when Workbook

Practice problems for Kotlin’s when Is the New switch. Each takes a minute or two. Write your own answer first, then click Show answer — nothing here is a trick question, just direct practice of the syntax from the lesson.

with a subject

1. Day classification

Given day (1–7), return "weekday" for 1–5 and "weekend" for 6–7, using when with a subject and ranges.

when (day) {
    in 1..5 -> "weekday"
    in 6..7 -> "weekend"
    else -> "invalid"
}

2. Multiple values per branch

Given a Char c, return true when it’s a lowercase vowel, false otherwise.

when (c) {
    'a', 'e', 'i', 'o', 'u' -> true
    else -> false
}

3. Assign from a when

Assign size the value "small", "medium", or "large" based on count (< 10, < 100, otherwise), using a subjectless when.

val size = when {
    count < 10 -> "small"
    count < 100 -> "medium"
    else -> "large"
}

type matching

4. Describe an Any

Given x: Any, return "int", "text", or "other" depending on whether it’s an Int, a String, or neither.

when (x) {
    is Int -> "int"
    is String -> "text"
    else -> "other"
}

5. Use the smart-cast value

Given x: Any, return its length if it’s a String, otherwise 0. Use the smart cast inside the branch.

when (x) {
    is String -> x.length
    else -> 0
}

ranges and capture

6. Range branches

Given n: Int, return "digit" for 0–9, "teens" for 10–19, else "big".

when (n) {
    in 0..9 -> "digit"
    in 10..19 -> "teens"
    else -> "big"
}

7. Capture the subject

Rewrite this so the result of readLine() is captured in the when header and reused:

val line = readLine()
when (line) { ... }

when (val line = readLine()) {
    null -> println("no input")
    else -> println(line.length)
}

exhaustiveness

8. No else for enums

Given enum class Direction { NORTH, SOUTH, EAST, WEST }, write a when expression mapping each to its opposite — with no else branch.

fun opposite(d: Direction) = when (d) {
    Direction.NORTH -> Direction.SOUTH
    Direction.SOUTH -> Direction.NORTH
    Direction.EAST -> Direction.WEST
    Direction.WEST -> Direction.EAST
}

9. Replace instanceof + cast

Rewrite this Java-style chain in idiomatic Kotlin:

if (shape is Circle) return shape.radius
if (shape is Square) return shape.side

when (shape) {
    is Circle -> shape.radius
    is Square -> shape.side
    else -> 0.0
}

10. Statement, not expression

Write a when that just prints a message for each level (1, 2, else) — used as a statement, returning nothing.

when (level) {
    1 -> println("low")
    2 -> println("high")
    else -> println("unknown")
}

Back to the lesson, Kotlin’s when Is the New switch, or on to the next one: smart casts.