if Expressions

if (condition) {
    // process
} else if (condition) {
    // process
} else {
    // process
}

// Used as an expression (returns a value)
val variable = if (condition) valueA else valueB

sample_if_expression.kt

fun main() {
    val score = 75

    // Basic if / else if / else
    if (score >= 90) {
        println("Excellent.")
    } else if (score >= 60) {
        println("Pass.") // Prints "Pass."
    } else {
        println("Fail.")
    }

    // Assign to a variable using an if expression (instead of a ternary operator)
    val result = if (score >= 60) "Pass" else "Fail"
    println(result) // Prints "Pass"

    // When using blocks, the last expression becomes the value
    val grade = if (score >= 90) {
        println("Excellent!")
        "A"
    } else if (score >= 70) {
        println("Good!") // Prints "Good!"
        "B"
    } else {
        "C"
    }
    println(grade) // Prints "B"

    // You can also use an if expression in a function return
    val x = 10
    val abs = if (x >= 0) x else -x
    println("Absolute value: $abs") // Prints "Absolute value: 10"

    // Also useful for null checks
    val name: String? = "Kotlin"
    val greeting = if (name != null) "Hello, $name" else "No name provided"
    println(greeting) // Prints "Hello, Kotlin"
}

if_expression.kt

kotlinc if_expression.kt -include-runtime -d if_expression.jar
java -jar if_expression.jar
Pass.
Pass
Good!
B
Absolute value: 10
Hello, Kotlin

sample_if_mistakes.kt

fun main() {
    val score = 75

    // Using if as an expression without an else branch
    // val grade = if (score >= 60) "Pass" // Compile error (else is required)

    // Misunderstanding which expression is returned in a block
    val label = if (score >= 90) {
        println("Excellent") // println is not the return value
        "A" // This is the return value
    } else {
        "B"
    }
    println(label) // B

    // Writing if as a statement and trying to assign it to a variable
    // var result: String
    // if (score >= 60) result = "Pass" else result = "Fail" // Works but verbose
    // val result2 = if (score >= 60) "Pass" else "Fail" // Idiomatic Kotlin

    // Using a Unit-returning block as an expression unintentionally
    val x = if (score > 0) println("Positive") else println("Non-positive")
    println(x) // kotlin.Unit
}

The command looks like this:

kotlinc sample_if_mistakes.kt -include-runtime -d sample_if_mistakes.jar
java -jar sample_if_mistakes.jar
B
Positive
kotlin.Unit

Kotlin does not have Java's ternary operator (? :). Instead, you can use if as an expression to achieve the same result. When using if as an expression, an else branch is always required. Without else, the value cannot be determined and a compile error will occur.

For branching on multiple conditions, the when expression is a better fit. The ?: Elvis operator is also handy when working with nullable types.