { y -> x + y }
}

fun main() {
    val add5 = getAdder(5)
    println(add5(10))
}

A Syntax error: unexpected token '->' in return type

B Runtime error: NullPointerException

C No error, prints 15

D Compilation error: Type mismatch in return statement

💡 Hint

Check the syntax for function types in Kotlin return types.

The return type Int -> Int is invalid syntax in Kotlin. The correct syntax uses parentheses: (Int) -> Int. The arrow must be inside parentheses to denote a function type.

🚀 Application

advanced

How many times is the inner function called in this Kotlin code?

Analyze this Kotlin code that returns a function and calls it multiple times. How many times is the inner function executed?

Kotlin

fun counter(): () -> Int {
    var count = 0
    return {
        count += 1
        count
    }
}

fun main() {
    val c = counter()
    println(c())
    println(c())
    println(c())
}

A 1

B 3

C 0

D Compilation error

💡 Hint

Each call to c() runs the inner function once.

The function counter returns a function that increments and returns count. Each call to c() runs this inner function, so it is called 3 times.

❓ Predict Output

expert

What is the output of this Kotlin code with nested returned functions?

Examine this Kotlin code with nested functions returned from functions. What will it print?

Kotlin

fun outer(x: Int): (Int) -> (Int) -> Int {
    return { y -> { z -> x + y + z } }
}

fun main() {
    val f = outer(1)
    val g = f(2)
    println(g(3))
}

A 5

B 9

C 6

D Compilation error

💡 Hint

Trace the values captured by each returned function.

The outer function returns a function that takes y and returns another function taking z. The final sum is x + y + z. Here, x=1, y=2, z=3, so the output is 6.