Control Flow: if, when & loops

Control flow decides what runs

Programs need to make decisions (“if the user is logged in, show the dashboard”) and repeat work (“for each item, draw a row”). Kotlin’s control-flow tools are familiar but more powerful than in many languages, because most of them are expressions — they return a value.

if as an expression

In Kotlin, if can return a value, so you rarely need a separate ternary operator:

val a = 7
val b = 12
val max = if (a > b) a else b      // max gets 12

// it can span multiple lines too
val label = if (max > 10) {
    "big"
} else {
    "small"
}

when: a supercharged switch

when replaces long if-else chains and is far more flexible — it can match values, ranges, types and conditions.

val score = 82
val grade = when (score) {
    in 90..100 -> "A"
    in 75..89  -> "B"
    in 50..74  -> "C"
    else       -> "Fail"
}

// when without an argument acts like if-else
val status = when {
    score >= 50 -> "Pass"
    else        -> "Retry"
}

Example — matching types:

fun describe(x: Any): String = when (x) {
    is Int    -> "a number: $x"
    is String -> "text of length ${x.length}"
    else      -> "something else"
}

Loops

// ranges
for (i in 1..5) print(i)          // 12345
for (i in 5 downTo 1) print(i)    // 54321
for (i in 0..10 step 2) print(i)  // 0246810
for (i in 1 until 5) print(i)     // 1234 (excludes 5)

// iterate a collection
val fruits = listOf("apple", "mango", "kiwi")
for (fruit in fruits) println(fruit)

// with index
for ((index, fruit) in fruits.withIndex()) {
    println("$index: $fruit")
}

while and do-while

var n = 3
while (n > 0) { println(n); n-- }

do { println("runs at least once") } while (false)

break and continue

for (i in 1..10) {
    if (i == 3) continue   // skip 3
    if (i == 6) break      // stop at 6
    print(i)               // 1245
}

Common mistakes

• Writing long if-else ladders instead of a clean when.
• Using .. when you mean until (off-by-one errors on list indices).
• Forgetting that if/when return values — you can assign them directly.

Summary: Use if/when as expressions to assign values directly, and prefer ranges (1..n, until, step) for clean, readable loops.