Kotlin when Expressions

Choose a result from several conditions with when.

Kotlin

fun grade(score: Int): String = when {
    score >= 90 -> "A"
    score >= 80 -> "B"
    else -> "C"
}

fun main() {
    println(grade(95))
    println(grade(82))
    println(grade(60))
}

Output

A
B
C

Explanation

when is Kotlin's multi-way branch. Used without an argument, each branch is a Boolean condition, and the first one that is true supplies the result. else handles every remaining case.

when is an expression, so it produces a value and can be the body of a function with =. The compiler requires else in that case, because the expression must always have a result.

Explore the dry run

Guided walkthroughReady

1fun grade(score: Int): String = when {

2 score >= 90 -> "A"

3 score >= 80 -> "B"

4 else -> "C"

5}

7fun main() {

8 println(grade(95))

9 println(grade(82))

10 println(grade(60))

11}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
The whole body is one when expression.	`—`	—
95 >= 90 is true, so the result is A.	`score = 95`	A
The first test fails; 82 >= 80 matches.	`score = 82`	A B
No test matches, so else gives C.	`score = 60`	A B C

Cost and common mistakes

O(1) time and space.

Branches are checked top to bottom, so putting score >= 80 first would label 95 as B.
A when used as an expression without else does not compile unless every case is covered.

Try it yourself

Add a D grade for scores of 70 or more.

Reveal answer and reasoning

Insert score >= 70 -> "D" before else. grade(60) still returns C.

Kotlin when Expressions

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading