How to Use When with Ranges in Kotlin: Simple Guide

In Kotlin, you can use the when expression with ranges by placing a range like 1..10 as a condition. The when block checks if the value falls inside the range and executes the matching branch. This makes it easy to test if a number is within certain intervals.

Syntax

The when expression uses ranges as conditions by specifying a range like start..end. If the value matches the range, the corresponding block runs.

• value: The variable you want to check.
• start..end: Defines the range to test.
• Branches: Code executed when the value is in the range.
• else: Optional default case if no ranges match.

kotlin

when (value) {
    in start..end -> {
        // code if value is in the range
    }
    else -> {
        // code if value is not in any range
    }
}

Example

This example shows how to use when with ranges to print messages based on a number's range.

kotlin

fun main() {
    val number = 25
    when (number) {
        in 1..10 -> println("Number is between 1 and 10")
        in 11..20 -> println("Number is between 11 and 20")
        in 21..30 -> println("Number is between 21 and 30")
        else -> println("Number is out of range")
    }
}

Output

Number is between 21 and 30

Common Pitfalls

Common mistakes include:

• Using == instead of in to check ranges.
• Forgetting that ranges are inclusive of both ends.
• Not covering all possible values, missing an else branch.

Always use in to check if a value is inside a range.

kotlin

fun main() {
    val number = 15
    // Wrong: using == instead of in
    when (number) {
        1..10 -> println("Between 1 and 10") // This compares reference, not range
        else -> println("Other")
    }

    // Right way:
    when (number) {
        in 1..10 -> println("Between 1 and 10")
        else -> println("Other")
    }
}

Output

Other
Other

Quick Reference

Tips for using when with ranges:

• Use in start..end to check if a value is inside a range.
• Ranges include both start and end values.
• Always add an else branch to handle unexpected values.
• when can replace multiple if-else checks for cleaner code.

Key Takeaways

Use the keyword in inside when to check if a value falls within a range.

Ranges in Kotlin are inclusive, meaning both start and end values are included.

Always provide an else branch in when to cover all cases.

Using when with ranges makes your code cleaner and easier to read than multiple if-else statements.