How to Use When Without Argument in Kotlin: Simple Guide

In Kotlin, you can use when without an argument to evaluate boolean conditions directly. Each branch uses a condition that returns true or false, and the first true condition's block runs. This makes when act like an elegant replacement for multiple if-else statements.

Syntax

The when expression without an argument uses boolean conditions in each branch. It looks like this:

• when { starts the expression.
• Each branch has a condition that returns true or false.
• The first branch with a true condition executes.
• else branch is optional but recommended as a fallback.

kotlin

when {
    condition1 -> {
        // code if condition1 is true
    }
    condition2 -> {
        // code if condition2 is true
    }
    else -> {
        // code if none above are true
    }
}

Example

This example shows how to use when without an argument to check a number's range and print a message accordingly.

kotlin

fun main() {
    val number = 15
    when {
        number < 0 -> println("Number is negative")
        number in 0..10 -> println("Number is between 0 and 10")
        number in 11..20 -> println("Number is between 11 and 20")
        else -> println("Number is greater than 20")
    }
}

Output

Number is between 11 and 20

Common Pitfalls

Common mistakes when using when without an argument include:

• Forgetting that each branch must be a boolean condition.
• Not including an else branch, which can cause a runtime error if no conditions match.
• Using expressions that do not return boolean values in branches.

kotlin

fun main() {
    val x = 5
    // Wrong: branch condition is not boolean
    /*
    when {
        x -> println("x") // Error: x is Int, not Boolean
        else -> println("else")
    }
    */

    // Right way:
    when {
        x > 0 -> println("x is positive")
        else -> println("x is zero or negative")
    }
}

Output

x is positive

Quick Reference

Tips for using when without argument:

• Use boolean expressions in each branch.
• Always provide an else branch to handle unexpected cases.
• It works like a cleaner if-else if chain.

Key Takeaways

Use when without argument to evaluate boolean conditions directly.

Each branch must be a boolean expression returning true or false.

The first true condition's block runs; else branch handles all other cases.

It simplifies multiple if-else statements into a clean, readable structure.

Always include an else branch to avoid missing cases.