control-flow.md

If expression

In Kotlin, if is an expression: it returns a value. Therefore, there is no ternary operator (condition ? then : else) because ordinary if works fine in this role.

fun main() {
    val a = 2
    val b = 3

    //sampleStart
    var max = a
    if (a < b) max = b

    // With else
    if (a > b) {
      max = a
    } else {
      max = b
    }

    // As expression
    max = if (a > b) a else b

    // You can also use `else if` in expressions:
    val maxLimit = 1
    val maxOrLimit = if (maxLimit > a) maxLimit else if (a > b) a else b

    //sampleEnd
    println("max is $max")
    println("maxOrLimit is $maxOrLimit")
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="if-else-if-kotlin"}

Branches of an if expression can be blocks. In this case, the last expression is the value of a block:

val max = if (a > b) {
    print("Choose a")
    a
} else {
    print("Choose b")
    b
}

If you're using if as an expression, for example, for returning its value or assigning it to a variable, the else branch is mandatory.

When expression

when defines a conditional expression with multiple branches. It is similar to the switch statement in C-like languages. Its simple form looks like this.

when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> {
        print("x is neither 1 nor 2")
    }
}

when matches its argument against all branches sequentially until some branch condition is satisfied.

when can be used either as an expression or as a statement. If it is used as an expression, the value of the first matching branch becomes the value of the overall expression. If it is used as a statement, the values of individual branches are ignored. Just like with if, each branch can be a block, and its value is the value of the last expression in the block.

The else branch is evaluated if none of the other branch conditions are satisfied.

If when is used as an expression, the else branch is mandatory, unless the compiler can prove that all possible cases are covered with branch conditions, for example, with enum class entries and sealed class subtypes).

enum class Bit {
    ZERO, ONE
}

val numericValue = when (getRandomBit()) {
    Bit.ZERO -> 0
    Bit.ONE -> 1
    // 'else' is not required because all cases are covered
}

In when statements, the else branch is mandatory in the following conditions:

• when has a subject of a Boolean, enum, or sealed type, or their nullable counterparts.
• branches of when don't cover all possible cases for this subject.

enum class Color {
    RED, GREEN, BLUE
}

when (getColor()) {  
    Color.RED -> println("red")
    Color.GREEN -> println("green")   
    Color.BLUE -> println("blue")
    // 'else' is not required because all cases are covered
}

when (getColor()) {
    Color.RED -> println("red") // no branches for GREEN and BLUE
    else -> println("not red") // 'else' is required
}

To define a common behavior for multiple cases, combine their conditions in a single line with a comma:

when (x) {
    0, 1 -> print("x == 0 or x == 1")
    else -> print("otherwise")
}

You can use arbitrary expressions (not only constants) as branch conditions

when (x) {
    s.toInt() -> print("s encodes x")
    else -> print("s does not encode x")
}

You can also check a value for being in or !in a range or a collection:

when (x) {
    in 1..10 -> print("x is in the range")
    in validNumbers -> print("x is valid")
    !in 10..20 -> print("x is outside the range")
    else -> print("none of the above")
}

Another option is checking that a value is or !is of a particular type. Note that, due to smart casts, you can access the methods and properties of the type without any extra checks.

fun hasPrefix(x: Any) = when(x) {
    is String -> x.startsWith("prefix")
    else -> false
}

when can also be used as a replacement for an if-else if chain. If no argument is supplied, the branch conditions are simply boolean expressions, and a branch is executed when its condition is true:

when {
    x.isOdd() -> print("x is odd")
    y.isEven() -> print("y is even")
    else -> print("x+y is odd")
}

You can capture when subject in a variable using following syntax:

fun Request.getBody() =
    when (val response = executeRequest()) {
        is Success -> response.body
        is HttpError -> throw HttpException(response.status)
    }

The scope of variable introduced in when subject is restricted to the body of this when.

For loops

The for loop iterates through anything that provides an iterator. This is equivalent to the foreach loop in languages like C#. The syntax of for is the following:

for (item in collection) print(item)

The body of for can be a block.

for (item: Int in ints) {
    // ...
}

As mentioned before, for iterates through anything that provides an iterator. This means that it:

• has a member or an extension function iterator() that returns Iterator<>:
  • has a member or an extension function next()
  • has a member or an extension function hasNext() that returns Boolean.

All of these three functions need to be marked as operator.

To iterate over a range of numbers, use a range expression:

fun main() {
//sampleStart
    for (i in 1..3) {
        println(i)
    }
    for (i in 6 downTo 0 step 2) {
        println(i)
    }
//sampleEnd
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

A for loop over a range or an array is compiled to an index-based loop that does not create an iterator object.

If you want to iterate through an array or a list with an index, you can do it this way:

fun main() {
val array = arrayOf("a", "b", "c")
//sampleStart
    for (i in array.indices) {
        println(array[i])
    }
//sampleEnd
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

Alternatively, you can use the withIndex library function:

fun main() {
    val array = arrayOf("a", "b", "c")
//sampleStart
    for ((index, value) in array.withIndex()) {
        println("the element at $index is $value")
    }
//sampleEnd
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

While loops

while and do-while loops execute their body continuously while their condition is satisfied. The difference between them is the condition checking time:

• while checks the condition and, if it's satisfied, executes the body and then returns to the condition check.
• do-while executes the body and then checks the condition. If it's satisfied, the loop repeats. So, the body of do-while executes at least once regardless of the condition.

while (x > 0) {
    x--
}

do {
    val y = retrieveData()
} while (y != null) // y is visible here!

Break and continue in loops

Kotlin supports traditional break and continue operators in loops. See Returns and jumps.