Control Flow

If/else

Works as both a statement and an expression:

// Statement
if (x > 0) {
    println("positive")
} else {
    println("non-positive")
}

// Expression (returns a value)
val abs = if (x >= 0) x else -x

Else-if chains

if (score >= 90) {
    println("A")
} else if (score >= 80) {
    println("B")
} else if (score >= 70) {
    println("C")
} else {
    println("F")
}

When

Pattern matching with optional subject:

// With subject
when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    6, 7 -> println("Weekend")
    else -> println("Other")
}

// Without subject (acts as if/else chain)
when {
    temperature < 0 -> println("freezing")
    temperature < 20 -> println("cold")
    else -> println("warm")
}

Range patterns

when (score) {
    in 90..100 -> println("A")
    in 80..89 -> println("B")
    in 70..79 -> println("C")
    else -> println("F")
}

When as expression

val label = when (n) {
    0 -> "zero"
    1 -> "one"
    else -> "many"
}

Nested when

when (category) {
    "fruit" -> when (name) {
        "apple" -> println("red")
        "banana" -> println("yellow")
        else -> println("unknown fruit")
    }
    else -> println("not a fruit")
}

For loops

Iterate over integer ranges:

for (i in 1..10) {
    println(i)
}

While loops

var i = 0
while (i < 10) {
    println(i)
    i += 1
}

Do-while loops

var input = ""
do {
    println("enter 'quit' to exit")
    // ... read input ...
} while (input != "quit")

Break and continue

for (i in 1..100) {
    if (i % 15 == 0) continue
    if (i > 50) break
    println(i)
}

break and continue work inside for, while, and do-while loops, including when nested inside if blocks within the loop body.

`in` Operator

Check containment in ranges and collections:

val r = 1..10
println(5 in r)    // true
println(11 in r)   // false
println(r.first)   // 1
println(r.last)    // 10

The in operator desugars to collection.contains(element). The !in operator negates the result.

Return

Early return from functions:

fun findFirst(items: Int): Int {
    if (items <= 0) return -1
    // ... rest of function ...
    return items
}