Kotlin, Theory

Kotlin. Functions with a single expression. Unit type

Functions with a single expression. Unit type. Functions that return the Unit type. Examples

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1. General concepts about using functions with a single expression

A function body can contain any number of operators. However, there are cases when a function contains one expression (one line). If a function contains one expression, then in the Kotlin language it can be styled in a special way. The general form of a function declaration with a single expression is as follows

fun FuncName(parameters) = expression

here

  • FuncName – the name of function;
  • parameters – list of function parameters;
  • expression – an expression that defines the body of the function.

If several expressions (operators) need to be executed in the body of a function, then the usual syntax for declaring a function is used.

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2. Examples of functions with a single expression

Example 1. A function that calculates the modulus of a complex number. The function receives as parameters the real (re) and imaginary (im) parts of a complex number.

// A function that calculates the modulus of a complex number
fun AbsComplex(re:Double, im:Double) =
    Math.sqrt(re*re+im*im)

fun main(args:Array<String>)
{
  // Demonstration of using the AbsComplex() function
  var module : Double
  var re : Double
  var im : Double

  print("re = ")
  re = readLine().toString().toDouble()
  print("im = ")
  im = readLine().toString().toDouble()

  module = AbsComplex(re,im)
  print("module = " + module)
}

Test example

re = 3
im = 4
module = 5.0

Example 2. A function that calculates the volume of a sphere based on a given radius.

// A function that determines the volume of a sphere by a given radius
fun Volume(radius:Double) =
    4.0/3 * Math.PI * Math.pow(radius, 3.0)

fun main(args:Array<String>)
{
  // Demonstration of using the Volume() function
  var volume : Double
  var radius : Double

  print("radius = ")
  radius = readLine().toString().toDouble()

  volume = Volume(radius)
  println("volume = " + volume)
}

Test example

radius = 2.5
volume = 65.44984694978736

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3. An example of using a function with a single expression combined with the when statement. Calculate the number of days in a month

Task.

Develop a function that determines the number of days in this month by the month number (1..12) and year number.

Solution.

// Function that determines the number of days in a month
fun GetNDays(month : Int, year : Int) =
  when (month) {
    4, 6, 9, 11 -> 30
    1, 3, 5, 7, 8, 10, 12 -> 31
    2 -> {
      if (year % 400 == 0) 29
      else
      if (year % 100 == 0) 28
      else
      if (year % 4 == 0) 29
      else
        28
    }
    else -> -1
  }

fun main(args:Array<String>)
{
  // Demonstration of using the GetNDays() function

  // 1. Input data
  print("year = ")
  val year = readLine().toString().toInt()
  print("month = ")
  val month = readLine().toString().toInt()

  // 2. Calling the GetNDays() function
  val nDays = GetNDays(month, year)

  // 3. Display the result
  if (nDays==-1)
    println("Incorrect input.")
  else
    println("days = " + nDays)
}

Test example

year = 2008
month = 2
days = 29

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4. An example of using a function with a single expression in combination with the if statement

Task. A positive number between 0 and 999999 is specified. Develop a function with a single expression that calculates the number of digits in a number.

Solution.

// Function that returns the number of digits in a number
fun GetDigits(number : Int) =
  if (number < 0) -1
  else
  if (number<10) 1
  else
  if (number<100) 2
  else
  if (number<1000) 3
  else
  if (number<10000) 4
  else
  if (number<100000) 5
  else -1

fun main(args:Array<String>)
{
  // Demonstration of using the GetDigits() function

  // 1. Input number
  print("number = ")
  val number = readLine().toString().toInt()

  // 2. Invoke the GetDigits() function
  val nDigits = GetDigits(number)

  // 3. Display the result
  if (nDigits==-1)
    println("Incorrect input")
  else
    println("nDigits = " + nDigits)
}

Test example

number = 5564
nDigits = 4

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5. The Unit type. Functions that return the Unit type

If a function does not return a value, then in Kotlin this function is defined such that it returns the Unit type. If the return keyword is not used in the function body, then the function is considered to return a value of the Unit type.

In Kotlin, the Unit type indicates a function that does not return anything, but can be applied to generic functions. Returning at least some type is required in generic functions.

In the most general case, a function that returns the Unit type has the form

fun FuncName(parameters) : Unit {
  // ...
}

here

  • FuncName – the name of function;
  • parameters – function parameters.

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6. Examples of functions that return the Unit type
6.1. Display the binary representation of a decimal number

Task. Design a function that takes an integer and displays the binary representation of that number.

Solution.

// A function that returns the Unit type.
// A function that outputs the binary equivalent of an integer.
fun Print_10_to_2(number : Int) : Unit {
  var t = number
  var s = ""

  while (t>0) {
    s = (t%2).toString() + s
    t = t/2
  }

  println(number.toString() + " => " + s)
}

fun main(args:Array<String>)
{
  // Demonstration of using the Print_10_to_2() function

  // 1. Input a number
  print("number = ")
  val number = readLine().toString().toInt()

  // 2. Invoke the function
  Print_10_to_2(number)
}

Test example

number = 78
78 => 1001110

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6.2. A function that prints a number in reverse order (1234 => 4321)

Task. Develop a function that prints an integer in reverse order.

Solution.

// A function that returns the Unit type.
// The function displays the number in reverse order
fun Print_Reverse_Number(number : Int) : Unit {
  var t = number
  var s = ""

  while (t>0) {
    s += (t%10).toString()
    t = t/10
  }

  println(number.toString() + " => " + s)
}

fun main(args:Array<String>)
{
  // Demonstration of using the Print_Reverse_Number() function
  // 1. Input number
  print("number = ")
  val number = readLine().toString().toInt()

  // 2. Invoke the function
  Print_Reverse_Number(number)
}

Test example

number = 1234
1234 => 4321

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Related topics

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