Control Flow

In Kotlin, if and when are expressions — they return a value.

if as an expression

val max = if (a > b) a else b          // returns a value

// still usable as a statement
if (ready) {
    start()
} else {
    wait()
}

There is no ternary ?: operator for this — if/else does the job.

when

when is a powerful switch. As an expression it must be exhaustive:

val label = when (x) {
    0 -> "zero"
    1, 2, 3 -> "small"          // multiple values
    in 4..10 -> "medium"        // a range
    else -> "large"
}

when without a subject acts like a chain of conditions:

val sign = when {
    x > 0 -> "positive"
    x < 0 -> "negative"
    else -> "zero"
}

It can also test types (covered with smart casts in Part 3):

when (obj) {
    is String -> println("length ${obj.length}")
    is Int -> println("doubled ${obj * 2}")
    else -> println("other")
}

Loops and ranges

for (i in 1..5) print(i)        // 12345  (inclusive)
for (i in 0 until 5) print(i)   // 01234  (exclusive end)
for (i in 10 downTo 1 step 2) print(i)  // 10 8 6 4 2

for (item in listOf("a", "b")) println(item)

for ((index, value) in listOf("a", "b").withIndex()) {
    println("$index: $value")
}

in also tests membership:

if (x in 1..100) println("in range")

while and do/while

while (count < 10) { count++ }

do {
    count--
} while (count > 0)

break, continue, and labels

outer@ for (i in 1..3) {
    for (j in 1..3) {
        if (i + j == 4) continue@outer
        if (i * j > 6) break@outer
    }
}

Exercises

1. Use a when expression to map a day number (1–7) to its name.

2. Write fizzbuzz(n: Int): String returning "FizzBuzz" if n is divisible by 15, "Fizz" if by 3, "Buzz" if by 5, otherwise the number as a string.

   Solution

   fun fizzbuzz(n: Int): String = when {
       n % 15 == 0 -> "FizzBuzz"
       n % 3 == 0 -> "Fizz"
       n % 5 == 0 -> "Buzz"
       else -> n.toString()
   }
   

</details>

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