when expression

Learn to use when as an expression.

The when expression

Like if, when can also be an expression that returns a value. For this, branches must produce values and else is required.

val day = 3
val name = when (day) {
    1 -> "Monday"
    2 -> "Tuesday"
    3 -> "Wednesday"
    else -> "Unknown"
}
println(name)  // Wednesday

Each branch's right side becomes a value; that value is assigned to name. day is 3, so "Wednesday" is chosen.

The expression form is ideal for storing the result in a variable or passing it to a method.

Exercise Assign day name with a when expression

Write a fun main(). Assign 2 to val day. Write val name = when (day) { 1 -> "Monday" 2 -> "Tuesday" 3 -> "Wednesday" else -> "Unknown" }. Print the result with println(name).

Show example solution

fun main() {
    val day = 2
    val name = when (day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3 -> "Wednesday"
        else -> "Unknown"
    }
    println(name)
}

Exhaustiveness

In a when expression, the compiler checks that all possible cases are covered. This is called exhaustiveness.

val status = when (code) {
    200 -> "OK"
    404 -> "Not found"
    500 -> "Server error"
    else -> "Unknown"
}

Since code is an Int, listing all values is impossible, so else is required. But for binary types like Boolean, else is not needed.

If else is missing and a value falls through, you get a compile error.

Test yourself

Match the when forms

val n = when (x) { ... }

Expression
when (x) { println(...) }

Statement
1 -> "A"

Value branch
else -> "?"

Default

Complete the when branch

val name = when (day) {
    1 -> "Monday"
    ____ -> "Tuesday"
    else -> "Unknown"
}

Show answer

val name = when (day) {
    1 -> "Monday"
    2 -> "Tuesday"
    else -> "Unknown"
}

When is else required in a when expression?
- Never
- Only with two branches
- When the result is assigned to a value
- Always required
Show answer

When the result is assigned to a value — When when returns a value, else is required.

Next lesson: Summary - Conditionals

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