Run function behavior and use cases in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run the block and return its result.

Kotlin

val result = run [1] {
    5 + 3
}
println(result)

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

A() -> Int

B{ Int -> Int }

C() -> Unit

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using a lambda type with parameters instead of none.

Confusing the return type with the lambda parameter type.

The run function takes a lambda with no parameters and returns the lambda's result. The lambda type is () -> Int here.

2fill in blank

medium

Complete the code to run a block on a nullable object safely.

Kotlin

val name: String? = "Kotlin"
val length = name?.run [1] {
    length
} ?: 0
println(length)

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

A() -> Int

B{ String -> Int }

C() -> String

D{ Int -> Int }

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a parameter to the lambda when using run on an object.

Not handling the nullable case with ?: 0.

The run function called on an object uses the object as this inside the lambda, which takes no parameters. The lambda returns the length of the string.

3fill in blank

hard

Fix the error in the run block to correctly return the last character.

Kotlin

val word = "hello"
val lastChar = run [1] {
    word[word.length]
}
println(lastChar)

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

A() -> Int

B() -> String

C() -> Char

D() -> Unit

Attempts:

3 left

💡 Hint

Common Mistakes

Using an incorrect lambda type with parameters.

Confusing the return type with the parameter type.

The lambda returns a Char and has no parameters. The index should be word.length - 1 to avoid an error, but the blank is for the lambda type.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths using run.

Kotlin

val words = listOf("apple", "banana", "cherry")
val lengths = words.associateWith [1] {
    it.[2]
}
println(lengths)

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

Arun

Blength

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using count or size instead of length for strings.

Not using run in the first blank.

The associateWith function takes a lambda that returns a value for each element. Using run runs the block on each word, and length gets the word's length.

5fill in blank

hard

Fill all three blanks to filter and map words using run with conditions.

Kotlin

val words = listOf("dog", "cat", "elephant", "fox")
val filtered = words.filter [1] {
    it.length [2] 3
}.map [3] {
    it.uppercase()
}
println(filtered)

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

Arun

B>

Cfilter

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using filter or map as the lambda instead of run.

Using the wrong comparison operator.

The filter function takes a lambda to select words longer than 3. Using run runs the lambda on each word. Then map transforms each word to uppercase using run.

Practice

1. What does the Kotlin run function do when called without an object?

easy

A. Creates a new thread to run the block asynchronously

B. Delays the execution of the block until explicitly called

C. Executes the given block and returns the last expression's value

D. Throws an exception if the block contains no return statement

Solution

1. Step 1: Understand run without receiver

   When run is called without an object, it simply executes the block of code inside it.

2. Step 2: Identify the return value

   The function returns the value of the last expression inside the block.

3. Final Answer:

   Executes the given block and returns the last expression's value -> Option C

4. Quick Check:

   run without object returns last expression [OK]

Hint: run returns last expression value in its block [OK]

Common Mistakes:

• Thinking run creates a new thread
• Assuming run delays execution
• Believing run throws error without explicit return

2. Which of the following is the correct syntax to use run with an object in Kotlin?

easy

A. val result = run.obj { property + 1 }

B. val result = obj.run { this.property + 1 }

C. val result = run { obj.property + 1 }

D. val result = obj.run() { property + 1 }

Solution

1. Step 1: Identify correct run usage with receiver

   The run function is called as an extension on the object: obj.run { ... }.

2. Step 2: Check syntax correctness

   val result = obj.run { this.property + 1 } correctly uses obj.run { this.property + 1 } without extra parentheses or misplaced dots.

3. Final Answer:

   val result = obj.run { this.property + 1 } -> Option B

4. Quick Check:

   run called as extension function on object [OK]

Hint: run is called as extension: obj.run { ... } [OK]

Common Mistakes:

• Using dot after run like run.obj
• Adding empty parentheses after run
• Calling run without object but expecting receiver

3. What is the output of this Kotlin code?

val result = run {
    val x = 5
    val y = 10
    x + y
}
println(result)

medium

A. Compilation error

B. 510

C. null

D. 15

Solution

1. Step 1: Execute the run block

   The block declares x = 5 and y = 10, then returns x + y which is 15.

2. Step 2: Print the result

   The variable result holds 15, so println(result) outputs 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   run returns last expression value 15 [OK]

Hint: run returns last expression value inside block [OK]

Common Mistakes:

• Concatenating numbers as strings
• Expecting null because no explicit return
• Thinking run causes compilation error

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

val text = "Hello"
val length = text.run {
    val len = length
}
println(length)

medium

A. The run block does not return a value, so length is assigned Unit

B. The variable len is not accessible outside run block

C. The property length is undefined for String

D. The run function cannot be called on a String

Solution

1. Step 1: Analyze run block return

   The run block declares val len = length but does not return any expression explicitly, so it returns Unit.

2. Step 2: Check assignment to length variable

   Variable length is assigned the result of run, which is Unit, not the integer length of the string.

3. Final Answer:

   The run block does not return a value, so length is assigned Unit -> Option A

4. Quick Check:

   run returns last expression; no explicit return means Unit [OK]

Hint: run returns last expression; no explicit return means Unit [OK]

Common Mistakes:

• Assuming len is returned automatically
• Confusing property length with variable
• Thinking run cannot be called on String

5. Given a list of nullable integers val numbers = listOf(1, null, 3, null, 5), how can you use run to compute the sum of non-null values safely in Kotlin?

hard

A. val sum = numbers.run { filterNotNull().sum() }

B. val sum = run(numbers) { filterNotNull().sum() }

C. val sum = numbers.run { sum() }

D. val sum = run { filterNotNull().sum() }

Solution

1. Step 1: Use run as extension on list

   Calling numbers.run { ... } lets you access list functions directly inside the block.

2. Step 2: Filter non-null and sum

   Inside the block, filterNotNull() removes nulls, then sum() adds remaining numbers.

3. Final Answer:

   val sum = numbers.run { filterNotNull().sum() } -> Option A

4. Quick Check:

   run with receiver simplifies code, returns last expression [OK]

Hint: Use run as extension to chain calls on object [OK]

Common Mistakes:

• Passing list as argument to run incorrectly
• Calling sum() without filtering nulls
• Using run without receiver and accessing list members directly