Control flow

Like other programming languages, Kotlin is capable of making decisions based on whether a piece of code is evaluated to be true. Such pieces of code are called conditional expressions. Kotlin is also able to create and iterate through loops.

Conditional expressions

Kotlin provides if and when for checking conditional expressions.

If

To use if, add the conditional expression within parentheses () and the action to take if the result is true within curly braces {}:

fun main() {
//sampleStart
    val d: Int
    val check = true

    if (check) {
        d = 1
    } else {
        d = 2
    }

    println(d)
    // 1
//sampleEnd
}

There is no ternary operator condition ? then : else in Kotlin. Instead, if can be used as an expression. When using if as an expression, there are no curly braces {}:

fun main() { 
//sampleStart
    val a = 1
    val b = 2

    println(if (a > b) a else b) // Returns a value: 2
//sampleEnd
}

When

Use when when you have a conditional expression with multiple branches. when can be used either as a statement or as an expression.

Here is an example of using when as a statement:

Place the conditional expression within parentheses () and the actions to take within curly braces {}.
Use -> in each branch to separate each condition from each action.

fun main() {
//sampleStart
    val obj = "Hello"

    when (obj) {
        // Checks whether obj equals to "1"
        "1" -> println("One")
        // Checks whether obj equals to "Hello"
        "Hello" -> println("Greeting")
        // Default statement
        else -> println("Unknown")     
    }
    // Greeting
//sampleEnd
}

Here is an example of using when as an expression. The when syntax is assigned immediately to a variable:

fun main() {
//sampleStart    
    val obj = "Hello"    
    
    val result = when (obj) {
        // If obj equals "1", sets result to "one"
        "1" -> "One"
        // If obj equals "Hello", sets result to "Greeting"
        "Hello" -> "Greeting"
        // Sets result to "Unknown" if no previous condition is satisfied
        else -> "Unknown"
    }
    println(result)
    // Greeting
//sampleEnd
}

If when is used as an expression, the else branch is mandatory, unless the compiler can detect that all possible cases are covered by the branch conditions.

The previous example showed that when is useful for matching a variable. when is also useful when you need to check a chain of Boolean expressions:

fun main() {
//sampleStart
    val temp = 18

    val description = when {
        // If temp < 0 is true, sets description to "very cold"
        temp < 0 -> "very cold"
        // If temp < 10 is true, sets description to "a bit cold"
        temp < 10 -> "a bit cold"
        // If temp < 20 is true, sets description to "warm"
        temp < 20 -> "warm"
        // Sets description to "hot" if no previous condition is satisfied
        else -> "hot"             
    }
    println(description)
    // warm
//sampleEnd
}

Ranges

Before talking about loops, it's useful to know how to construct ranges for loops to iterate over.

The most common way to create a range in Kotlin is to use the .. operator. For example, 1..4 is equivalent to 1, 2, 3, 4.

To declare a range that doesn't include the end value, use the ..< operator. For example, 1..<4 is equivalent to 1, 2, 3.

To declare a range in reverse order, use downTo. For example, 4 downTo 1 is equivalent to 4, 3, 2, 1.

To declare a range that increments in a step that isn't 1, use step and your desired increment value. For example, 1..5 step 2 is equivalent to 1, 3, 5.

You can also do the same with Char ranges:

'a'..'d' is equivalent to 'a', 'b', 'c', 'd'
'z' downTo 's' step 2 is equivalent to 'z', 'x', 'v', 't'

Loops

The two most common loop structures in programming are for and while. Use for to iterate over a range of values and perform an action. Use while to continue an action until a particular condition is satisfied.

For

Using your new knowledge of ranges, you can create a for loop that iterates over numbers 1 to 5 and prints the number each time.

Place the iterator and range within parentheses () with keyword in. Add the action you want to complete within curly braces {}:

fun main() {
//sampleStart
    for (number in 1..5) { 
        // number is the iterator and 1..5 is the range
        print(number)
    }
    // 12345
//sampleEnd
}

Collections can also be iterated over by loops:

fun main() { 
//sampleStart
    val cakes = listOf("carrot", "cheese", "chocolate")

    for (cake in cakes) {
        println("Yummy, it's a $cake cake!")
    }
    // Yummy, it's a carrot cake!
    // Yummy, it's a cheese cake!
    // Yummy, it's a chocolate cake!
//sampleEnd
}

While

while can be used in two ways:

To execute a code block while a conditional expression is true. (while)
To execute the code block first and then check the conditional expression. (do-while)

In the first use case (while):

Declare the conditional expression for your while loop to continue within parentheses ().
Add the action you want to complete within curly braces {}.

fun main() {
//sampleStart
    var cakesEaten = 0
    while (cakesEaten < 3) {
        println("Eat a cake")
        cakesEaten++
    }
    // Eat a cake
    // Eat a cake
    // Eat a cake
//sampleEnd
}

In the second use case (do-while):

Declare the conditional expression for your while loop to continue within parentheses ().
Define the action you want to complete within curly braces {} with the keyword do.

fun main() {
//sampleStart
    var cakesEaten = 0
    var cakesBaked = 0
    while (cakesEaten < 3) {
        println("Eat a cake")
        cakesEaten++
    }
    do {
        println("Bake a cake")
        cakesBaked++
    } while (cakesBaked < cakesEaten)
    // Eat a cake
    // Eat a cake
    // Eat a cake
    // Bake a cake
    // Bake a cake
    // Bake a cake
//sampleEnd
}

For more information and examples of conditional expressions and loops, see Conditions and loops.

Now that you know the fundamentals of Kotlin control flow, it's time to learn how to write your own functions.

Practice

Exercise 1

Using a when expression, update the following program so that when you input the names of GameBoy buttons, the actions are printed to output.

Button	Action
A	Yes
B	No
X	Menu
Y	Nothing
Other	There is no such button

fun main() {
    val button = "A"

    println(
        // Write your code here
    )
}

fun main() { val button = "A" println( when (button) { "A" -> "Yes" "B" -> "No" "X" -> "Menu" "Y" -> "Nothing" else -> "There is no such button" } ) }

Exercise 2

You have a program that counts pizza slices until there’s a whole pizza with 8 slices. Refactor this program in two ways:

Use a while loop.
Use a do-while loop.

fun main() {
    var pizzaSlices = 0
    // Start refactoring here
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    println("There's only $pizzaSlices slice/s of pizza :(")
    pizzaSlices++
    // End refactoring here
    println("There are $pizzaSlices slices of pizza. Hooray! We have a whole pizza! :D")
}

fun main() { var pizzaSlices = 0 while ( pizzaSlices < 7 ) { pizzaSlices++ println("There's only $pizzaSlices slice/s of pizza :(") } pizzaSlices++ println("There are $pizzaSlices slices of pizza. Hooray! We have a whole pizza! :D") }

fun main() { var pizzaSlices = 0 pizzaSlices++ do { println("There's only $pizzaSlices slice/s of pizza :(") pizzaSlices++ } while ( pizzaSlices < 8 ) println("There are $pizzaSlices slices of pizza. Hooray! We have a whole pizza! :D") }

Exercise 3

Write a program that simulates the Fizz buzz game. Your task is to print numbers from 1 to 100 incrementally, replacing any number divisible by three with the word "fizz", and any number divisible by five with the word "buzz". Any number divisible by both 3 and 5 must be replaced with the word "fizzbuzz".

Hint: Use a for loop to count numbers and a when expression to decide what to print at each step.

fun main() {
    // Write your code here
}

fun main() { for (number in 1..100) { println( when { number % 15 == 0 -> "fizzbuzz" number % 3 == 0 -> "fizz" number % 5 == 0 -> "buzz" else -> number.toString() } ) } }

Exercise 4

You have a list of words. Use for and if to print only the words that start with the letter l.

Hint: Use the .startsWith() function for String type.

fun main() {
    val words = listOf("dinosaur", "limousine", "magazine", "language")
    // Write your code here
}

fun main() { val words = listOf("dinosaur", "limousine", "magazine", "language") for (w in words) { if (w.startsWith("l")) println(w) } }

Next step

Functions

Last modified: 17 July 2023