Trailing lambda convention in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the function using the trailing lambda convention.

Kotlin

fun greet(action: () -> Unit) {
    action()
}

greet() [1] {
    println("Hello!")
}

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

A()

B() -> Unit

C() ->

D() -> {}

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the parentheses entirely when the function requires them.

Trying to put the lambda inside the parentheses incorrectly.

The trailing lambda convention allows you to pass the lambda outside the parentheses if it is the last argument. So you just call greet() followed by the lambda block.

2fill in blank

medium

Complete the code to define a function that takes a lambda and call it using trailing lambda syntax.

Kotlin

fun repeatAction(times: Int, action: () -> Unit) {
    for (i in 1..times) {
        action()
    }
}

repeatAction(3) [1] {
    println("Hi")
}

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

A()

B() -> Unit

C() ->

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Removing the parentheses entirely.

Putting the lambda inside the parentheses incorrectly.

When calling repeatAction, the lambda is the last argument, so you put it outside the parentheses. The parentheses must still be present for the first argument.

3fill in blank

hard

Fix the error in calling the function with trailing lambda syntax.

Kotlin

fun calculate(operation: (Int, Int) -> Int): Int {
    return operation(5, 3)
}

val result = calculate [1] { (a, b) -> a + b }
println(result)

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

A[

B(

C{

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of curly braces for the lambda.

Using square brackets or angle brackets.

The lambda expression must be enclosed in curly braces {} when used as a trailing lambda.

4fill in blank

hard

Fill both blanks to define and call a function using trailing lambda syntax correctly.

Kotlin

fun operate(x: Int, y: Int, operation: (Int, Int) -> Int): Int {
    return operation(x, y)
}

val sum = operate(4, 5[1] [2] a, b -> a + b)

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

A,

B)

C{

D]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a comma instead of closing parenthesis before the lambda.

Omitting the closing parenthesis before the trailing lambda.

Close the parentheses after the regular arguments with ')', then start the trailing lambda with '{' outside the parentheses.

5fill in blank

hard

Fill all three blanks to create a function call with trailing lambda and print the result.

Kotlin

fun transform(value: Int, operation: (Int) -> Int): Int {
    return operation(value)
}

val result = transform[1] 10[2] [3] x -> x * 2
println(result)

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

A(

B,

C{

D)

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the closing parenthesis before the trailing lambda.

Using a comma instead of ')' after the argument.

Open the parentheses with '(', pass the first argument '10', close with ')', then start the trailing lambda with '{' outside the parentheses.

Practice

1. What is the main purpose of the trailing lambda convention in Kotlin?

easy

A. To allow placing the lambda expression outside the parentheses when it is the last argument

B. To require all lambda expressions to be inside parentheses

C. To prevent using lambda expressions as function arguments

D. To force lambda expressions to be declared as variables before use

Solution

1. Step 1: Understand the trailing lambda convention and compare options

   The trailing lambda convention lets you write a lambda expression outside the parentheses if it is the last argument of a function. Only "To allow placing the lambda expression outside the parentheses when it is the last argument" correctly describes this behavior. Other options contradict the convention.

2. Final Answer:

   To allow placing the lambda expression outside the parentheses when it is the last argument -> Option A

3. Quick Check:

   Trailing lambda = lambda outside parentheses [OK]

Hint: Last lambda argument can go outside parentheses [OK]

Common Mistakes:

• Thinking lambdas must always be inside parentheses
• Confusing trailing lambda with lambda variable declaration
• Believing trailing lambda disallows lambdas as arguments

2. Which of the following Kotlin function calls correctly uses the trailing lambda convention?

easy

A. list.filter { it > 0 }

B. list.filter({ it > 0 })

C. list.filter(it > 0)

D. list.filter it > 0

Solution

1. Step 1: Identify correct trailing lambda syntax and check other options

   Trailing lambda means the lambda is placed outside the parentheses. list.filter { it > 0 } shows this correctly. list.filter({ it > 0 }) uses lambda inside parentheses (valid but not trailing). list.filter(it > 0) and D are syntax errors.

2. Final Answer:

   list.filter { it > 0 } -> Option A

3. Quick Check:

   Trailing lambda = lambda outside parentheses [OK]

Hint: Trailing lambda goes after parentheses, no extra parentheses needed [OK]

Common Mistakes:

• Putting lambda inside parentheses when trailing lambda is expected
• Omitting parentheses incorrectly
• Writing lambda without braces

3. What is the output of this Kotlin code?

fun greet(action: () -> String) = println(action())
greet { "Hello, Kotlin!" }

medium

A. No output

B. kotlin.Unit

C. Syntax error

D. Hello, Kotlin!

Solution

1. Step 1: Understand the function call and evaluate the output

   The function greet takes a lambda returning a String and prints its result. The call uses trailing lambda syntax. The lambda returns "Hello, Kotlin!" which is printed by println.

2. Final Answer:

   Hello, Kotlin! -> Option D

3. Quick Check:

   Trailing lambda call prints lambda result [OK]

Hint: Trailing lambda calls run the lambda and print result [OK]

Common Mistakes:

• Expecting syntax error due to trailing lambda
• Confusing return value with Unit
• Missing that println prints the lambda result

4. Identify the error in this Kotlin code using trailing lambda:

fun calculate(x: Int, operation: (Int) -> Int): Int {
    return operation(x)
}

val result = calculate(5) operation { it * 2 }

medium

A. Missing parentheses around lambda argument

B. Incorrect function declaration

C. Trailing lambda must be outside parentheses but parentheses are missing

D. No error, code is correct

Solution

1. Step 1: Analyze the function call syntax and correct trailing lambda usage

   The function call calculate(5) operation { it * 2 } is invalid because the trailing lambda must be placed immediately after the parentheses without extra tokens. The correct call is calculate(5) { it * 2 }. The word operation should not appear between parentheses and lambda.

2. Final Answer:

   Trailing lambda must be outside parentheses but parentheses are missing -> Option C

3. Quick Check:

   Trailing lambda goes right after parentheses [OK]

Hint: Trailing lambda must follow parentheses directly, no extra tokens [OK]

Common Mistakes:

• Inserting parameter name between parentheses and lambda
• Forgetting to remove parentheses when using trailing lambda
• Confusing function parameter names with call syntax

5. Given this Kotlin function:

fun processNumbers(numbers: List<Int>, filter: (Int) -> Boolean, action: (Int) -> Unit) {
    for (number in numbers) {
        if (filter(number)) action(number)
    }
}

Which is the correct way to call processNumbers using trailing lambda convention for both lambdas?

hard

A. processNumbers(listOf(1,2,3)) { it > 1 } { println(it) }

B. processNumbers(listOf(1,2,3), { it > 1 }) { println(it) }

C. processNumbers(listOf(1,2,3)) { println(it) } { it > 1 }

D. processNumbers(listOf(1,2,3), { println(it) }, { it > 1 })

Solution

1. Step 1: Understand trailing lambda rules for multiple lambdas and apply to function call

   Kotlin allows only the last lambda argument to be passed outside parentheses as a trailing lambda. Since action is the last lambda, it can be trailing. The first lambda filter must be inside parentheses. processNumbers(listOf(1,2,3), { it > 1 }) { println(it) } correctly places filter inside parentheses and action outside.

2. Final Answer:

   processNumbers(listOf(1,2,3), { it > 1 }) { println(it) } -> Option B

3. Quick Check:

   Only last lambda can be trailing [OK]

Hint: Only last lambda argument goes outside parentheses [OK]

Common Mistakes:

• Trying to put multiple lambdas outside parentheses
• Swapping lambda order incorrectly
• Putting non-lambda arguments outside parentheses