Kotlin - Control Flows

3 분 소요

If Expression

코틀린에서 if 표현식은 조건절을 표현식에 사용할 수 있고 반환값을 가진다. 그래서 코틀린엔 삼항연산자 ( condition ? then : else) 가 없다.

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

//전통적인 사용방법
var max = a 
if (a < b) max = b

// With else 
var max: Int
if (a > b) {
    max = a
} else {
    max = b
// 표현식으로 
val max = if (a > b) a else b

if 표현식은 블럭으로 분기 할 수 있고, 블럭에서 마지막 구문이 값이 된다.

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

If you’re using if as an expression rather than a statement (for example, returning its value or assigning it to a variable), the expression is required to have an else branch.

See the grammar for if.

When Expression

when replaces the switch operator of C-like languages. In the simplest form it looks like this

when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> { // Note the block
        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 satisfied 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.

If many cases should be handled in the same way, the branch conditions may be combined with a comma:

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

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

when (x) {
    parseInt(s) -> print("s encodes x")
    else -> print("s does not encode x")

We 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 possibility is to check 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")
    x.isEven() -> print("x is even")
    else -> print("x is funny")

See the grammar for when.

For Loops

for loop iterates through anything that provides an iterator. The syntax is as follows:

for (item in collection) print(item)

The body can be a block.

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

As mentioned before, for iterates through anything that provides an iterator, i.e.

  • has a member- or extension-function iterator(), whose return type
    • has a member- or extension-function next(), and
    • has a member- or extension-function hasNext() that returns Boolean.

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

A for loop over 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:

for (i in array.indices) {

Note that this “iteration through a range” is compiled down to optimal implementation with no extra objects created.

Alternatively, you can use the withIndex library function:

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")

See the grammar for for.

While Loops

while and do..while work as usual

while (x > 0) {

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

See the grammar for while.

Break and continue in loops

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