Number Types

Different types of numbers are stored in different ways.

If you create an identifier and assign an integer value to it, Kotlin infers the Int type:

// NumberTypes/InferInt.kt

fun main() {
  val million = 1_000_000  // Infers Int
  println(million)
}
/* Output:
1000000
*/

For readability, Kotlin allows underscores within numerical values.

The basic mathematical operators for numbers are the ones available in most programming languages: addition (+), subtraction (-), division (/), multiplication (*) and modulus (%), which produces the remainder from integer division:

// NumberTypes/Modulus.kt

fun main() {
  val numerator: Int = 19
  val denominator: Int = 10
  println(numerator % denominator)
}
/* Output:
9
*/

Integer division truncates its result:

// NumberTypes/IntDivisionTruncates.kt

fun main() {
  val numerator: Int = 19
  val denominator: Int = 10
  println(numerator / denominator)
}
/* Output:
1
*/

If the operation had rounded the result, the output would be 2.

The order of operations follows basic arithmetic:

// NumberTypes/OpOrder.kt

fun main() {
  println(45 + 5 * 6)
}
/* Output:
75
*/

The multiplication operation 5 * 6 is performed first, followed by the addition 45 + 30.

If you want 45 + 5 to happen first, use parentheses:

// NumberTypes/OpOrderParens.kt

fun main() {
  println((45 + 5) * 6)
}
/* Output:
300
*/

Now let’s calculate body mass index (BMI), which is weight in kilograms divided by the square of the height in meters. If you have a BMI of less than 18.5, you are underweight. Between 18.5 and 24.9 is normal weight. BMI of 25 and higher is overweight. This example also shows the preferred formatting style when you can’t fit the function’s parameters on a single line:

// NumberTypes/BMIMetric.kt

fun bmiMetric(
  weight: Double,
  height: Double
): String {
  val bmi = weight / (height * height)  // [1]
  return if (bmi < 18.5) "Underweight"
    else if (bmi < 25) "Normal weight"
    else "Overweight"
}

fun main() {
  val weight = 72.57 // 160 lbs
  val height = 1.727 // 68 inches
  val status = bmiMetric(weight, height)
  println(status)
}
/* Output:
Normal weight
*/

• [1] If you remove the parentheses, you divide weight by height then multiply that result by height. That’s a much larger number, and the wrong answer.

bmiMetric() uses Doubles for the weight and height. A Double holds very large and very small floating-point numbers.

Here’s a version using English units, represented by Int parameters:

// NumberTypes/BMIEnglish.kt

fun bmiEnglish(
  weight: Int,
  height: Int
): String {
  val bmi =
    weight / (height * height) * 703.07 // [1]
  return if (bmi < 18.5) "Underweight"
    else if (bmi < 25) "Normal weight"
    else "Overweight"
}

fun main() {
  val weight = 160
  val height = 68
  val status = bmiEnglish(weight, height)
  println(status)
}
/* Output:
Underweight
*/

Why does the result differ from bmiMetric(), which uses Doubles? When you divide an integer by another integer, Kotlin produces an integer result. The standard way to deal with the remainder during integer division is truncation, meaning “chop it off and throw it away” (there’s no rounding). So if you divide 5 by 2 you get 2, and 7/10 is zero. When Kotlin calculates bmi in expression [1], it divides 160 by 68 * 68 and gets zero. It then multiplies zero by 703.07 to get zero.

To avoid this problem, move 703.07 to the front of the calculation. The calculations are then forced to be Double:

val bmi = 703.07 * weight / (height * height)

The Double parameters in bmiMetric() prevent this problem. Convert computations to the desired type as early as possible to preserve accuracy.

All programming languages have limits to what they can store within an integer. Kotlin’s Int type can take values between -2³¹ and +2³¹-1, a constraint of the Int 32-bit representation. If you sum or multiply two Ints that are big enough, you’ll overflow the result:

// NumberTypes/IntegerOverflow.kt

fun main() {
  val i: Int = Int.MAX_VALUE
  println(i + i)
}
/* Output:
-2
*/

Int.MAX_VALUE is a predefined value which is the largest number an Int can hold.

The overflow produces a result that is clearly incorrect, as it is both negative and much smaller than we expect. Kotlin issues a warning whenever it detects a potential overflow.

Preventing overflow is your responsibility as a developer. Kotlin can’t always detect overflow during compilation, and it doesn’t prevent overflow because that would produce an unacceptable performance impact.

If your program contains large numbers, you can use Longs, which accommodate values from -2⁶³ to +2⁶³-1. To define a val of type Long, you can specify the type explicitly or put L at the end of a numeric literal, which tells Kotlin to treat that value as a Long:

// NumberTypes/LongConstants.kt

fun main() {
  val i = 0          // Infers Int
  val l1 = 0L        // L creates Long
  val l2: Long = 0   // Explicit type
  println("$l1 $l2")
}
/* Output:
0 0
*/

By changing to Longs we prevent the overflow in IntegerOverflow.kt:

// NumberTypes/UsingLongs.kt

fun main() {
  val i = Int.MAX_VALUE
  println(0L + i + i)              // [1]
  println(1_000_000 * 1_000_000L)  // [2]
}
/* Output:
4294967294
1000000000000
*/

Using a numeric literal in both [1] and [2] forces Long calculations, and also produces a result of type Long. The location where the L appears is unimportant. If one of the values is Long, the resulting expression is Long.

Although they can hold much larger values than Ints, Longs still have size limitations:

// NumberTypes/BiggestLong.kt

fun main() {
  println(Long.MAX_VALUE)
}
/* Output:
9223372036854775807
*/

Long.MAX_VALUE is the largest value a Long can hold.

Exercises and solutions can be found at www.AtomicKotlin.com.