Run function behavior and use cases in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using run?

Consider the following Kotlin code snippet using the run function. What will be printed when this code runs?

Kotlin

val result = run {
    val x = 10
    val y = 20
    x + y
}
println(result)

A Compilation error

B 20

C 30

D 10

💡 Hint

Remember that run executes the block and returns the last expression.

The run function executes the block and returns the value of the last expression inside it. Here, x + y is the last expression, so result is 30.

❓ Predict Output

intermediate

What does this run block return?

Look at this Kotlin code using run. What is the value of output after running this?

Kotlin

val output = "Hello".run {
    println(this)
    this.length
}
println(output)

A Compilation error

B Hello\n5

C 5

D Hello\nHello

💡 Hint

The run block returns the last expression's value, but println prints to console.

The run block prints Hello first, then returns this.length which is 5. So the console shows Hello and then 5.

🧠 Conceptual

advanced

Which statement about Kotlin's run function is true?

Choose the correct statement about the behavior of Kotlin's run function.

A <code>run</code> is a keyword and cannot be used as a function.

B <code>run</code> can only be used with nullable types.

C <code>run</code> modifies the receiver object permanently.

D <code>run</code> executes a block and returns the last expression's value.

💡 Hint

Think about what run returns after executing the block.

run is a standard library function that executes a block and returns the last expression's value. It does not modify the receiver permanently and is not limited to nullable types.

🔧 Debug

advanced

What error does this Kotlin code cause?

Examine this Kotlin code using run. What error will it produce when compiled or run?

Kotlin

val number = 5
val result = number.run {
    val number = "ten"
    number.length
}
println(result)

A Prints 3

B Prints 5

C Compilation error due to variable shadowing

D Runtime NullPointerException

💡 Hint

Check how variable shadowing works inside the run block.

Inside the run block, the local number variable shadows the outer one. The string "ten" has length 3, so result is 3 and prints 3.

🚀 Application

expert

How many items are in the map created by this run block?

What is the number of entries in the map returned by this Kotlin run block?

Kotlin

val map = run {
    val tempMap = mutableMapOf<Int, String>()
    tempMap[1] = "one"
    tempMap[2] = "two"
    tempMap[3] = "three"
    tempMap
}
println(map.size)

A 3

B 0

C 1

D Compilation error

💡 Hint

The run block returns the map after adding three entries.

The run block creates a mutable map, adds three entries, and returns it. So the map size is 3.