# Operator Functions

Learn how to define your own operators, which limitations Kotlin poses on such operators, and how they lead to succinct code.

## We'll cover the following

Operator functions allow you to define the meaning of well-known symbols such as `+`

and `-`

for your own types.

## Introducing Operators #

Kotlin’s operators allow calling functions using certain symbols. For instance, the infix `+`

operator actually calls a `plus`

function so that `a + b`

translates to `a.plus(b)`

under the hood.

Many operators use infix notation and therefore can be seen as an extension of infix functions. Instead of a function name, operator functions instead use an *operator symbol*. However, there are also operators using prefix and postfix notation:

- Prefix:
`+a`

,`-a`

,`!a`

- Infix:
`a * b`

,`a..b`

,`a == b`

, … - Postfix:
`a[i]`

,`a()`

Kotlin has a predefined list of possible operators you can use (in contrast to languages like Scala or Haskell).

## Important Kotlin Operators #

Note:Some of the examples below require a custom operator definition to run. For instance,`"Coffee" * 3`

doesn’t work out of the box because Kotlin provides no such operator implementation.

Function Name Operator Example Equivalent Arithmetic Operators`plus`

`+`

`2 + 3`

`2.plus(3)`

`minus`

`-`

`"Kotlin" – "in"`

`"Kotlin".minus("in")`

`times`

`*`

`"Coffee" * 2`

`"Coffee".times(2)`

`div`

`/`

`7.0 / 2.0`

`(7.0).div(2.0)`

`rem`

`%`

`"android" % 'a'`

`"android".rem('a')`

Assignment Operators`plusAssign`

`+=`

`x += "Island"`

`x.plusAssign`

(“island”)`minusAssign`

`-=`

`x -= 1.06`

`x.minusAssign(1.06)`

`timesAssign`

`*=`

`x *= 3`

`x.timesAssign(3)`

`divAssign`

`/=`

`x /= 2`

`x.divAssign(2)`

`remAssign`

`%=`

`x %= 10`

`x.remAssign(10)`

Miscellaneous Operators`inc`

`++`

`grade++`

`grade.inc()`

`dec`

`--`

`'c'--`

`'c'.dec()`

`contains`

`in`

`"A" in list`

`list.contains("A")`

`rangeTo`

`..`

`now..then`

`now.rangeTo(then)`

`get`

`[…]`

`skipList[4]`

`skipList.get(4)`

`set`

`[…] = …`

`array[5] = 42`

`array.set(5, 42)`

For a full list of all operators, check out the Kotlin documentation.

## Implementing an Operator Function #

You can define the meaning of any of the allowed operators for any of your own types. **The declaration is similar as for infix functions, except that you use the operator modifier**:

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