Why lambdas enable functional style in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a lambda that adds two numbers.

Kotlin

val sum = { a: Int, b: Int -> a [1] b }

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.

Forgetting the operator inside the lambda.

The lambda uses the plus operator + to add two numbers.

2fill in blank

medium

Complete the code to filter a list using a lambda that keeps even numbers.

Kotlin

val evens = numbers.filter { [1] % 2 == 0 }

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

Ait

Bx

Cnum

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name without declaring it.

Not using the implicit it parameter.

The implicit lambda parameter it represents each element in the list.

3fill in blank

hard

Fix the error in the lambda that multiplies each number by 2.

Kotlin

val doubled = numbers.map { [1] * 2 }

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

Anumber

Bit

Cnum

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names not declared in the lambda.

Forgetting to use it for single-parameter lambdas.

The lambda uses the implicit parameter it to refer to each element.

4fill in blank

hard

Fill both blanks to create a lambda that filters words longer than 3 letters.

Kotlin

val longWords = words.filter { word -> word.[1] > [2] }

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

Alength

Bsize

C3

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using size which is not a string property.

Comparing to 4 instead of 3.

The property length gives the number of letters in a word, and we compare it to 3.

5fill in blank

hard

Fill all three blanks to create a map of word lengths for words longer than 4 letters.

Kotlin

val wordLengths = words.filter { [1] -> [1].[2] > [3] }.associateWith { it.length }

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

Aw

Blength

C4

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size which is not a string property.

Using inconsistent variable names.

Use w as the lambda parameter, length to get string length, and compare to 4.

Practice

1. Why do lambdas enable a functional programming style in Kotlin?

easy

A. Because they automatically optimize code for speed

B. Because they replace all loops with recursion

C. Because they require explicit variable declarations

D. Because they allow passing functions as arguments to other functions

Solution

1. Step 1: Understand what lambdas do

   Lambdas are small functions that can be passed around like data.

2. Step 2: Connect lambdas to functional style

   Functional programming relies on passing behavior (functions) as arguments, which lambdas enable.

3. Final Answer:

   Because they allow passing functions as arguments to other functions -> Option D

4. Quick Check:

   Lambdas enable passing behavior = C [OK]

Hint: Lambdas let you pass functions like data [OK]

Common Mistakes:

• Thinking lambdas only replace loops
• Believing lambdas optimize speed automatically
• Confusing lambdas with variable declarations

2. Which of the following is the correct syntax for a lambda expression in Kotlin?

easy

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

B. val sum = (a: Int, b: Int) => a + b

C. val sum = fun(a: Int, b: Int) { return a + b }

D. val sum = lambda(a: Int, b: Int) -> a + b

Solution

1. Step 1: Recall Kotlin lambda syntax

   Kotlin lambdas use curly braces with parameters before the arrow, like { a, b -> ... }.

2. Step 2: Compare options

   val sum = { a: Int, b: Int -> a + b } matches Kotlin syntax. Options B, C, and D use syntax from other languages or incorrect forms.

3. Final Answer:

   val sum = { a: Int, b: Int -> a + b } -> Option A

4. Quick Check:

   Kotlin lambdas use { params -> body } = A [OK]

Hint: Kotlin lambdas use curly braces and arrow -> [OK]

Common Mistakes:

• Using => instead of -> in Kotlin
• Writing full function syntax inside lambda
• Using 'lambda' keyword which Kotlin doesn't have

3. What is the output of this Kotlin code?

val numbers = listOf(1, 2, 3, 4)
val doubled = numbers.map { it * 2 }
println(doubled)

medium

A. Compilation error

B. [1, 2, 3, 4]

C. [2, 4, 6, 8]

D. [3, 6, 9, 12]

Solution

1. Step 1: Understand the map function with lambda

   map applies the lambda to each element, here multiplying each by 2.

2. Step 2: Calculate the transformed list

   1*2=2, 2*2=4, 3*2=6, 4*2=8, so the list becomes [2, 4, 6, 8].

3. Final Answer:

   [2, 4, 6, 8] -> Option C

4. Quick Check:

   map doubles each element = [2, 4, 6, 8] [OK]

Hint: map applies lambda to each list item [OK]

Common Mistakes:

• Thinking map modifies original list
• Confusing map with filter
• Expecting syntax error due to lambda

4. Identify the error in this Kotlin code using lambdas:

val list = listOf(1, 2, 3)
val filtered = list.filter { it > 2 }
val result = filtered.map { it * 2 }
println(result)

medium

A. Syntax error in lambda expression

B. No error; code runs and prints [6]

C. filter should be replaced with map

D. map cannot be used after filter

Solution

1. Step 1: Analyze filter and map usage

   filter keeps elements greater than 2, so only 3 remains.

2. Step 2: Apply map to filtered list

   map multiplies 3 by 2, resulting in [6].

3. Final Answer:

   No error; code runs and prints [6] -> Option B

4. Quick Check:

   filter then map works fine = A [OK]

Hint: filter then map is valid chain [OK]

Common Mistakes:

• Assuming syntax error in lambdas
• Thinking map can't follow filter
• Confusing filter and map roles

5. Given a list of names, how can lambdas help you create a map of name lengths only for names longer than 3 characters?
Choose the correct Kotlin code snippet.

hard

A. val result = names.filter { it.length > 3 }.associate { it to it.length }

B. val result = names.map { it.length > 3 }.associateWith { it.length }

C. val result = names.associate { if (it.length > 3) it to it.length }

D. val result = names.map { it.length > 3 }.associate { it to it.length }

Solution

1. Step 1: Filter names longer than 3 characters

   Use filter { it.length > 3 } to keep only long names.

2. Step 2: Create a map of name to length

   Use associate { it to it.length } to map each name to its length.

3. Final Answer:

   val result = names.filter { it.length > 3 }.associate { it to it.length } -> Option A

4. Quick Check:

   filter then associate with pairs = B [OK]

Hint: Filter first, then associate with pairs [OK]

Common Mistakes:

• Using associateWith incorrectly
• Trying to map booleans instead of names
• Returning null in associate causing errors