How to Use if as Expression in Kotlin: Simple Guide

In Kotlin, if can be used as an expression that returns a value, allowing you to assign the result directly to a variable. This means you can write val result = if (condition) value1 else value2 to choose between values based on a condition.

Syntax

The if expression in Kotlin evaluates a condition and returns a value based on that condition. It has the form:

• if (condition) value1 else value2: returns value1 if the condition is true, otherwise value2.
• The else part is mandatory when using if as an expression.

kotlin

val result = if (score >= 50) "Pass" else "Fail"

Example

This example shows how to use if as an expression to assign a message based on a number's value.

kotlin

fun main() {
    val number = 7
    val message = if (number % 2 == 0) {
        "Even number"
    } else {
        "Odd number"
    }
    println(message)
}

Output

Odd number

Common Pitfalls

One common mistake is forgetting the else branch when using if as an expression, which causes a compilation error. Another is using if as a statement when you want a value, leading to unexpected Unit results.

kotlin

/* Wrong: Missing else branch */
// val result = if (score > 0) "Positive"

/* Right: Include else branch */
val result = if (score > 0) "Positive" else "Non-positive"

Quick Reference

Usage	Description
if (condition) value1 else value2	Returns value1 if condition is true, else value2
val x = if (a > b) a else b	Assigns the greater of a or b to x
if used without else	Compilation error when used as expression
if as statement	Does not return a value, used for side effects

Key Takeaways

Use if with else to return values as expressions in Kotlin.

Assign the result of if expression directly to variables for concise code.

Always include else branch when using if as an expression to avoid errors.

if expressions can have blocks with multiple statements inside braces.

Using if as a statement does not return a value and is different from expression usage.