It keyword for single parameter in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of lambda using 'it' keyword

What is the output of this Kotlin code snippet using the 'it' keyword in a lambda?

Kotlin

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

A [1, 2, 3, 4]

B [2, 4, 6, 8]

C [3, 6, 9, 12]

D Compilation error

💡 Hint

The 'it' keyword refers to the single parameter of the lambda function.

The 'map' function applies the lambda to each element. 'it' is each element, so multiplying by 2 doubles each number.

❓ Predict Output

intermediate

Using 'it' in filter lambda

What will this Kotlin code print?

Kotlin

val words = listOf("apple", "banana", "cherry", "date")
val filtered = words.filter { it.length > 5 }
println(filtered)

A ["banana", "cherry"]

B ["apple", "date"]

C ["banana", "date"]

D []

💡 Hint

The 'it' keyword is the current element in the list during filtering.

The filter keeps words whose length is greater than 5. 'banana' and 'cherry' have length 6.

🔧 Debug

advanced

Identify the error with 'it' usage

What error does this Kotlin code produce?

Kotlin

val numbers = listOf(1, 2, 3)
numbers.forEach { println(it) }
numbers.forEach { val it = 5; println(it) }

A Compilation error: 'it' cannot be redeclared

B Runtime error: NullPointerException

C Compilation error: Val cannot be reassigned

D No error, prints 1 2 3 5 5 5

💡 Hint

You cannot redeclare 'it' inside the lambda.

The 'it' keyword is implicitly declared as the lambda parameter and cannot be redeclared inside the lambda body.

❓ Predict Output

advanced

Output of nested lambdas with 'it'

What is the output of this Kotlin code using nested lambdas with 'it'?

Kotlin

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

A [2, 4, 6]

B [1, 2, 3]

C [4, 6]

D Compilation error due to ambiguous 'it'

💡 Hint

Each lambda has its own 'it' referring to the current element in that lambda.

The first lambda doubles each number: [2,4,6]. The second lambda filters numbers greater than 3: [4,6].

🧠 Conceptual

expert

Understanding 'it' scope in Kotlin lambdas

Consider this Kotlin code snippet. What is the value of 'result' after execution?

Kotlin

val result = listOf(1, 2, 3).fold(0) { acc, it -> acc + it }

A Compilation error: 'it' cannot be used with two parameters

B 0

C 3

D 6

💡 Hint

In fold, the lambda has two parameters, so 'it' is not used implicitly.

The fold function sums all elements starting from 0. The lambda uses 'acc' and 'it' explicitly, so 'it' is the current element.

Practice

1. What does the it keyword represent in a Kotlin lambda with a single parameter?

easy

A. The single implicit parameter of the lambda

B. A reserved word for loop counters

C. A keyword to declare variables

D. A function to print output

Solution

1. Step 1: Understand lambda parameters

   In Kotlin, if a lambda has only one parameter, you don't need to name it explicitly.

2. Step 2: Role of it

   The keyword it automatically refers to that single parameter inside the lambda.

3. Final Answer:

   The single implicit parameter of the lambda -> Option A

4. Quick Check:

   it = single lambda parameter [OK]

Hint: Remember: it means the one lambda input [OK]

Common Mistakes:

• Thinking it is a loop variable
• Confusing it with a keyword to declare variables
• Assuming it prints output

2. Which of the following is the correct syntax to use it in a Kotlin lambda that doubles each number in a list?

easy

A. list.map { it * 2 }

B. list.map { x -> x * 2 }

C. list.map { it -> it * 2 }

D. list.map { * 2 }

Solution

1. Step 1: Identify lambda with single parameter

   The lambda inside map has one parameter, so it can be used implicitly.

2. Step 2: Check syntax correctness

   list.map { it * 2 } correctly uses it without naming the parameter explicitly.

3. Final Answer:

   list.map { it * 2 } -> Option A

4. Quick Check:

   Implicit single parameter lambda syntax = list.map { it * 2 } [OK]

Hint: Use it only when lambda has one parameter [OK]

Common Mistakes:

• Using explicit parameter name and it together
• Omitting parameter name but adding arrow
• Trying to multiply without parameter

3. What is the output of this Kotlin code?

val numbers = listOf(1, 2, 3)
val result = numbers.filter { it > 1 }
println(result)

medium

A. [1, 2, 3]

B. [1]

C. [2, 3]

D. []

Solution

1. Step 1: Understand the filter condition

   The lambda filters numbers where it > 1, so only numbers greater than 1 remain.

2. Step 2: Apply filter to the list

   From the list [1, 2, 3], numbers 2 and 3 satisfy the condition.

3. Final Answer:

   [2, 3] -> Option C

4. Quick Check:

   Filter with it > 1 = [2, 3] [OK]

Hint: Filter keeps items where it meets condition [OK]

Common Mistakes:

• Including 1 in the result
• Confusing filter with map
• Expecting original list unchanged

4. Identify the error in this Kotlin code snippet:

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

medium

A. Missing parameter name before *

B. No error, code runs fine

C. Using it without declaring lambda parameter

D. Syntax error: incomplete expression after *

Solution

1. Step 1: Analyze the lambda expression

   The expression it * is incomplete; it lacks a value after the multiplication operator.

2. Step 2: Identify syntax error

   This causes a syntax error because Kotlin expects a complete expression after *.

3. Final Answer:

   Syntax error: incomplete expression after * -> Option D

4. Quick Check:

   Incomplete expression causes syntax error [OK]

Hint: Check for complete expressions after operators [OK]

Common Mistakes:

• Assuming it * is valid alone
• Thinking it must be declared explicitly
• Ignoring syntax errors

5. Given a list of strings val words = listOf("apple", "banana", "cat"), which Kotlin code uses it correctly to create a list of word lengths greater than 3?

hard

A. words.map { it.length > 3 }

B. words.filter { it.length > 3 }

C. words.map { it.length }.filter { it > 3 }

D. words.filter { length > 3 }.map { it }

Solution

1. Step 1: Understand the goal

   We want words whose lengths are greater than 3, so we filter words based on their length.

2. Step 2: Analyze each option

   words.filter { it.length > 3 }, words.filter { it.length > 3 }, correctly filters the words list to include only words with length greater than 3, using it properly.

3. Final Answer:

   words.filter { it.length > 3 } -> Option B

4. Quick Check:

   Filter words by length > 3 = words.filter { it.length > 3 } [OK]

Hint: Use filter with it.length > 3 to get words longer than 3 [OK]

Common Mistakes:

• Filtering on string directly without length
• Using it incorrectly in filter
• Mixing order of map and filter