Functional Kotlin - Lambdas and Higher-Order Functions

Author: smartherd

src/35_lambdas_higher_order_functions.kt

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+
+/*
+    EXAMPLE ONE
+
+*   1. Lambda Expression
+*   2. Higher-Order Function
+* */
+fun main(args: Array<String>) {
+
+    val program = Program()
+
+    program.addTwoNumbers(2, 7)     // Simple way... for better understanding
+
+    program.addTwoNumbers(2, 7, object : MyInterface {   // Using Interface / OOPs way
+
+        override fun execute(sum: Int) {
+            println(sum)    // Body
+        }
+    })
+
+    val test: String = "Hello"
+
+    val myLambda: (Int) -> Unit = { s: Int -> println(s)}   // Lambda Expression [ Function ]
+    program.addTwoNumbers(2, 7, myLambda)
+}
+
+class Program {
+
+    fun addTwoNumbers(a: Int, b: Int, action: (Int) -> Unit) {  // High Level Function with Lambda as Parameter
+
+        val sum = a + b
+        action(sum)     // println(sum)
+//        println(sum)  // Body
+    }
+
+    fun addTwoNumbers(a: Int, b: Int, action: MyInterface) {    // Using Interface / Object Oriented Way
+        val sum = a + b
+        action.execute(sum)
+    }
+
+    fun addTwoNumbers(a: Int, b: Int) {                         // Simple way.. Just for Better Understanding
+
+        val sum =  a + b
+        println(sum)
+    }
+}
+
+interface MyInterface {
+    fun execute(sum: Int)
+}

src/36_lambdas_example_two.kt

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+
+/*
+    EXAMPLE TWO
+
+*   1. Lambda Expression
+*   2. Higher-Order Function
+* */
+fun main(args: Array<String>) {
+
+    val program = MyProgram()
+
+//    val myLambda: (Int, Int) -> Int = { x, y -> x + y}  // Lambda Expression [ Function ]
+// OR,
+//    program.addTwoNumbers(2, 7, { x, y -> x + y })
+// OR,
+    program.addTwoNumbers(2, 7) {x, y -> x + y}
+}
+
+class MyProgram {
+
+    fun addTwoNumbers(a: Int, b: Int, action: (Int, Int) -> Int) {  // High Level Function with Lambda as Parameter
+
+        val result = action(a, b)
+        println(result)
+    }
+}

src/37_lambdas_closures.kt

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+
+/*
+*   1. Closures
+* */
+fun main(args: Array<String>) {
+
+    val program = TheProgram()
+
+    var result = 0
+
+    program.addTwoNumbers(2, 7) {x, y -> result = x + y}
+
+    println(result)
+}
+
+class TheProgram {
+
+    fun addTwoNumbers(a: Int, b: Int, action: (Int, Int) -> Unit) {  // High Level Function with Lambda as Parameter
+
+        action(a, b)
+    }
+}

src/38_it_keyword_lambdas.kt

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+
+
+/*
+*   1. 'it' keyword
+* */
+fun main(args: Array<String>) {
+
+    val program = Programs()
+    program.reverseAndDisplay("hello", { it.reversed() })
+}
+
+class Programs {
+
+    fun reverseAndDisplay(str: String, myFunc: (String) -> String) {  // High Level Function with Lambda as Parameter
+
+        val result = myFunc(str)    // it.reversed() ==> str.reversed() ==> "hello".reversed() = "olleh"
+        println(result)
+    }
+}

src/39_with_apply_functions.kt

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+
+/*
+*   1. 'with' function
+*   2. 'apply' function
+* */
+fun main(args: Array<String>) {
+
+    var person = Perrson()
+
+    with(person) {
+        name = "Steve"
+        age = 23
+    }
+
+    person.apply {
+        name = "Steve"
+        age = 23
+    }.startRun()
+
+    println(person.name)
+    println(person.age)
+}
+
+class Perrson {
+
+    var name: String = ""
+    var age: Int = -1
+
+    fun startRun() {
+        println("Now I am ready to run")
+    }
+}

src/40_arrays.kt

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+
+/*
+*   1. Arrays
+* */
+fun main(args: Array<String>) {
+
+    // Elements :   32  0   0   54  0
+    // Index    :   0   1   2   3   4
+
+    var myArray = Array<Int>(5) { 0 }   // Mutable. Fixed Size.
+    myArray[0] = 32
+    myArray[3] = 54
+    myArray[1] = 11
+
+    for (element in myArray) {              // Using individual elements (Objects)
+        println(element)
+    }
+
+    println()
+
+    for (index in 0..myArray.size - 1) {
+        println(myArray[index])
+    }
+}

src/41_list.kt

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+
+/*
+*   1. List and ArrayList 
+* */
+fun main(args: Array<String>) {
+
+    // Elements :
+    // Index    :   0   1   2   3   4
+
+//    var list = mutableListOf<String>()  // Mutable, No Fixed Size, Can Add or Remove Elements
+//    var list = arrayListOf<String>()    // Mutable, No Fixed Size, Can Add or Remove Elements
+    var list = ArrayList<String>()      // Mutable, No Fixed Size, Can Add or Remove Elements
+    list.add("Yogi")        // 0
+    list.add("Manmohan")    // 1
+    list.add("Vajpayee")    // 2
+
+//    list.remove("Manmohan")
+//    list.add("Vajpayee")
+
+    list[1] = "Modi"
+
+    for (element in list) {             // Using individual elements (Objects)
+        println(element)
+    }
+}

src/42_map_hashmap.kt

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+
+/*
+*   1. Map and HashMap
+* */
+fun main(args: Array<String>) {
+
+    // Map Tutorial: Key-Value pair
+//    var myMap = HashMap<Int, String>()      // Mutable, READ and WRITE both, No Fixed Size
+//    var myMap = mutableMapOf<Int, String>() // Mutable, READ and WRITE both, No Fixed Size
+    var myMap = hashMapOf<Int, String>()      // Mutable, READ and WRITE both, No Fixed Size
+
+    myMap.put(4, "Yogi")
+    myMap.put(43, "Manmohan")
+    myMap.put(7, "Vajpayee")
+
+    myMap.put(43, "Modi")
+
+    for (key in myMap.keys) {
+        println("Element at $key = ${myMap[key]}")  // myMap.get(key)
+    }
+}

src/43_set_hashset.kt

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+
+/*
+*   1. Set and HashSet
+* */
+fun main(args: Array<String>) {
+
+    // "Set" contains unique elements
+    // "HashSet" also contains unique elements but sequence is not guaranteed in output
+
+    var mySet = mutableSetOf<Int>( 2, 54, 3, 1, 0, 9, 9, 9, 8)  // Mutable Set, READ and WRITE both
+//    var mySet = hashSetOf<Int>( 2, 54, 3, 1, 0, 9, 9, 9, 8)     // Mutable Set, READ and WRITE both
+
+    mySet.remove(54)
+    mySet.add(100)
+
+    for (element in mySet) {
+        println(element)
+    }
+}

src/44_filter_map_sorting.kt

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+/** FILTER
+ * Returns a list containing only elements matching the given [predicate]
+ * */
+
+/** MAP
+ * Returns a list containing the results of applying the given [transform] function
+ * to each element in the original collection
+ * */
+
+fun main(args: Array<String>) {
+
+    val myNumbers: List<Int> = listOf(2, 3, 4, 6, 23, 90)
+
+    val mySmallNums = myNumbers.filter { it < 10 }    // OR { num -> num < 10 }
+    for (num in mySmallNums) {
+        println(num)
+    }
+
+    val mySquaredNums = myNumbers.map { it * it }     // OR { num -> num * num }
+    for (num in mySquaredNums) {
+        println(num)
+    }
+
+    var people = listOf<Pperson>(Pperson(10, "Steve"), Pperson(23, "Annie"), Pperson(17, "Sam"))
+    var names = people.filter { person ->person.name.startsWith("S") }.map { it.name }
+
+    for (name in names) {
+        println(name)
+    }
+}
+
+class Pperson(var age: Int, var name: String) {
+    // Some other code..
+}

src/45_predicate.kt

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+
+/**
+ *  PREDICATES
+ * */
+fun main(args: Array<String>) {
+
+    val myNumbers = listOf(2, 3, 4, 6, 23, 90)
+
+    val myPredicate = { num: Int -> num > 10 }
+
+    val check1 = myNumbers.all( myPredicate )       // Are all elements greater than 10?
+    println(check1)
+
+    val check2 = myNumbers.any(myPredicate)         // Does any of these elements satisfy the predicate?
+    println(check2)
+
+    val totalCount: Int = myNumbers.count(myPredicate) // Number of elements that satify the predicate.
+    println(totalCount)
+
+    val num = myNumbers.find(myPredicate)           // Returns the first number that matches the predicate
+    println(num)
+}