Kotlin Numbers and Operators

Calculate with Kotlin's arithmetic and comparison operators, avoid integer division surprises, and use the math functions from kotlin.math.

Lesson 3 of 43Beginner17 min read

Almost every program does some arithmetic: adding up a basket, working out a percentage, converting minutes into hours, picking a random number for a game. Kotlin uses the familiar symbols +, -, * and /, plus a few more you'll use constantly.

Like Java, Kotlin is strict about number types, and that affects results. Dividing one Int by another drops the decimal part, so 7 / 2 is 3. Mixing an Int with a Double gives a Double. Knowing these rules means your calculations come out right first time.

In this lesson you'll use the arithmetic operators, update variables with shortcuts such as +=, compare values and combine conditions, call math functions such as sqrt and round, generate random numbers, and format results to a fixed number of decimal places.

Arithmetic operators

OperatorMeaningInt exampleDouble example
+Add7 + 2 is 97.5 + 2 is 9.5
-Subtract7 - 2 is 57.5 - 2 is 5.5
*Multiply7 * 2 is 142.5 * 3 is 7.5
/Divide7 / 2 is 37.0 / 2 is 3.5
%Remainder7 % 2 is 17.5 % 2 is 1.5

When both sides are Int, / performs integer division and drops the decimals. Make one side a Double to keep them: 7.0 / 2 or total.toDouble() / count. The remainder operator % answers questions like "is this even?" (n % 2 == 0).

Shortcuts and precedence

Kotlin has shorthand for updating a var:

ShortcutSame as
x += 5x = x + 5
x -= 5x = x - 5
x *= 2, x /= 2, x %= 3and so on
x++ / x--add or subtract 1

Operators follow normal maths order: brackets first, then *, / and %, then + and -. So 2 + 3 * 4 is 14, while (2 + 3) * 4 is 20. Add brackets whenever they make your intention clearer.

Comparison and logic

Comparisons produce a Boolean: == (equal), != (not equal), <, >, <= and >=. In Kotlin, == compares values, even for strings, so "cat" == "cat" is true.

Combine conditions with && (and), || (or) and ! (not):

Kotlin
val age = 20
val hasTicket = true
println(age >= 18 && hasTicket)   // true
println(!hasTicket)               // false

&& and || stop early: if the left side of && is false, the right side isn't even checked.

Math functions, random numbers and formatting

Import what you need from kotlin.math:

  • sqrt(16.0) is 4.0, abs(-7) is 7, and 2.0.pow(10) is 1024.0.
  • round(2.5) rounds to the nearest whole number (ties go to the even neighbour, so 2.0), ceil rounds up and floor rounds down.
  • maxOf(a, b) and minOf(a, b) pick the larger or smaller value.

For random whole numbers, (1..6).random() picks one from the range. To show a fixed number of decimals, use "%.2f".format(price), which turns 3.5 into "3.50".

Rounding: round, roundToInt and formatting

Kotlin has several ways to round, and they don't all behave the same at exactly .5:

Kotlin
import kotlin.math.round
import kotlin.math.roundToInt

println(round(2.5))          // 2.0, rounds a tie to the even neighbour
println(2.5.roundToInt())    // 3, rounds a tie upwards
println(3.7.toInt())         // 3, simply cuts off the decimals
println("%.2f".format(3.14159))   // 3.14, for display only

round uses "round half to even", which avoids bias when many values are rounded, so round(2.5) is 2.0 and round(3.5) is 4.0. roundToInt() returns an Int and rounds halves up. For display, format the number instead of changing it, and for money use java.math.BigDecimal, where BigDecimal("0.10") + BigDecimal("0.20") is exactly 0.30.

Kotlin Operators Syntax

Operators

a + b   a - b   a * b   a / b   a % b
x += 1   x++   x--
a == b   a != b   a < b   a >= b
cond1 && cond2   cond1 || cond2   !cond
import kotlin.math.*
sqrt(x)  abs(x)  x.pow(2)  round(x)  ceil(x)  floor(x)
(1..6).random()
"%.2f".format(price)

Kotlin Operators Summary

  • / between two Ints drops the decimals; make one side a Double to keep them.
  • % gives the remainder; +=, -=, ++ and -- update a var.
  • ==, !=, <, > compare values; &&, || and ! combine Booleans.
  • kotlin.math provides sqrt, abs, pow, round, ceil and floor.
  • (1..6).random() gives a random number; "%.2f".format(x) formats decimals.

Kotlin Operators Examples

Each example shows the code, the output it produces and what each part does.

Example 1

Minutes, hours and even numbers

Kotlin
fun main() {
    val minutes = 135
    val hours = minutes / 60
    val left = minutes % 60
    println("$minutes minutes = $hours h $left min")

    for (n in 1..6) {
        if (n % 2 == 0) println("$n is even")
    }
}
Output
135 minutes = 2 h 15 min
2 is even
4 is even
6 is even
Line-by-line explanation
  1. Line 3Integer division gives the whole hours.
  2. Line 4The remainder is the minutes left over.
  3. Line 5$name inside a string inserts the variable's value, as the strings lesson explains.
  4. Line 8A number is even when dividing by 2 leaves no remainder.
Example 2

Averages and decimals

Kotlin
fun main() {
    val total = 17
    val count = 4

    println(total / count)
    println(total.toDouble() / count)
    println("%.1f".format(total.toDouble() / count))
    println(maxOf(total, 20))
}
Output
4
4.25
4.3
20
Line-by-line explanation
  1. Line 5Both are Ints, so the .25 is lost.
  2. Line 6Converting one side to Double keeps the decimals.
  3. Line 7Formatting to one decimal place rounds 4.25 up to 4.3.
  4. Line 8maxOf returns the larger of the two values.
Example 3

Math functions and a dice roll

Kotlin
import kotlin.math.pow
import kotlin.math.sqrt
import kotlin.math.abs

fun main() {
    var score = 0
    score += 50
    score *= 2
    score -= 15
    score++
    println("Score: $score")

    println(sqrt(144.0))
    println(2.0.pow(8))
    println(abs(-42))

    val roll = (1..6).random()
    println("You rolled $roll")
}
Output
Score: 86
12.0
256.0
42
You rolled 4
Line-by-line explanation
  1. Lines 7 to 100, then 50, 100, 85 and finally 86.
  2. Lines 13 to 14sqrt and pow work with Doubles and return Doubles.
  3. Line 17random picks any number from 1 to 6, so your roll may differ.

Common Kotlin Operators Mistakes

Integer division by accident

Don't
val half: Double = 1 / 2       // Type mismatch: Int can't be assigned to Double
Do
val half = 1.0 / 2             // 0.5

Expecting toInt to round

Don't
val stars = 4.7.toInt()                    // 4
Do
val stars = kotlin.math.round(4.7).toInt() // 5

Using = instead of == in a condition

Don't
if (score = 100) println("Perfect")      // error: assignments are not expressions
Do
if (score == 100) println("Perfect")

Kotlin refuses to compile the first version, which prevents a classic bug.

Kotlin Operators in Real Projects

Numbers in Kotlin apps

Shopping apps calculate totals and taxes, fitness apps convert distances and paces, and games roll dice and move characters with these operators every frame. For money, professional apps often store amounts as whole pence or cents in a Long, or use Java's BigDecimal, to avoid the tiny rounding errors of Double.

Kotlin Operators FAQ

Does Kotlin have a power operator?

No. Use the pow function from kotlin.math: 2.0.pow(10) is 1024.0. It works on Double and Float, so convert an Int first with toDouble(), and convert the result back with toInt() if you need a whole number.

Why does -7 % 3 give -1 in Kotlin?

The % operator gives the remainder, which takes the sign of the left-hand number. If you need a result between 0 and the divisor, for example to wrap around a circular list, use mod: (-7).mod(3) is 2.

How do I generate a random number in Kotlin?

Call random() on a range: (1..6).random() gives a dice roll from 1 to 6. Random.nextInt(1, 7) from kotlin.random does the same, but its upper bound is exclusive. For repeatable results in tests, create a seeded generator: Random(42).

How do I detect integer overflow?

Ordinary Int arithmetic wraps silently, so Int.MAX_VALUE + 1 becomes a large negative number. On the JVM, Math.addExact(a, b) and Math.multiplyExact(a, b) throw an ArithmeticException with the message "integer overflow" instead. Using Long gives a much larger range.

Why is 0.1 + 0.2 not exactly 0.3?

Double stores numbers in binary, and 0.1 can't be represented exactly, so the sum prints as 0.30000000000000004. Format results for display, compare decimals with a small tolerance, and use BigDecimal or whole cents for money.

Kotlin Operators Exercises

Answer each one and check it instantly. If you get stuck, open the hint before the answer.

Fill in the blankseasy

Complete the code so it prints the remainder of 17 divided by 5.

println(17  5)
Predict the outputeasy

What does println(9 / 2) print?

Predict the outputmedium

What does this print?

var x = 10
x -= 3
x *= 2
println(x)
Multiple choicemedium

Which expression is true only when age is from 13 to 19?

Multiple choicehard

How do you show 2.5 as "2.50"?

Kotlin Operators Quiz

5 questions to check what you learned.

  1. What is 2 + 3 * 4 in Kotlin?

    • 20
    • 14
    • 24
    • 9

  2. What does == compare for two strings in Kotlin?

    • Their memory addresses
    • Their lengths only
    • Nothing, it's an error
    • Their text values

  3. What does (1..6).random() return?

    • Always 1
    • A random Int from 1 to 6, inclusive
    • A random Double
    • A list of six numbers

  4. Which type should you use for a value like 5,000,000,000?

    • Int
    • Char
    • Boolean
    • Long

  5. What does true || println("hi") == Unit evaluate, and is "hi" printed?

    • true, and hi is not printed
    • false, and hi is printed
    • true, and hi is printed
    • An error

Related Kotlin topics

02Variables & TypesStore values with val and var, understand Kotlin's basic types, let the compiler infer types, and convert between them.06if & whenMake decisions with if, else if and else, use if as an expression that returns a value, and replace long chains with when.04StringsWork with text in Kotlin: string templates, length and indexes, common methods like uppercase, trim and replace, substrings, splitting and multi-line raw strings.
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