Run function behavior and use cases in Kotlin

Introduction

The run function helps you execute a block of code and return its result in a simple way. It makes your code cleaner and easier to read.

When you want to execute multiple statements and return the last expression as a result.

When you want to limit the scope of variables to a small block of code.

When you want to perform operations on an object and return a value without creating a separate function.

When you want to initialize an object and immediately use it in a concise way.

When you want to avoid repeating the object name multiple times in a block.

Syntax

Kotlin

val result = run {
    // code block
    // last expression is returned
}

The run function takes a lambda block and returns the value of the last expression inside it.

You can use run without an object or as an extension function on an object.

Examples

This runs a block that adds two numbers and returns the sum.

Kotlin

val result = run {
    val x = 5
    val y = 10
    x + y
}

This runs a block on the string "hello", prints it, and returns its length.

Kotlin

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

This creates a greeting message inside the block and returns it.

Kotlin

val message = run {
    val name = "Anna"
    "Hello, $name!"
}

Sample Program

This program shows two uses of run. First, it sums two numbers inside a block and prints the result. Second, it runs a block on a string, prints it, and returns its length.

Kotlin

fun main() {
    val sum = run {
        val a = 3
        val b = 7
        a + b
    }
    println("Sum is: $sum")

    val textLength = "Kotlin".run {
        println("Word: $this")
        this.length
    }
    println("Length is: $textLength")
}

Important Notes

The run function is useful to keep variables local and avoid polluting the outer scope.

Inside run, this refers to the object if used as an extension function.

Use run to make your code more readable and concise when you need a temporary scope.

Summary

run executes a block and returns the last expression.

It helps keep code clean by limiting variable scope.

You can use it with or without an object to simplify code.

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