Conditions: if and when

Making decisions

Programs often need to choose: show "Pass" or "Fail", charge delivery or not, allow login or not. A condition is a question with a true or false answer, like marks >= 33. Kotlin uses if for simple choices and when for many choices.

if, else if, else

fun main() {
    val marks = 72
    if (marks >= 75) {
        println("Distinction")
    } else if (marks >= 33) {
        println("Pass")
    } else {
        println("Fail")
    }
}

The braces after if hold the code to run. If there is only one line, you may leave the braces out, but many teams keep them anyway because adding a second line later without braces is a classic bug.

if as an expression

In Kotlin, if gives back a value, so you can use it on the right side of =. This is why Kotlin has no ? : ternary operator like Java. When used this way, else is required, so every case produces a value.

val orderTotal = 349
val deliveryFee = if (orderTotal >= 199) 0 else 30
println("Delivery: Rs $deliveryFee")   // Delivery: Rs 0

when: many choices

when compares a value with several branches. It is clearer than a long chain of else if. Each branch is condition -> result. else catches everything else.

fun main() {
    val day = 6
    val name = when (day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3, 4, 5 -> "Midweek"        // several values in one branch
        in 6..7 -> "Weekend"         // a range
        else -> "Invalid day"
    }
    println(name)                    // Weekend
}

when without an argument

You can also write when with no value after it. Then each branch is a full condition, and the first true one wins. This is a clean replacement for if-else chains.

fun fare(age: Int, isStudent: Boolean): Int = when {
    age < 5 -> 0
    age >= 60 -> 40
    isStudent -> 50
    else -> 100
}

Checking types with is

fun describe(x: Any): String = when (x) {
    is Int -> "a number: ${x + 1}"     // x is treated as Int here (smart cast)
    is String -> "text of length ${x.length}"
    else -> "something else"
}

Common mistakes

• Writing if (x = 5) instead of if (x == 5). = assigns, == compares; Kotlin rejects the first.
• Forgetting else when using if or when as an expression: "if must have both main and else branches if used as an expression".
• Long else-if ladders. Switch to when for readability.
• Wrong order of conditions: checking marks >= 33 before marks >= 75 means nobody gets Distinction. Put the strictest condition first.