Kotlin makes it possible to assign a value to a parameter in the function declaration. If the function is invoked without passing a value, then the compiler automatically assigns a default value to it. The hello function prints Hello Kotlin if no value is passed to the function:

fun hello(message : String = "Kotlin") : Unit{
     println("Hello $message")
}
 
fun main (args: Array<String>) {    hello()
     hello("World")
}

The default argument is a very helpful feature in many situations. For example, consider a situation in which we are writing a currency exchange function that converts dollars into another currency and applies service charges on the conversion:

fun currencyExchange(dollar: Double, currencyRate: Double, charges: Double): Double {
      var total = dollar * currencyRate
      var fees = total * charges / 100
      total = total - fees
      return total
 } 

Let's assume that we want to convert 100 US dollars to Swedish krona. 1 dollar is equal to 10 Swedish krona, and our company charges 5% on the total amount:

fun main (args: Array<String>) {
     var total = currencyExchange(100.0,10.0, 5.0)
     println(total)
}

This function works fine; it multiplies the dollar by the target currency, calculates the charges, and returns the total amount after deductions. In the currency market, currency prices move quite rapidly, so it is a good idea to check the currency rate before conversion. However, its highly likely that conversion charges (5% in this example) will remain the same for a long period of time. If this is true, then the default value can be assigned to the charges variable, as follows:

fun currencyExchange(dollar: Double, currencyRate: Double, charges: Double = 5.0): Double {
     var total = dollar * currencyRate
     var fees = total * charges / 100
     total = total - fees
     return total
 }

By setting the default value, the function can be invoked without a third parameter:

fun main (args: Array<String>) {
    var total = currencyExchange(100.0,10.0)
    println(total)     
    
    var total = currencyExchange(100.0,10.0, 3.0)
    println(total)
}