Kotlin when Expressions

Before you start

Kotlin functions and comparison operators.

Choose a result from several conditions with when.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

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.

Code example

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

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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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

Behind the scenes

How Kotlin processes this code

Kotlin commonly compiles to JVM bytecode, which the JVM executes.

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.