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. If there is only one line of code per action, the curly braces {} are optional:

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.

To use when:

Place the value you want to evaluate within parentheses ().
Place the branches within curly braces {}.
Use -> in each branch to separate each check from the action to take if the check is successful.

when can be used either as a statement or as an expression. A statement doesn't return anything but performs actions instead.

Here is an example of using when as a statement:

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
}

An expression returns a value that can be used later in your code.

Here is an example of using when as an expression. The when expression is assigned immediately to a variable which is later used with the println() function:

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
}

The examples of when that you've seen so far both had a subject: obj. But when can also be used without a subject.

This example uses a when expression without a subject to check a chain of Boolean expressions:

fun main() {
    val trafficLightState = "Red" // This can be "Green", "Yellow", or "Red"

    val trafficAction = when {
        trafficLightState == "Green" -> "Go"
        trafficLightState == "Yellow" -> "Slow down"
        trafficLightState == "Red" -> "Stop"
        else -> "Malfunction"
    }

    println(trafficAction)
    // Stop
}

However, you can have the same code but with trafficLightState as the subject:

fun main() {
    val trafficLightState = "Red" // This can be "Green", "Yellow", or "Red"

    val trafficAction = when (trafficLightState) {
        "Green" -> "Go"
        "Yellow" -> "Slow down"
        "Red" -> "Stop"
        else -> "Malfunction"
    }

    println(trafficAction)  
    // Stop
}

Using when with a subject makes your code easier to read and maintain. When you use a subject with a when expression, it also helps Kotlin check that all possible cases are covered. Otherwise, if you don't use a subject with a when expression, you need to provide an else branch.

Conditional expressions practice

Exercise 1

Create a simple game where you win if throwing two dice results in the same number. Use if to print You win :) if the dice match or You lose :( otherwise.

Hint: Use the equality operator (==) to compare the dice results.

import kotlin.random.Random

fun main() {
    val firstResult = Random.nextInt(6)
    val secondResult = Random.nextInt(6)
    // Write your code here
}

import kotlin.random.Random fun main() { val firstResult = Random.nextInt(6) val secondResult = Random.nextInt(6) if (firstResult == secondResult) println("You win :)") else println("You lose :(") }

Exercise 2

Using a when expression, update the following program so that it prints the corresponding actions when you input the names of game console buttons.

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" } ) }

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.

Loops practice

Exercise 1

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 2

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

Hint 2: Use the modulo operator (%) to return the remainder of a number being divided. Use the equality operator (==) to check if the remainder equals zero.

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" } ) } }

Exercise 3

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: 22 October 2024