When expressions and matching

A report needs grade messages for several ranges. Kotlin when compares a subject with branch conditions and can return the selected branch’s value. It provides a readable alternative to a long chain when the choices concern one subject.

Branches can match values, ranges or type checks. The first matching branch executes, and there is no automatic fallthrough. A value-producing when must cover the possibilities required by its type, often using else. A subjectless when can organize independent Boolean conditions.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

fun main() {
  val score = 82
  val result = when (score) {
    in 90..100 -> "Excellent"
    in 75..<90 -> "Pass"
    else -> "Review score"
  }
  println(result)
}

Expected output

Pass

Follow the execution

Eighty-two matches the second range and assigns Pass. The half-open range excludes 90, while the first closed range includes it. No later branch executes after the match.

Try this in the Kotlin compiler by pasting the complete program.

Your task: Which branch handles score 90?

Kotlin/JVM · complete program

fun main() {
  val score = 82
  val result = when (score) {
    in 90..100 -> "Excellent"
    in 75..<90 -> "Pass"
    else -> "Review score"
  }
  println(result)
}

Worked answer

The first branch handles 90 because 90..100 includes that endpoint.

A default branch can also catch impossible values such as 120. Validate the domain separately instead of calling every unmatched score an ordinary low grade.

The first branch handles 90 because 90..100 includes that endpoint. A default branch can also catch impossible values such as 120.

Further reading: official Kotlin documentation for this topic.