Named arguments and default values

Kotlin function calls can read like descriptions of a task. Argument labels are names used at the call site, while parameter names are used inside the body. A default parameter lets a caller omit one argument when a sensible standard value exists.

Named arguments identify a parameter at the call site, and defaults supply values for omitted arguments. Kotlin does not use Swift-style inout parameters for ordinary numeric variables; returning a corrected value keeps the state update visible in the caller.

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

Kotlin/JVM · complete program

fun addBonus(score: Int, amount: Int = 5): Int = score + amount
fun main() {
  val score = 70
  println(addBonus(score = score))
  println(addBonus(amount = 8, score = score))
  println(score)
}

Expected output

75
78
70

Follow the execution

The first call uses default five and returns 75. The named second call uses eight and returns 78. Neither changes the original score, which remains 70; the caller can assign a returned result to a var when updating state is intended.

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

Your task: How do you request an eight-point bonus without mutating the input?

Kotlin/JVM · complete program

fun addBonus(score: Int, amount: Int = 5): Int = score + amount
fun main() {
  val score = 70
  println(addBonus(score = score))
  println(addBonus(amount = 8, score = score))
  println(score)
}

Worked answer

Call addBonus(score = score, amount = 8) and use its returned value. Named arguments clarify which quantity is changing.

Defaults should express a meaningful common policy rather than hide required information. Named arguments are not generally available when calling Java methods, whose parameter-name contract differs. Use vararg only when a variable argument count fits the function.

Call addBonus(score = score, amount = 8) and use its returned value. Named arguments clarify which quantity is changing. Defaults should express a meaningful common policy rather than hide required information.

Further reading: official Kotlin documentation for this topic.