Kotlin Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

Functions are used to perform certain actions, and they are also known as methods.

Predefined Functions

So it turns out you already know what a function is. You have been using it the whole time through this tutorial!

For example, println() is a function. It is used to output/print text to the screen:

fun main() {
  println("Hello World")
}

Create Your Own Functions

To create your own function, use the fun keyword, and write the name of the function, followed by parantheses ():

fun myFunction() {
  println("I just got executed!")
} 

Call a Function

Now that you have created a function, you can execute it by calling it.

To call a function in Kotlin, write the name of the function followed by two parantheses ().

In the following example, myFunction() will print some text (the action), when it is called:

fun main() {
  myFunction() // Call myFunction
}

// Outputs "I just got executed!" 

A function can be called multiple times if you want:

fun main() {
  myFunction()
  myFunction()
  myFunction()
}

// I just got executed!
// I just got executed!
// I just got executed! 

Function Parameters

Information can be passed to functions as parameter.

Parameters are specified after the function name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma. Just note that you must specify the type of each parameter (Int, String, etc).

The following example has a function that takes a String called fname as parameter. When the function is called, we pass along a first name, which is used inside the function to print the full name:

fun myFunction(fname: String) {
  println(fname + " Doe")
}

fun main() {
  myFunction("John")
  myFunction("Jane")
  myFunction("George")
}

// John Doe
// Jane Doe
// George Doe 

Multiple Parameters

You can have as many parameters as you like:

fun myFunction(fname: String, age: Int) {
  println(fname + " is " + age)
}

fun main() {
  myFunction("John", 35)
  myFunction("Jane", 32)
  myFunction("George", 15)
}

// John is 35
// Jane is 32
// George is 15 

Return Values

In the examples above, we used functions to output a value. In the following example, we will use a function to return a value and assign it to a variable.

To return a value, use the return keyword, and specify the return type after the function's parantheses (Int in this example):

fun myFunction(x: Int): Int {
  return (x + 5)
}

fun main() {
  var result = myFunction(3)
  println(result)
}

// 8 (3 + 5) 

Using two parameters:

fun myFunction(x: Int, y: Int): Int {
  return (x + y)
}

fun main() {
  var result = myFunction(3, 5)
  println(result)
}

// 8 (3 + 5) 

Shorter Syntax for Return Values

There is also a shorter syntax for returning values. You can use the = operator instead of return without specifying the return type. Kotlin is smart enough to automatically find out what it is:

fun myFunction(x: Int, y: Int) = x + y

fun main() {
  var result = myFunction(3, 5)
  println(result)
}

// 8 (3 + 5)