How to Use run Function in Kotlin: Simple Guide

In Kotlin, the run function executes a block of code and returns its result. It is useful for scoping variables or running code expressions in a concise way.

Syntax

The run function takes a lambda block and executes it, returning the last expression's value inside the block.

Syntax parts:

• run { ... }: Runs the code inside the curly braces.
• The block can contain multiple statements.
• The result of the block is returned by run.

kotlin

val result = run {
    val x = 10
    val y = 20
    x + y // last expression returned
}

Example

This example shows how run executes a block and returns the sum of two numbers. It also demonstrates scoping variables inside run.

kotlin

fun main() {
    val sum = run {
        val a = 5
        val b = 7
        a + b
    }
    println("Sum is $sum")
}

Output

Sum is 12

Common Pitfalls

Common mistakes when using run include:

• Not returning a value explicitly if the last expression is missing.
• Using run unnecessarily when simple expressions suffice.
• Confusing run with other scope functions like let or apply.

Example of a pitfall and fix:

kotlin

// Wrong: no value returned explicitly, returns Unit
val result = run {
    val x = 10
    val y = 20
    println(x + y) // prints but returns Unit
}

// Right: last expression is returned
val correctResult = run {
    val x = 10
    val y = 20
    x + y
}

Quick Reference

Feature	Description
Purpose	Execute a block and return its result
Return value	Last expression inside the block
Scope	Variables inside block are local
Common use	Scoping, initialization, chaining expressions

Key Takeaways

The run function executes a block and returns the last expression's value.

Use run to limit variable scope and write concise code blocks.

Always ensure the last expression in run returns the desired value.

Avoid using run when a simple expression or other scope functions fit better.

run is useful for initialization and chaining expressions cleanly.