Hàm (Function) trong Kotlin

fun sayHello() {
    println("Hello, Kotlin!")
}
 
fun main() {
    sayHello()  // Gọi hàm
}

fun greet(name: String) {
    println("Hello, $name!")
}
 
fun main() {
    greet("Alice")
    greet("Bob")
}

fun sum(a: Int, b: Int) {
    println("$a + $b = ${a + b}")
}
 
fun main() {
    sum(5, 3)  // 5 + 3 = 8
}

fun sum(a: Int, b: Int): Int {
    return a + b
}
 
fun main() {
    val result = sum(10, 20)
    println(result)  // 30
}

fun sum(a: Int, b: Int): Int = a + b
 
// Có thể bỏ kiểu trả về (type inference)
fun sum(a: Int, b: Int) = a + b

fun greet(name: String = "Guest", greeting: String = "Hello") {
    println("$greeting, $name!")
}
 
fun main() {
    greet()                          // Hello, Guest!
    greet("Alice")                   // Hello, Alice!
    greet("Bob", "Hi")              // Hi, Bob!
    greet(greeting = "Hey")         // Hey, Guest! (named argument)
}

fun createUser(name: String, age: Int, city: String) {
    println("Name: $name, Age: $age, City: $city")
}
 
fun main() {
    // Gọi theo thứ tự
    createUser("Alice", 25, "Hanoi")
 
    // Named arguments - rõ ràng hơn
    createUser(
        name = "Bob",
        age = 30,
        city = "HCMC"
    )
 
    // Có thể đổi thứ tự
    createUser(
        age = 22,
        city = "Danang",
        name = "Charlie"
    )
}

fun sum(vararg numbers: Int): Int {
    var total = 0
    for (num in numbers) {
        total += num
    }
    return total
}
 
fun main() {
    println(sum(1, 2, 3))           // 6
    println(sum(1, 2, 3, 4, 5))    // 15
    println(sum(10))                // 10
}

// Cách 1: Khai báo Unit
fun printMessage(): Unit {
    println("Message")
}
 
// Cách 2: Bỏ qua Unit (khuyến nghị)
fun printMessage() {
    println("Message")
}

fun fail(message: String): Nothing {
    throw IllegalStateException(message)
}
 
fun validateAge(age: Int) {
    if (age < 0) {
        fail("Age cannot be negative")
    }
}

fun outerFunction(x: Int) {
    fun innerFunction(y: Int): Int {
        return x + y
    }
 
    println(innerFunction(5))
}

infix fun Int.times(str: String) = str.repeat(this)
 
fun main() {
    println(3 times "Hello ")  // Hello Hello Hello
}

fun String.addExclamation(): String {
    return "$this!"
}
 
fun main() {
    val message = "Hello"
    println(message.addExclamation())  // Hello!
}

fun calculateBMI(weight: Double, height: Double): Double {
    return weight / (height * height)
}
 
fun getBMICategory(bmi: Double): String = when {
    bmi < 18.5 -> "Gầy"
    bmi < 25.0 -> "Bình thường"
    bmi < 30.0 -> "Thừa cân"
    else -> "Béo phì"
}
 
fun main() {
    val weight = 70.0  // kg
    val height = 1.75  // m
 
    val bmi = calculateBMI(weight, height)
    val category = getBMICategory(bmi)
 
    println("BMI: %.2f".format(bmi))
    println("Phân loại: $category")
}

fun isPrime(n: Int): Boolean {
    if (n < 2) return false
    for (i in 2..kotlin.math.sqrt(n.toDouble()).toInt()) {
        if (n % i == 0) return false
    }
    return true
}
 
fun main() {
    println(isPrime(17))   // true
    println(isPrime(20))   // false
}