Function Implementations in Kotlin
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Basic functions
Infix functions

There are number of diffrent functions in kotlin which varies according to their syntaxes. Let's take them one by one.

Default Parameter Values and Named Arguments
fun printMessage(message: String): Unit {                    
	
   println(message)
   
}

This is a simple function that takes a parameter of type String and returns Unit (In kotlin Unit is datatype which return no value).


fun printMessageWithPrefix(message: String, prefix: String = "Info") {  

    println("[$prefix] $message")
    
}

A function that takes a second optional parameter prefix with default value Info. The return type is omitted, meaning that it's actually Unit.


fun sum(x: Int, y: Int): Int {                                

    return x + y
}

A function that returns an integer value. this function is also can be written as :

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

A single-expression function that returns an integer (inferred).


Now, lets write a main function and call above function frome there :

fun main() {
    printMessage("Hello")                                         //1
    printMessageWithPrefix("Hello", "Log")                        //2     
    printMessageWithPrefix("Hello")                               //3
    printMessageWithPrefix(prefix = "Log", message = "Hello")     //4     
    println(sum(1, 2))                                            //5     
}

1. Calls the first function with the argument Hello.
2. Calls the function with two parameters, passing values for both of them.
3. Calls the same function omitting the second one. The default value Info is used.
4. Calls the same function using named arguments and changing the order of the arguments.
5. Prints the result of a function call sum(1, 2).

Infix Functions

Functions marked with the infix keyword can also be called using the infix notation (omitting the dot and the parentheses for the call). Infix functions must satisfy the following requirements:

  • They must be member functions or extension functions;
  • They must have a single parameter;
  • The parameter must not accept variable number of arguments and must have no default value.
infix fun Int.shl(x: Int): Int { ... }

// calling the function using the infix notation
1 shl 2

// is the same as
1.shl(2)

Note that infix functions always require both the receiver and the parameter to be specified. When you're calling a method on the current receiver using the infix notation, you need to use this explicitly; unlike regular method calls, it cannot be omitted.

Practical Approach to Infix Function

Member functions and extensions with a single parameter can be turned into infix functions.

infix fun Int.times(str: String) = str.repeat(this)          //1

println(2 times "Bye ")   //2

1. Defines an infix extension function on Int.
2. Calls the infix function.
Above code print Bye Bye in output console


val pair = "Ferrari" to "Katrina"                        // 3

println(pair)

Creates a Pair by calling the infix function to from the standard library.

  infix fun String.onto(other: String) = Pair(this, other)   //4
  val myPair = "McLaren" onto "Lucas"                        //5
  println(myPair)

Here's your own implementation of to creatively called onto.

Operator Functions

Certain functions can be "upgraded" to operators, allowing their calls with the corresponding operator symbol. Functions that overload operators need to be marked with the operator modifier as bellow :

operator fun Int.times(str: String) = str.repeat(this)       // 1

println(2 * "Bye ")

This takes the infix function from above one step further using the operator modifier. The operator symbol for times() is * so that you can call the function using 2 * "Bye".


operator fun String.get(range: IntRange) = substring(range)  

val str = "Always forgive your enemies; nothing annoys them so much."

println(str[0..14]) 

An operator function allows easy range access on strings. The get() operator enables bracket-access syntax. Here, output will be generated by substring from 0 to 14 as Always forgive from given str string.