Numbers and explicit conversions

A class total is a whole number, while its average can contain a fraction. Kotlin provides numeric types such as Int, Long and Double. The chosen type affects valid operations and results, so integer arithmetic is not interchangeable with floating-point arithmetic.

Convert deliberately with toDouble() before fractional division. Parse arbitrary text with toIntOrNull() when invalid input should become absence rather than an exception. A successful numeric conversion still does not validate whether a mark is within the school’s allowed range.

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 total = 155
  println(total / 2)
  println(total.toDouble() / 2)
  println("eighty".toIntOrNull() ?: -1)
}

Expected output

77
77.5
-1

Follow the execution

Integer division produces 77. Converting the total gives 77.5. The invalid text has no parsed integer, so the Elvis fallback displays -1. That fallback is a demonstration marker, not a valid student mark.

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

Your task: Why does the first division not display 77.5?

Kotlin/JVM · complete program

fun main() {
  val total = 155
  println(total / 2)
  println(total.toDouble() / 2)
  println("eighty".toIntOrNull() ?: -1)
}

Worked answer

Both operands use integer arithmetic, which truncates the fractional part here. Convert a suitable operand to Double before division when a fractional result is required.

Int arithmetic can overflow, and floating-point numbers approximate many decimal values. Choose types and range checks from the domain. Converting a large value to a smaller numeric type can lose information.

Both operands use integer arithmetic, which truncates the fractional part here. Convert a suitable operand to Double before division when a fractional result is required. Int arithmetic can overflow, and floating-point numbers approximate many decimal values.

Further reading: official Kotlin documentation for this topic.