Lesson 2: Functions - Android Development with Kotlin PDF

Lesson 2: Functions This work is licensed under the Android Development with Kotlin Apache 2 license. 1 v1.0 About this lesson Lesson 2: Functions ○ Programs in Kotlin ○ (Almost) Everything has a value...

Lesson 2: Functions This work is licensed under the Android Development with Kotlin Apache 2 license. 1 v1.0 About this lesson Lesson 2: Functions ○ Programs in Kotlin ○ (Almost) Everything has a value ○ Functions in Kotlin ○ Compact functions ○ Lambdas and higher-order functions ○ List filters ○ Summary This work is licensed under the Android Development with Kotlin Apache 2 license. 2 Programs in Kotlin This work is licensed under the Android Development with Kotlin Apache 2 license. 3 Setting up Before you can write code and run programs, you need to: Create a file in your project Create a main() function Pass arguments to main()(Optional) Use any passed arguments in function calls (Optional) Run your program This work is licensed under the Android Development with Kotlin Apache 2 license. 4 Create a new Kotlin file In IntelliJ IDEA's Project pane, under Hello World, right-click the src folder. Select New > Kotlin File/Class. Select File, name the file Hello, and press Enter. This work is licensed under the Android Development with Kotlin Apache 2 license. 5 Create a Kotlin file You should now see a file in the src folder called Hello.kt. This work is licensed under the Android Development with Kotlin Apache 2 license. 6 Create a main() function main() is the entry point for execution for a Kotlin program. In the Hello.kt file: fun main(args: Array) { println("Hello, world!") } The args in the main() function are optional. This work is licensed under the Android Development with Kotlin Apache 2 license. 7 Run your Kotlin program To run your program, click the Run icon ( ) to the left of the main() function. IntelliJ IDEA runs the program, and displays the results in the console. This work is licensed under the Android Development with Kotlin Apache 2 license. 8 Pass arguments to main() Select Run > Edit Configurations to open the Run/Debug Configurations window. This work is licensed under the Android Development with Kotlin Apache 2 license. 9 Use arguments in main() 1. Val i: Int = 0 2. fun main(args: Array) { 3. for (element in args) { 4. println("Hello, ${args[i]}") 5. } 6. } This work is licensed under the Android Development with Kotlin Apache 2 license. Use arguments in main() Use args to access the first input argument passed to main(). fun main(args: Array) { println("Hello, ${args}") } ⇒ Hello, Kotlin! This work is licensed under the Android Development with Kotlin Apache 2 license. 11 (Almost) Everything has a value This work is licensed under the Android Development with Kotlin Apache 2 license. 12 (Almost) Everything is an expression In Kotlin, almost everything is an expression and has a value. Even an if expression has a value. val temperature = 20 val isHot = if (temperature > 40) true else false println(isHot) ⇒ false This work is licensed under the Android Development with Kotlin Apache 2 license. 13 Expression values Sometimes, that value is kotlin.Unit. val isUnit = println("This is an expression") println(isUnit) ⇒ This is an expression kotlin.Unit This work is licensed under the Android Development with Kotlin Apache 2 license. 14 Expression values In Kotlin, Unit is a type that represents the absence of a meaningful value. It is analogous to void in languages like Java or C#. However, unlike void, Unit is a real type and can be used as a return type of a function. fun printMessage(message: String): Unit { println(message) } This work is licensed under the Android Development with Kotlin Apache 2 license. 15 Expression values In this example, the printMessage function has a return type of Unit, which indicates that it doesn't return any value. However, specifying Unit is optional, as it is the default return type for functions with no return value in Kotlin. This work is licensed under the Android Development with Kotlin Apache 2 license. 16 Functions in Kotlin This work is licensed under the Android Development with Kotlin Apache 2 license. 17 About functions A block of code that performs a specific task Breaks a large program into smaller modular chunks Declared using the fun keyword Can take arguments with either named or default values 18 This work is licensed under the Android Development with Kotlin Apache 2 license. Parts of a function Earlier, you created a simple function that printed "Hello World". fun printHello() { println("Hello World") } printHello() This work is licensed under the Android Development with Kotlin Apache 2 license. 19 Unit returning functions If a function does not return any useful value, its return type is Unit. fun printHello(name: String?): Unit { println("Hi there!") } Unit is a type with only one value: Unit. This work is licensed under the Android Development with Kotlin Apache 2 license. 20 Unit returning functions The Unit return type declaration is optional. fun printHello(name: String?): Unit { println("Hi there!") } is equivalent to: fun printHello(name: String?) { println("Hi there!") } This work is licensed under the Android Development with Kotlin Apache 2 license. 21 Function arguments Functions may have: Default parameters Required parameters Named arguments This work is licensed under the Android Development with Kotlin Apache 2 license. 22 Default parameters Default values provide a fallback if no parameter value is passed. fun drive(speed: String = "fast") { println("driving $speed") } Use "=" after the type drive() ⇒ driving fast to define default values drive("slow") ⇒ driving slowly drive(speed = "turtle-like") ⇒ driving turtle-like This work is licensed under the Android Development with Kotlin Apache 2 license. 23 Required parameters If no default is specified for a parameter, the corresponding argument is required. Required parameters fun tempToday(day: String, temp: Int) { println("Today is $day and it's $temp degrees.") } tempToday ("Tuesday", 20) This work is licensed under the Android Development with Kotlin Apache 2 license. 24 Default versus required parameters Functions can have a mix of default and required parameters. fun reformat(str: String, divideByCamelHumps: Boolean, wordSeparator: Char, Has default value normalizeCase: Boolean = true){ Pass in required arguments. reformat("Today is a day like no other day", false, '_') This work is licensed under the Android Development with Kotlin Apache 2 license. 25 Named arguments To improve readability, use named arguments for required arguments. reformat(str, divideByCamelHumps = false, wordSeparator = '_') It's considered good style to put default arguments after positional arguments, that way callers only have to specify the required arguments. This work is licensed under the Android Development with Kotlin Apache 2 license. 26 Compact functions This work is licensed under the Android Development with Kotlin Apache 2 license. 27 Single-expression functions Compact functions, or single-expression functions, make your code more concise and readable. fun double(x: Int): Int { Complete x * 2 version } fun double(x: Int):Int = x * 2 Compact version This work is licensed under the Android Development with Kotlin Apache 2 license. 28 Lambdas and higher-order functions This work is licensed under the Android Development with Kotlin Apache 2 license. 29 Kotlin functions are first-class Kotlin functions can be stored in variables and data structures They can be passed as arguments to, and returned from, other higher-order functions You can use higher-order functions to create new "built- in" functions This work is licensed under the Android Development with Kotlin Apache 2 license. 30 Lambda functions A lambda is an expression that makes a function that has no name. Parameter and type Function var dirtLevel = 20 arrow val waterFilter = {level: Int -> level / 2} println(waterFilter(dirtLevel)) ⇒ 10 Code to execute This work is licensed under the Android Development with Kotlin Apache 2 license. 31 Syntax for function types Kotlin's syntax for function types is closely related to its syntax for lambdas. Declare a variable that holds a function. val waterFilter: (Int) -> Int = {level -> level / 2} Variable name Data type of Function variable (function type) This work is licensed under the Android Development with Kotlin Apache 2 license. 32 Higher-order functions Higher-order functions take functions as parameters, or return a function. fun encodeMsg(msg: String, encode: (String) -> String): String {​ return encode(msg) } body of the code calls the function that was passed as the second The argument, and passes the first argument along to it. This work is licensed under the Android Development with Kotlin Apache 2 license. 33 Higher-order functions To call this function, pass it a string and a function. val enc1: (String) -> String = { input -> input.toUpperCase() } println(encodeMsg("abc", enc1)) Using a function type separates its implementation from its usage. This work is licensed under the Android Development with Kotlin Apache 2 license. 34 Passing a function reference Use the :: operator to pass a named function as an argument to another function. fun enc2(input:String): String = input.reversed() encodeMessage("abc", ::enc2) Passing a named function, not a lambda The :: operator lets Kotlin know that you are passing the function reference as an argument, and not trying to call the function. This work is licensed under the Android Development with Kotlin Apache 2 license. 35 Last parameter call syntax Kotlin prefers that any parameter that takes a function is the last parameter. encodeMessage("acronym", { input -> input.toUpperCase() }) You can pass a lambda as a function parameter without putting it inside the parentheses. encodeMsg("acronym") { input -> input.toUpperCase() } This work is licensed under the Android Development with Kotlin Apache 2 license. 36 Using higher-order functions Many Kotlin built-in functions are defined using last parameter call syntax. inline fun repeat(times: Int, action: (Int) -> Unit) repeat(3) { println("Hello") } This work is licensed under the Android Development with Kotlin Apache 2 license. 37  List filters This work is licensed under the Android Development with Kotlin Apache 2 license. 38 List filters Get part of a list based on some condition bright red red-orange dark red orange saffron orange Apply filter() on list Condition: element contains “red” red red-orange dark red This work is licensed under the Android Development with Kotlin Apache 2 license. 39 Iterating through lists If a function literal has only one parameter, you can omit its declaration and the "->". The parameter is implicitly declared under the name it. val ints = listOf(1, 2, 3) ints.filter { it > 0 } Filter iterates through a collection, where it is the value of the element during the iteration. This is equivalent to: ints.filter { n: Int -> n > ints.filter { n -> n > 0 0 } OR } This work is licensed under the Android Development with Kotlin Apache 2 license. 40 List filters The filter condition in curly braces {} tests each item as the filter loops through. If the expression returns true, the item is included. val books = listOf("nature", "biology", "birds") println(books.filter { it == 'b' }) ⇒ [biology, birds] This work is licensed under the Android Development with Kotlin Apache 2 license. 41 Eager and lazy filters Evaluation of expressions in lists: Eager: occurs regardless of whether the result is ever used Lazy: occurs only if necessary at runtime Lazy evaluation of lists is useful if you don't need the entire result, or if the list is exceptionally large and multiple copies wouldn't wouldn't fit into RAM. This work is licensed under the Android Development with Kotlin Apache 2 license. 42 Eager filters Filters are eager by default. A new list is created each time you use a filter. val instruments = listOf("viola", "cello", "violin") val eager = instruments.filter { it == 'v' } println("eager: " + eager) ⇒ eager: [viola, violin] This work is licensed under the Android Development with Kotlin Apache 2 license. 43 Lazy filters Sequences are data structures that use lazy evaluation, and can be used with filters to make them lazy. val instruments = listOf("viola", "cello", "violin") val filtered = instruments.asSequence().filter { it == 'v'} println("filtered: " + filtered) ⇒ filtered: kotlin.sequences.FilteringSequence@386cc1c4 This work is licensed under the Android Development with Kotlin Apache 2 license. 44 Sequences -> lists Sequences can be turned back into lists using toList(). val filtered = instruments.asSequence().filter { it == 'v'} val newList = filtered.toList() println("new list: " + newList) ⇒ new list: [viola, violin] This work is licensed under the Android Development with Kotlin Apache 2 license. 45 Other list transformations map() performs the same transform on every item and returns the list. val numbers = setOf(1, 2, 3) println(numbers.map { it * 3 }) => [3, 6, 9] flatten() returns a single list of all the elements of nested collections. val numberSets = listOf(setOf(1, 2, 3), setOf(4, 5), setOf(1, 2)) println(numberSets.flatten()) => [1, 2, 3, 4, 5, 1, 2] This work is licensed under the Android Development with Kotlin Apache 2 license. 46  Summary This work is licensed under the Android Development with Kotlin Apache 2 license. 47 Summary In Lesson 2, you learned how to: Create a file and a main() function in your project, and run a program Pass arguments to the main() function Use the returned value of an expression Use default arguments to replace multiple versions of a functi on Use compact functions, to make code more readable Use lambdas and higher-order functions Use eager and lazy Android list filters Development with Kotlin This work is licensed under the Apache 2 license. 48 Pathway Practice what you’ve learned by completing the pathway: Lesson 2: Functions This work is licensed under the Android Development with Kotlin Apache 2 license. 49