#Introduction to Kotlin

#main()

fun main() {
      println("Greetings, CareerInEdu.com!")
      // Code goes here
    }

The main() function is the starting point of every Kotlin program and must be included in the code before execution

println("Greetings, earthling!")
    print("Take me to ")
    print("your leader.")
    
    /*
    Print:
    Greetings, earthling!
    Take me to your leader.
    */

#Notes

// this is a single line comment
    
    /*
    this
    note
    for
    many
    */

#Execution order

fun main() {
      println("I will be printed first.")
      println("I will be printed second.")
      println("I will be printed third.")
    }

#Data Types and Variables

#Mutable variables

var age = 25
    age = 26

#Immutable variables

val goldenRatio = 1.618

#Type inference

// The following variables are assigned a literal value inside double quotes
    // so the inferred type is String
    
    var color = "Purple"

#String concatenation

var streetAddress = "123 Main St."
    var cityState = "Brooklyn, NY"
    
    println(streetAddress + " " + cityState)
    // Print: 123 Main St. Brooklyn, NY

#String Templates

var address = "123 Main St."
    println("The address is $address")
    // prints: The address is 123 Main St.

#Built-in Properties and Functions

var monument = "the Statue of Liberty"
    
    println(monument. capitalize())
    // print: The Statue of Liberty
    println(monument. length)
    // print: 21

#Character escape

print("\"Excellent!\" I cried. \"Elementary,\" said he.")
    
    // Print: "Excellent!" I cried. "Elementary," said he.
  • \n insert new line
  • \t inserts a tab
  • \r inserts carriage return
  • \' inserts a single quote
  • \" inserts a double quote
  • \\ inserts a backslash
  • \$ inserts a dollar sign

#Arithmetic Operators

5 + 7  // 12
    9 -2   // 7
    8 *4   // 32
    25 /5  // 5
    31 % 2 // 1

+ addition, - subtraction, * multiplication, / division, and % modulus

#Order of operations

5 + 8 *2 /4 -3 // 6
    3 + (4 + 4) /2 // 7
    4 *2 + 1 *7    // 15
    3 + 18 /2 *1   // 12
    6 -3 % 2 + 2   // 7

#Enhanced assignment operator

var batteryPercentage = 80
    
    // long syntax
    batteryPercentage = batteryPercantage + 10
    
    // short syntax with augmented assignment operator
    batteryPercentage += 10

#Increment and decrement operators

var year = 2019
    year++   // 2020
    year--   // 2019

#Math library

Math.pow(2.0, 3.0) // 8.0
    Math.min(6, 9)     // 6
    Math.max(10, 12)   // 12
    Math. round(13.7)  // 14

#Conditional Expression

#If expression

var morning = true
    
    if (morning) {
      println("Rise and shine!")
    }
    // Print: Rise and shine!

#Else-expression

var rained = false
    
    if (rained) {
      println("No need to water the plants today.")
    } else {
      println("The plant needs to be watered!")
    }
    // print: The plant needs watering!

#Else-If expressions

var age = 65
    
    if (age < 18 ) {
      println("You are considered a minor")
    } else if (age < 60) {
      println("You are considered an adult")
    } else {
      println("You are considered senior")
    }
    
    // print: you are considered senior

#Comparison Operators

var myAge = 19
    var sisterAge = 11
    var cousinAge = 11
    
    myAge > sisterAge  // true
    myAge < cousinAge  // false
    myAge >= cousinAge // true
    myAge <= sisterAge // false

#Logical Operators

var humid = true
    var raining = true
    var jacket = false
    
    println(!humid)
    // print: false
    println(jacket && raining)
    // print: true
    println(humid || raining)
    // print: true

#AND operator: &&

var humid = true
    var raining = true
    var shorts = false
    var sunny = false
    
    // true AND true
    println(humid && raining) // true
    // true AND false
    println(humid && shorts)  // false
    // false AND true
    println(sunny && raining) // false
    // false AND false
    println(shorts && sunny)  // false

#Or operator:||

var late = true
    var skipBreakfast = true
    var underslept = false
    var checkEmails = false
    
    // true OR true
    println(skipBreakfast || late) // true
    // true OR false
    println(late || checkEmails)   // true
    // false OR true
    println(underslept || late)    // true
    // false OR false
    println(checkEmails || underslept) // false

#NOT operator

var hungry = true
    var full = false
    
    println(!hungry) // false
    println(!full)   // true

#Evaluation order

!true && (false || true) // false
    /*
    (false || true) is evaluated first to return true.
    Then, evaluate !true && true and return the final result false
    */
    
    !false && true || false // true
    /*
    !false is evaluated first to return true.
    Then true && true is evaluated, returning true.
    then, true || evaluates to false and eventually returns true
    */

#Nested conditions

var studied = true
    var well Rested = true
    
    if (wellRested) {
      println("Good luck today!")
      if (studied) {
        println("You should prepare for the exam!")
      } else {
        println("Spend a few hours studying before the exam!")
      }
    }
    
    // Print: Good luck today!
    // print: You should be ready for the exam!

#When expression

var grade = "A"
    
    when (grade) {
      "A" -> println("Great job!")
      "B" -> println("Great job!")
      "C" -> println("You passed!")
      else -> println("Close! Be sure to prepare more next time!")
    }
    // print: Great job!

#Range operator

var height = 46 // inches
    
    if (height in 1..53) {
      println("Sorry, you must be at least 54 inches to ride the coaster")
    }
    // Prints: Sorry, you must be at least 54 inches to ride the roller coaster

#Equality Operators

var myAge = 22
    var sisterAge = 21
    
    myAge == sisterAge // false
    myAge !== sisterAge // true

#Collections

#Immutable list

var programmingLanguages = listOf("C#", "Java", "Kotlin", "Ruby")

#Mutable List

var fruits = mutableListOf("Orange", "Apple", "Banana", "Mango")

#Access List

var cars = listOf("BMW", "Ferrari", "Volvo", "Tesla")
    
    println(cars[2]) // Prints: Volvo

#Size Attribute

var worldContinents = listOf("Asia", "Africa", "North America", "South America", "Antarctica", "Europe", "Australia")
    
    println(worldContinents.size) // Prints: 7

#List Manipulation

var seas = listOf("Black Sea", "Caribbean Sea", "North Sea")
    println(seas. contains("North Sea")) // Prints: true
    
    // The contains() function performs a read operation on any list and determines if the element exists
    seas.add("Baltic Sea") // Error: cannot write to immutable list
    // The add() function can only be called on mutable lists, so the code above throws an error

#Immutable Sets

var primaryColors = setOf("Red", "Blue", "Yellow")

#Mutable Sets

var womenInTech = mutableSetOf("Ada Lovelace", "Grace Hopper", "Radia Perlman", "Sister Mary Kenneth Keller")

#Access Collection Elements

var companies = setOf("Facebook", "Apple", "Netflix", "Google")
    
    println(companies.elementAt(3))
    // Prints: Google
    println(companies.elementAt(4))
    // Returns and Error
    println(companies.elementAtOrNull(4))
    // Prints: null

#Immutable Map

var averageTemp = mapOf("winter" to 35,  "spring" to 60,  "summer" to 85, "fall" to 55)

#Mutable Mapping

var europeanDomains = mutableMapOf("Germany" to "de", "Slovakia" to "sk", "Hungary" to "hu", "Norway" to "no")

#Retrieve map keys and values

var oscarWinners = mutableMapOf("Parasite" to "Bong Joon-ho", "Green Book" to "Jim Burke", "The Shape Of Water" to "Guillermo del Toro")
    
    println(oscarWinners.keys)
    // Prints: [Parasite, Green Book, The Shape Of Water]
    
    println(oscarWinners.values)
    // Prints: [Bong Joon-ho, Jim Burke, Guillermo del Toro]
    println(oscarWinners["Parasite"])
    // Prints: Bong Joon-ho

#Add and remove map entries

var worldCapitals = mutableMapOf("United States" to "Washington D.C.", "Germany" to "Berlin", "Mexico" to "Mexico City", "France" to "Paris")
    
    worldCapitals.put("Brazil", "Brasilia")
    println(worldCapitals)
    // Prints: {United States=Washington D.C., Germany=Berlin, Mexico=Mexico City, France=Paris, Brazil=Brasilia}
    
    worldCapitals.remove("Germany")
    println(worldCapitals)
    // Prints: {United States=Washington D.C., Mexico=Mexico City, France=Paris, Brazil=Brasilia}

#Function

#Function

fun greet() {
      println("Hey there!")
    }
    
    fun main() {
      //Function call
      greet() //Prints: Hey there!
    }

#Function Parameters

fun birthday(name: String, age: Int) {
       println("Happy birthday $name! You turn $age today!")
    }
    
    fun main() {
      birthday("Oscar", 26) 
      //Prints: Happy birthday Oscar! You turn 25 today!
      birthday("Amarah", 30) 
      //Prints: Happy birthday Amarah! You turn 30 today!
    }

#Default Parameters

fun favoriteLanguage(name, language = "Kotlin") {
      println("Hello, $name. Your favorite programming language is $language")  
    }
    
    fun main() {
      favoriteLanguage("Manon") 
      //Prints: Hello, Manon. Your favorite programming language is Kotlin
      
      favoriteLanguage("Lee", "Java") 
      //Prints: Hello, Lee. Your favorite programming language is Java
    }

#Named Parameters

fun findMyAge(currentYear: Int, birthYear: Int) {
       var myAge = currentYear -birthYear
    println("I am $myAge years old.")
    }
    
    fun main() {
      findMyAge(currentYear = 2020, birthYear = 1995)
      //Prints: I am 25 years old.
      findMyAge(birthYear = 1920, currentYear = 2020)
      //Prints: I am 100 years old.
    }

#Return Statement

//Return type is declared outside the parentheses
    fun getArea(length: Int, width: Int): Int {
      var area = length *width
    
      //return statement
      return area
    }
    
    fun main() {
      var myArea = getArea(10, 8)
    println("The area is $myArea.")
    //Prints: The area is 80.
    }

#Single expression function

fun fullName(firstName: String, lastName: String) = "$firstName $lastName"
    
    fun main() {
      println(fullName("Ariana", "Ortega"))
      //Prints: Ariana Ortega
      println(fullName("Kai", "Gittens"))
      //Prints: Kai Gittens
    }

#Function Literals

fun main() {
      //Anonymous Function:
      var getProduct = fun(num1: Int, num2: Int): Int {
    return num1 *num2
      }
      println(getProduct(8, 3))
      //Prints: 24
    //Lambda Expression
      var getDifference = { num1: Int, num2: Int -> num1 -num2 }
      println(getDifference(10, 3))
      //Prints: 7
    }

#Class

#Class Example

//class with properties containing default values
    class Student {
      var name = "Lucia"
      var semester = "Fall"
      var gpa = 3.95
    }
    
    //shorthand syntax without class body
    class Student

#Class Instance

// Class
    class Student {
      var name = "Lucia"
      var semester = "Fall"
    var gpa = 3.95
    }
    
    fun main() {
      var student = Student()   
      // Instance
      println(student.name)     
      // Prints: Lucia
      println(student.semester) 
      // Prints: Fall
      println(student.gpa)      
      // Prints: 3.95  
    }

#Primary Constructor

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) 
    
    fun main() {
      var student = Student("Lucia", 3.95, "Fall", 2022) 
      println(student.name)     
      //Prints: Lucia
      println(student.gpa)
      //Prints: 3.95
      println(student.semester) 
      //Prints: Fall
      println(student.estimatedGraduationYear) 
      //Prints: 2022
    }

#Initialization Block

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) {
      init {
        println("$name has ${estimatedGraduationYear -2020} years left in college.")
      }
    }
    
    fun main() {
      var student = Student("Lucia", 3.95, "Fall", 2022)
    //Prints: Lucia has 2 years left in college. 
    }

#Member Function

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) {
    
      init {
    println("$name has ${estimatedGraduationYear -2020} years left in college.")
      }
    
      //member function
      fun calculateLetterGrade(): String {
        return when {
          gpa >= 3.0 -> "A"
          gpa >= 2.7 -> "B"
          gpa >= 1.7 -> "C"
    gpa >= 1.0 -> "D"
          else -> "E"
        }
      }
    }
    
    //When the instance is created and the function is called, the when expression will be executed and return the letter grade
    fun main() {
      var student = Student("Lucia", 3.95, "Fall", 2022)
    //Prints: Lucia has 2 years left in college.
      println("${student.name}'s letter grade is ${student.calculateLetterGrade()}.")
      //Prints: Lucia's letter grade is A.
    }

#See Also