when

Learn multi-way selection with when.

The when statement

when compares a value against several possible cases. It is like Java's switch but more powerful. Each branch uses ->.

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

day is 3, so the 3 -> branch runs and prints "Wednesday". If no branch matches, the else branch runs.

when is more readable; it replaces long if/else if chains.

Exercise Convert day number to name

Write a fun main(). Assign 3 to val day. Use when (day) to print the day name: 1 "Monday", 2 "Tuesday", 3 "Wednesday", else "Unknown".

Show example solution

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

Multiple values and ranges in when

when branches can list several values with commas or use ranges with in.

val n = 5
when (n) {
    1, 2, 3 -> println("Small")
    in 4..7 -> println("Medium")
    else -> println("Large")
}

n is 5, so the in 4..7 branch runs and prints "Medium". Ranges use ..; you'll see them in detail later.

If no branch matches, else runs. else is optional, but with no match nothing runs.

Test yourself

What does when (day) { ... } compare against?
- A fixed value
- The day variable
- A Boolean
- A random value
Show answer

The day variable — when (day) compares day's value against the branches.

Complete the when branch

when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    ____ -> println("Wednesday")
}

Show answer

when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    3 -> println("Wednesday")
}

Match the when forms

1 -> ...

Single value branch
1, 2 -> ...

Multi value branch
in 4..7 -> ...

Range branch
else -> ...

Default branch

Next lesson: when expression

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