Returning functions from functions in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return a function that adds 5 to its input.

Kotlin

fun addFive(): (Int) -> Int {
    return { x -> x + 5 }
}

Drag options to blanks, or click blank then click option'

A{ x -> x + 5 }

B{ x -> x - 5 }

C{ x -> x * 5 }

D{ x -> x / 5 }

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a lambda that subtracts or multiplies instead of adding.

Not returning a lambda at all.

The function must return a lambda that adds 5 to the input number.

2fill in blank

medium

Complete the code to return a function that multiplies its input by 3.

Kotlin

fun multiplyByThree(): (Int) -> Int {
    return { x -> x * 3 }
}

Drag options to blanks, or click blank then click option'

A{ x -> x / 3 }

B{ x -> 3 - x }

C{ x -> x + 3 }

D{ x -> x * 3 }

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or subtraction instead of multiplication.

Dividing instead of multiplying.

The returned lambda multiplies the input by 3.

3fill in blank

hard

Fix the error in the code to correctly return a function that subtracts 2 from its input.

Kotlin

fun subtractTwo(): (Int) -> Int {
    return { x -> x - 2 }
}

Drag options to blanks, or click blank then click option'

A{ x -> x + 2 }

B{ x -> x - 2 }

C{ x -> 2 - x }

D{ x -> x * 2 }

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 2 instead of subtracting.

Subtracting input from 2 instead of the other way around.

The lambda must subtract 2 from the input number.

4fill in blank

hard

Fill both blanks to create a function that returns a function which adds a given number to its input.

Kotlin

fun adder(n: Int): (Int) -> Int {
    return { x -> x [1] n[2] }
}

Drag options to blanks, or click blank then click option'

A*

B-

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

Adding extra operators after n.

The function returns a lambda that adds n to x using the + operator. The second blank is a space to complete the expression.

5fill in blank

hard

Fill all three blanks to create a function that returns a function which multiplies its input by a given number and then adds another number.

Kotlin

fun multiplyAndAdd(multiplier: Int, addend: Int): (Int) -> Int {
    return { x -> x [1] multiplier [2] addend[3] }
}

Drag options to blanks, or click blank then click option'

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or division instead of addition.

Mixing the order of operations.

The lambda multiplies x by multiplier using * and then adds addend using +. The last blank is a space to complete the expression.

Practice

1. What does it mean when a Kotlin function returns another function?

easy

A. The function can only return primitive data types like Int or String.

B. The function cannot have parameters if it returns another function.

C. The outer function produces a function as its result that can be called later.

D. The returned function must be called immediately inside the outer function.

Solution

1. Step 1: Understand function return types

   In Kotlin, a function can return any type, including another function type.

2. Step 2: Interpret returning a function

   Returning a function means the outer function gives back a function that can be stored or called later.

3. Final Answer:

   The outer function produces a function as its result that can be called later. -> Option C

4. Quick Check:

   Returning a function means output is a callable function [OK]

Hint: Returning function means output is a callable function [OK]

Common Mistakes:

• Thinking only primitive types can be returned
• Assuming returned function must be called immediately
• Believing returned functions cannot have parameters

2. Which of the following is the correct syntax to declare a Kotlin function that returns another function taking an Int and returning a String?

easy

A. fun outer(): (Int) -> String = { i -> i.toString }

B. fun outer(): (Int) -> String { return { i -> i.toString() } }

C. fun outer(): (Int) String { return { i -> i.toString() } }

D. fun outer(): Int -> String = return { i -> i.toString() } }

Solution

1. Step 1: Check function return type syntax

   In Kotlin, a function returning another function uses syntax like: fun name(): (ParamType) -> ReturnType

2. Step 2: Verify the return statement

   The function returns a lambda matching (Int) -> String, e.g., { i -> i.toString() }

3. Final Answer:

   fun outer(): (Int) -> String { return { i -> i.toString() } } -> Option B

4. Quick Check:

   Correct function type syntax = fun outer(): (Int) -> String { return { i -> i.toString() } } [OK]

Hint: Return type for function is (ParamType) -> ReturnType [OK]

Common Mistakes:

• Using arrow -> incorrectly in return type
• Mixing return type syntax with function body
• Omitting parentheses around function type

3. What is the output of this Kotlin code?

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

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

medium

A. 35

B. 8

C. Compilation error

D. 15

Solution

1. Step 1: Understand makeMultiplier function

   makeMultiplier returns a function that multiplies its input by x (captured from outer function).

2. Step 2: Trace main function calls

   multiplyBy3 is a function multiplying by 3; multiplyBy3(5) = 3 * 5 = 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   3 * 5 = 15 [OK]

Hint: Returned function uses captured outer variable [OK]

Common Mistakes:

• Confusing parameter names
• Forgetting the closure captures x
• Expecting immediate multiplication without calling returned function

4. Identify the error in this Kotlin code that tries to return a function:

fun outer(): (Int) -> Int {
    val inner = { x: Int -> x * 2 }
    return inner()
}

medium

A. Calling inner() instead of returning the function itself

B. Missing return type declaration for outer function

C. Incorrect lambda parameter syntax

D. Cannot return a lambda from a function

Solution

1. Step 1: Analyze return statement

   inner is a lambda function; inner() calls it and returns Int, not a function.

2. Step 2: Correct return value

   To return the function itself, return inner without parentheses.

3. Final Answer:

   Calling inner() instead of returning the function itself -> Option A

4. Quick Check:

   Return function, not function call [OK]

Hint: Return function name without () to return function itself [OK]

Common Mistakes:

• Returning function call instead of function
• Omitting function return type
• Misunderstanding lambda syntax

5. Given this Kotlin code, what will be the output?

fun createCounter(start: Int): () -> Int {
    var count = start
    return {
        count += 1
        count
    }
}

fun main() {
    val counter = createCounter(10)
    println(counter())
    println(counter())
}

hard

A. 11
12

B. 10
11

C. 12
13

D. Compilation error

Solution

1. Step 1: Understand closure capturing variable

   The returned lambda captures 'count' variable and increments it each call.

2. Step 2: Trace calls to counter()

   First call: count = 10 + 1 = 11; prints 11. Second call: count = 11 + 1 = 12; prints 12.

3. Final Answer:

   11 12 -> Option A

4. Quick Check:

   Closure increments count on each call [OK]

Hint: Closures keep state between calls [OK]

Common Mistakes:

• Assuming count resets each call
• Confusing initial value with first output
• Expecting compilation error due to var usage