Testing scope functions and lambdas in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the length of the string using let.

Kotlin

val text = "hello"
text.[1] {
    println(it.length)
}

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

Aapply

Brun

Clet

Dalso

Attempts:

3 left

💡 Hint

Common Mistakes

Using apply which uses this instead of it.

Using also which returns the original object but is meant for side effects.

The let function passes the object as it to the lambda, allowing us to access it.length.

2fill in blank

medium

Complete the code to initialize and print the length of the string using run.

Kotlin

val length = "Kotlin".[1] {
    length
}
println(length)

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

Aalso

Blet

Cwith

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using also which returns the original object, not the lambda result.

Using let which uses it instead of this.

run executes the lambda with this as the receiver and returns the lambda result, here the string length.

3fill in blank

hard

Fix the error in the lambda to correctly filter even numbers.

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.filter { it [1] 2 == 0 }
println(evens)

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

A+

B%

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using / which performs division and returns a quotient.

Using + or * which are arithmetic but not for checking evenness.

The modulo operator % returns the remainder. it % 2 == 0 checks if a number is even.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, filtering words longer than 3 characters.

Kotlin

val words = listOf("cat", "house", "dog", "elephant")
val lengths = words.associateWith { [1] }
val filtered = lengths.filter { it.value [2] 3 }
println(filtered)

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

Ait.length

B>

C<=

Dit.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using it.size which is not a string property.

Using wrong comparison operators like <= instead of >.

associateWith maps each word to its length using it.length. Then filter keeps entries where length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, filtering lengths greater than 4.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val result = words.associate { [1] to [2] }
    .filter { it.value [3] 4 }
println(result)

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

Ait.uppercase()

Bit.length

C>

Dit.toString()

Attempts:

3 left

💡 Hint

Common Mistakes

Using it.toString() which returns the same string.

Using < or <= instead of > for filtering.

associate creates pairs of uppercase words and their lengths. Then filter keeps pairs with length greater than 4.

Practice

1. Which Kotlin scope function returns the object it was called on after executing the given lambda?

easy

A. run

B. also

C. let

D. apply

Solution

1. Step 1: Understand scope function behavior

   The also function executes the lambda with the object as it and returns the original object.

2. Step 2: Compare with other functions

   let returns the lambda result, run returns lambda result, apply returns the object but uses this inside lambda.

3. Final Answer:

   also -> Option B

4. Quick Check:

   Returns original object after lambda = also [OK]

Hint: Remember: also returns object, let returns lambda result [OK]

Common Mistakes:

• Confusing 'also' with 'let' return values
• Mixing 'apply' and 'also' usage
• Assuming 'run' returns the object

2. Which of the following is the correct syntax to use the let scope function on a nullable variable name?

easy

A. name?.let { println(it) }

B. name.let { println(it) }

C. let(name) { println(it) }

D. name?.let println(it)

Solution

1. Step 1: Check nullable safe call usage

   To safely call let on a nullable variable, use the safe call operator ?..

2. Step 2: Validate lambda syntax

   The lambda must be enclosed in braces {} and use it as the implicit parameter.

3. Final Answer:

   name?.let { println(it) } -> Option A

4. Quick Check:

   Safe call with lambda braces = name?.let { println(it) } [OK]

Hint: Use ?.let { } for nullable variables to avoid errors [OK]

Common Mistakes:

• Omitting safe call operator on nullable
• Incorrect lambda syntax without braces
• Trying to call let as a standalone function

3. What is the output of this Kotlin code?

val result = "hello".run {
    println(this.uppercase())
    this.length
}
println(result)

medium

A. 5
HELLO

B. hello
5

C. HELLO
hello

D. HELLO
5

Solution

1. Step 1: Understand run behavior

   The run function executes the lambda with this as the receiver and returns the lambda's last expression.

2. Step 2: Trace the code execution

   Inside lambda, println(this.uppercase()) prints "HELLO". Then this.length returns 5, assigned to result. Finally, println(result) prints 5.

3. Final Answer:

   HELLO 5 -> Option D

4. Quick Check:

   run prints uppercase then returns length = HELLO and 5 [OK]

Hint: run returns last lambda value; println outputs immediately [OK]

Common Mistakes:

• Assuming run returns the printed string
• Confusing this with it inside lambda
• Mixing order of printed lines

4. Identify the error in this Kotlin code snippet:

val numbers = listOf(1, 2, 3)
numbers.also {
    it.add(4)
}
println(numbers)

medium

A. Missing lambda parameter name

B. also does not return the list

C. Cannot call add on immutable list

D. Syntax error in lambda

Solution

1. Step 1: Check list mutability

   listOf creates an immutable list; calling add is not allowed.

2. Step 2: Understand scope function effect

   also runs the lambda but does not change immutability; trying to add causes a runtime error.

3. Final Answer:

   Cannot call add on immutable list -> Option C

4. Quick Check:

   Immutable list blocks add() method = Cannot call add on immutable list [OK]

Hint: listOf creates immutable list; use mutableListOf to add [OK]

Common Mistakes:

• Assuming also changes list mutability
• Ignoring immutable list restrictions
• Thinking lambda syntax is wrong

5. Given a list of nullable integers val nums = listOf(1, null, 3, null, 5), which code snippet correctly uses scope functions and lambdas to create a list of doubled non-null values?

hard

A. nums.filterNotNull().map { it * 2 }

B. nums.map { it * 2 }.filterNotNull()

C. nums.also { it.map { it * 2 } }

D. nums.run { map { it!! * 2 } }

Solution

1. Step 1: Remove nulls before mapping

   Use filterNotNull() to exclude null values safely.

2. Step 2: Map remaining values to double

   Apply map { it * 2 } on non-null list to double each integer.

3. Final Answer:

   nums.filterNotNull().map { it * 2 } -> Option A

4. Quick Check:

   Filter nulls then map doubling = nums.filterNotNull().map { it * 2 } [OK]

Hint: Filter nulls first, then map to avoid null errors [OK]

Common Mistakes:

• Mapping before filtering nulls causes null results
• Using unsafe !! operator risking exceptions
• Misusing also or run without returning correct list