when Expressions and Ranges

Branching as an expression

when matches a value against branches and, like if, is an expression that produces a value. Branches can be constants, several values separated by commas, in a range or collection, is a type check, or arbitrary boolean conditions when when has no subject. When used as an expression, when must be exhaustive, usually via an else branch, or by covering every case of an enum or sealed type. Ranges such as 1..10 (inclusive), 0 until n or 0..<n (end exclusive), 10 downTo 1 and step 2 are used in loops and in checks.

Grading with when and ranges

when returns a value.

fun grade(score: Int): String = when (score) {
    in 90..100 -> "A"
    in 75..<90 -> "B"
    in 50..<75 -> "C"
    in 0..<50 -> "F"
    else -> throw IllegalArgumentException("score out of range: $score")
}

fun describe(x: Any): String = when {
    x is String && x.isEmpty() -> "empty string"
    x is Int && x % 2 == 0 -> "even number"
    else -> "something else"
}

fun main() {
    for (i in 10 downTo 0 step 5) println(i)
    for (i in 0..<3) println(grade(40 + i * 25))
}

A mail sorter

when is like a sorting desk: each letter is checked against labelled slots in order and lands in the first slot that matches, with else as the catch-all tray.

Quick check: Which range excludes its end value?

• `0..5`
• `0..<5` (or `0 until 5`)
• `5 downTo 0`
• `0..5 step 1`