Closures and variable capture in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a lambda that adds 5 to its input.

Kotlin

val addFive = { x: Int -> x [1] 5 }

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

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causes subtraction.

Using '*' or '/' changes the operation to multiplication or division.

The lambda adds 5 to the input using the + operator.

2fill in blank

medium

Complete the code to capture the variable 'factor' inside the lambda.

Kotlin

fun makeMultiplier(factor: Int): (Int) -> Int = { x -> x [1] factor }

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' adds instead of multiplies.

Using '-' or '/' changes the operation incorrectly.

The lambda multiplies the input by the captured 'factor' using '*'.

3fill in blank

hard

Fix the error in the lambda to correctly capture the variable 'count'.

Kotlin

var count = 0
val increment = { count [1] 1 }

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

A=

B-=

C+=

D*=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' alone does not increment the value.

Using '-=' or '*=' changes the value incorrectly.

Use '+=' to increment the variable 'count' inside the lambda.

4fill in blank

hard

Fill both blanks to create a lambda that filters a list by length greater than 3.

Kotlin

val words = listOf("cat", "house", "dog", "elephant")
val filtered = words.filter { word -> word.length [1] [2] }

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

A>

B3

C<

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' filters words shorter than 3.

Using 4 instead of 3 changes the filter condition.

The lambda filters words with length greater than 3 using '>' and 3.

5fill in blank

hard

Fill all three blanks to create a map of words to their uppercase versions for words longer than 4.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val result = words.filter { [1] -> [1].length [2] 4 }
    .associateWith { [3].uppercase() }

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

Aword

B>

Cit

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'it' in filter parameter causes error.

Using '<' filters wrong words.

Using 'length' as variable name is incorrect.

The lambda filters words with length > 4 and maps each word to its uppercase version using 'word' and '>'.

Practice

1. What does a closure in Kotlin do with variables from its surrounding scope?

easy

A. It copies the variables but cannot modify them.

B. It deletes those variables to save memory.

C. It ignores variables outside its own body.

D. It remembers and can use those variables even after the outer function ends.

Solution

1. Step 1: Understand closure behavior

   A closure is a function that keeps access to variables from where it was created, even if that place no longer exists.

2. Step 2: Apply to Kotlin closures

   In Kotlin, lambdas capture variables from the surrounding scope and can use them later.

3. Final Answer:

   It remembers and can use those variables even after the outer function ends. -> Option D

4. Quick Check:

   Closure remembers and can use variables even after outer ends = C [OK]

Hint: Closures keep variables alive beyond their original scope [OK]

Common Mistakes:

• Thinking closures delete outside variables
• Believing closures only copy variables without access
• Assuming closures ignore outside variables

2. Which of the following is the correct syntax to create a lambda that captures a variable count in Kotlin?

easy

A. val lambda = (count) -> println(count)

B. val lambda = { println(count) }

C. val lambda = fun(count) { println(count) }

D. val lambda = { count -> println() }

Solution

1. Step 1: Identify lambda syntax capturing external variable

   In Kotlin, a lambda can access variables from outside without parameters by just using them inside curly braces.

2. Step 2: Check each option

   val lambda = { println(count) } correctly uses count from outside. val lambda = (count) -> println(count) tries to declare a parameter but syntax is wrong. val lambda = fun(count) { println(count) } uses fun keyword incorrectly. val lambda = { count -> println() } declares a parameter but does not use it properly.

3. Final Answer:

   val lambda = { println(count) } -> Option B

4. Quick Check:

   Lambda uses outside variable directly = A [OK]

Hint: Use curly braces and refer to outside variables directly [OK]

Common Mistakes:

• Trying to declare external variables as lambda parameters
• Using incorrect lambda syntax with fun keyword
• Not referencing the variable inside the lambda body

3. What will be the output of the following Kotlin code?

fun main() {
    var x = 5
    val closure = { x += 1; println(x) }
    closure()
    closure()
    println(x)
}

medium

A. 6\n7\n7

B. 6\n6\n5

C. 6\n7\n5

D. 5\n5\n5

Solution

1. Step 1: Trace the closure calls and variable changes

   Initially, x = 5. First closure call increments x to 6 and prints 6. Second call increments x to 7 and prints 7.

2. Step 2: Print final value of x

   After two increments, x is 7, so printing x outputs 7.

3. Final Answer:

   6\n7\n7 -> Option A

4. Quick Check:

   Closure increments x twice, final x = 7 [OK]

Hint: Closures can modify captured vars; track changes stepwise [OK]

Common Mistakes:

• Assuming closure uses a copy of x, not the original
• Forgetting x increments on each closure call
• Confusing printed values with initial x

4. Identify the error in this Kotlin code that tries to capture a variable inside a loop:

fun main() {
    val funcs = mutableListOf<() -> Unit>()
    for (i in 1..3) {
        funcs.add { println(i) }
    }
    funcs.forEach { it() }
}

medium

A. All lambdas print 3 instead of 1, 2, 3

B. Runtime error: variable i not found

C. Lambdas print 1, 2, 3 correctly

D. Syntax error: cannot capture loop variable

Solution

1. Step 1: Understand variable capture in Kotlin loops

   Kotlin captures the loop variable i correctly for each lambda, so each lambda remembers its own i value.

2. Step 2: Check output of lambdas

   When calling each lambda, they print 1, 2, and 3 respectively, no error occurs.

3. Final Answer:

   Lambdas print 1, 2, 3 correctly -> Option C

4. Quick Check:

   Kotlin captures loop vars per iteration = B [OK]

Hint: Kotlin captures loop vars freshly each iteration [OK]

Common Mistakes:

• Assuming all lambdas print the last loop value
• Expecting syntax or runtime errors for capturing loop vars
• Confusing Kotlin behavior with other languages like JavaScript

5. You want to create a list of functions in Kotlin, each printing a different number from 1 to 3. Which code correctly captures the variable to avoid all functions printing the same number?

val funcs = mutableListOf<() -> Unit>()
for (i in 1..3) {
    // What to do here?
}
funcs.forEach { it() }

hard

A. for (i in 1..3) { funcs.add { println(i) } }

B. for (i in 1..3) { val x = i + 1; funcs.add { println(x) } }

C. for (i in 1..3) { funcs.add { println(i + 1) } }

D. for (i in 1..3) { val x = i; funcs.add { println(x + 1) } }

Solution

1. Step 1: Understand variable capture in Kotlin for loops

   In Kotlin, lambdas added inside a for (i in 1..3) capture the i from their iteration correctly.

2. Step 2: Check each snippet

   funcs.add { println(i) } prints 1 then 2 then 3. The snippet with val x = i + 1; funcs.add { println(x) } prints 2 3 4. funcs.add { println(i + 1) } prints 2 3 4. val x = i; funcs.add { println(x + 1) } prints 2 3 4.

3. Final Answer:

   for (i in 1..3) { funcs.add { println(i) } } -> Option A

4. Quick Check:

   Direct println(i) in for loop lambda = B [OK]

Hint: Directly use loop var i inside lambda [OK]

Common Mistakes:

• Declaring x = i + 1 and printing x (prints 2 3 4)
• Printing i + 1 directly (prints 2 3 4)
• Copying to x = i but printing x + 1 (prints 2 3 4)