Control Flow and Functions

if Expressions

In Kotlin, if is an expression — it returns a value:

val score = 85
val grade = if (score >= 80) {
    "A"
} else if (score >= 70) {
    "B"
} else {
    "C"
}
println(grade)  // A

You can also write it on one line for simple cases:

val status = if (age >= 18) "Adult" else "Minor"

when Expressions

when is Kotlin's version of switch. It is more powerful:

val day = 3
val name = when (day) {
    1 -> "Monday"
    2 -> "Tuesday"
    3 -> "Wednesday"
    4 -> "Thursday"
    5 -> "Friday"
    else -> "Weekend"
}
println(name)  // Wednesday

You can match multiple values, ranges, and conditions:

val score = 75
val result = when {
    score >= 90 -> "Excellent"
    score in 70..89 -> "Good"
    score in 50..69 -> "Pass"
    else -> "Fail"
}
println(result)  // Good

Loops

// for loop with range
for (i in 1..5) {
    print("$i ")      // 1 2 3 4 5
}

// for loop with step
for (i in 1..10 step 2) {
    print("$i ")      // 1 3 5 7 9
}

// for loop with list
val fruits = listOf("apple", "banana", "cherry")
for ((index, fruit) in fruits.withIndex()) {
    println("$index: $fruit")
}
// 0: apple
// 1: banana
// 2: cherry

// while loop
var count = 3
while (count > 0) {
    println("Count: $count")
    count--
}

Functions

Basic Functions

fun greet(name: String): String {
    return "Hello, $name!"
}

println(greet("Alice"))  // Hello, Alice!

Single-Expression Functions

Short functions can skip the braces and return:

fun add(a: Int, b: Int): Int = a + b

println(add(3, 4))  // 7

Default Arguments

fun greet(name: String, greeting: String = "Hello"): String {
    return "$greeting, $name!"
}

println(greet("Bob"))               // Hello, Bob!
println(greet("Bob", "Hi"))         // Hi, Bob!

Named Arguments

When a function has many parameters, use names for clarity:

fun createUser(name: String, age: Int, active: Boolean, admin: Boolean) {
    // ...
}

createUser(
    name = "Alice",
    age = 30,
    active = true,
    admin = false
)

Vararg (Variable Number of Arguments)

fun sum(vararg numbers: Int): Int {
    return numbers.sum()
}

println(sum(1, 2, 3))          // 6
println(sum(10, 20, 30, 40))   // 100

Practice Exercise

Write a function that classifies a number:

fun classify(n: Int): String = when {
    n > 0 -> "Positive"
    n < 0 -> "Negative"
    else -> "Zero"
}

fun main() {
    println(classify(5))    // Positive
    println(classify(-3))   // Negative
    println(classify(0))    // Zero
}

Try modifying classify to also handle ranges like "Small positive" (1-9) and "Large positive" (10+).