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when

Learn multi-way selection with when.

The when statement

when compares a value against several possible cases. It is like Java's switch but more powerful. Each branch uses ->.

val day = 3
when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    3 -> println("Wednesday")
    else -> println("Unknown")
}

day is 3, so the 3 -> branch runs and prints "Wednesday". If no branch matches, the else branch runs.

when is more readable; it replaces long if/else if chains.

Practice Convert day number to name

Write a fun main(). Assign 3 to val day. Use when (day) to print the day name: 1 "Monday", 2 "Tuesday", 3 "Wednesday", else "Unknown".

Starter code

Main.kt
fun main() {
    // Convert day number to name
}

Sample solution

Main.kt
fun main() {
    val day = 3
    when (day) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        else -> println("Unknown")
    }
}

Expected output

Wednesday

Multiple values and ranges in when

when branches can list several values with commas or use ranges with in.

val n = 5
when (n) {
    1, 2, 3 -> println("Small")
    in 4..7 -> println("Medium")
    else -> println("Large")
}

n is 5, so the in 4..7 branch runs and prints "Medium". Ranges use ..; you'll see them in detail later.

If no branch matches, else runs. else is optional, but with no match nothing runs.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does when (day) { ... } compare against?

  • A fixed value
  • The day variable Correct answer
  • A Boolean
  • A random value

Explanation: when (day) compares day's value against the branches.

2 Complete the when branch

Question 
when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    ____ -> println("Wednesday")
}
Answer 
when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    3 -> println("Wednesday")
}

Explanation: 3 -> println("Wednesday") matches the third day.

3 Match the when forms

Term Meaning
1 -> ... Single value branch
1, 2 -> ... Multi value branch
in 4..7 -> ... Range branch
else -> ... Default branch

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together