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Kotlin Numbers

Numbers are values that Kotlin can add, subtract, multiply, and divide. Whole numbers and decimal numbers follow different rules when a division leaves a part over.

Int arithmetic

The four signs work on whole numbers like plain maths. Each print shows the result of one sign.

fun main() {
    val a = 12
    val b = 4
    println(a + b)
    println(a - b)
    println(a * b)
    println(a / b)
}

Int division drops the leftover part

Dividing whole numbers gives a whole number back. The result moves toward zero, so -7 / 2 gives -3 and not -4.

fun main() {
    println(7 / 2)
    println(-7 / 2)
}

Remainder with %

The % sign gives what is left after a division. A negative first number keeps its sign in the leftover.

fun main() {
    println(17 % 5)
    println(10 % 2)
    println(-5 % 3)
}

Double arithmetic keeps decimals

One value with a decimal point makes the calculation decimal. The last line shows exactly what the computer prints for that decimal sum.

fun main() {
    println(7.0 / 2)
    println(2.5 * 4)
    println(0.1 + 0.2)
}

Changing a stored value

The signs +=, -=, and *= change a stored value in place. The sign ++ at the end adds one more.

fun main() {
    var score = 10
    score += 5
    println(score)
    score -= 3
    println(score)
    score *= 2
    println(score)
    score++
    println(score)
}

Example

fun main() {
    val price = 4
    val count = 3
    println(price * count)
    var change = 20 - price * count
    println(change)
    println(change % 3)
    println(change / 2.0)
    change++
    println(change)
}

Key points

  • Int arithmetic: whole numbers work with +, -, *, and / like plain maths.
  • Int division drops the part after the point and moves toward zero.
  • Remainder: % gives what is left after one number is divided by another.
  • Double arithmetic keeps decimal places in the result.
  • Compound assignment: +=, -=, and *= change a stored value and keep the new value.
  • ++ adds one to a stored value.

In this example, price is 4 and count is 3, so the first print shows the total cost 12. The change from 20 is 8. The remainder of 8 % 3 is 2, and 8 divided by 2.0 gives the decimal 4.0. The last line adds one to change and shows 9.

Note

A decimal sum can print with many digits after the point because a computer stores decimal values in a fixed space.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

What does println(9 / 2) print?