Functions and return values

A report calculation should not be copied for every student. A function groups statements behind a name, accepts inputs and can return a result. Its parameter types and return type make the contract visible before you inspect its body.

A function declaration uses fun, parameter types and an optional return type. Returning a value lets a caller use it independently of printing. Kotlin can use an expression body for a short function; local names remain within their declared scope.

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

Kotlin/JVM · complete program

fun average(total: Int, count: Int): Double {
  require(count > 0) { "Count must be positive" }
  return total.toDouble() / count
}
fun main() { println(average(155, 2)) }

Expected output

77.5

Follow the execution

The precondition accepts count two, then converting total avoids integer truncation. The returned Double is 77.5. Printing is performed by main, so the function itself can also support another kind of caller.

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

Your task: What happens if count is zero?

Kotlin/JVM · complete program

fun average(total: Int, count: Int): Double {
  require(count > 0) { "Count must be positive" }
  return total.toDouble() / count
}
fun main() { println(average(155, 2)) }

Worked answer

Require rejects the input with an IllegalArgumentException before division. The caller needs a suitable policy for invalid inputs.

Choose explicit error or result contracts for ordinary invalid user input. Recursion needs a stopping case, and tailrec optimization applies only to suitable recursive calls. A function’s type contract does not prove its domain calculation is correct.

Require rejects the input with an IllegalArgumentException before division. The caller needs a suitable policy for invalid inputs. Choose explicit error or result contracts for ordinary invalid user input.

Further reading: official Kotlin documentation for this topic.