Kotlin if Expression

The if statement lets your program make decisions. Based on whether a condition is true or false, the program takes a different path. In Kotlin, if is also an expression — it can return a value, not just execute code.

Basic if Statement

val temperature = 35

if (temperature > 30) {
    println("It is hot today.")
}

If temperature > 30 is true, the block runs. If it is false, nothing happens.

if-else Statement

val marks = 45

if (marks >= 50) {
    println("Passed")
} else {
    println("Failed")
}

if-else if-else Chain

val score = 72

if (score >= 90) {
    println("Grade: A")
} else if (score >= 75) {
    println("Grade: B")
} else if (score >= 60) {
    println("Grade: C")
} else {
    println("Grade: F")
}

Output: Grade: C

How the Decision Tree Works


             score = 72
                │
         score >= 90?
         NO ↓
         score >= 75?
         NO ↓
         score >= 60?
         YES → print "Grade: C" ← stops here

if as an Expression (Returns a Value)

In Kotlin, if can return a value. This makes it function like the ternary operator in other languages.

val a = 15
val b = 22

// Traditional approach
var max: Int
if (a > b) {
    max = a
} else {
    max = b
}

// Kotlin expression approach
val max2 = if (a > b) a else b

println(max2)  // 22

Multi-Line if Expressions

When using if as an expression with multiple lines in each branch, the last expression in each block is the returned value:

val age = 20
val ticket = if (age >= 18) {
    println("Adult verified.")
    "Full price"
} else {
    println("Child verified.")
    "Half price"
}
println("Ticket: $ticket")

Output:

Adult verified.
Ticket: Full price

Nested if Statements

val isWeekend = true
val isRaining = false

if (isWeekend) {
    if (isRaining) {
        println("Stay home and watch a movie.")
    } else {
        println("Go for a walk in the park.")
    }
} else {
    println("Go to work.")
}

if with Logical Operators

val salary = 80000
val experience = 5

if (salary > 50000 && experience >= 3) {
    println("Eligible for senior role.")
} else {
    println("Not eligible yet.")
}

Practical Example: BMI Calculator

fun main() {
    val weight = 70.0    // kg
    val height = 1.75    // meters

    val bmi = weight / (height * height)
    val category = if (bmi < 18.5) {
        "Underweight"
    } else if (bmi < 25.0) {
        "Normal weight"
    } else if (bmi < 30.0) {
        "Overweight"
    } else {
        "Obese"
    }

    println("BMI: ${"%.2f".format(bmi)}")
    println("Category: $category")
}