Kotlin when Statement

Introduction

The when statement in Kotlin is a powerful control flow statement that allows you to choose between multiple possible execution paths based on the value of an expression. It is similar to the switch statement found in other programming languages, but more expressive and versatile. This chapter will cover the syntax and usage of the when statement with examples.

Basic when Statement

The when statement evaluates a given expression and executes the corresponding branch of code based on the value of the expression.

Syntax

when (expression) {
    value1 -> {
        // Code to execute if expression == value1
    }
    value2 -> {
        // Code to execute if expression == value2
    }
    else -> {
        // Code to execute if expression does not match any value
    }
}

Example

fun main() {
    val dayOfWeek = 3

    when (dayOfWeek) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        4 -> println("Thursday")
        5 -> println("Friday")
        6 -> println("Saturday")
        7 -> println("Sunday")
        else -> println("Invalid day")
    }
}

Explanation:

• when (dayOfWeek): The when statement evaluates the value of dayOfWeek.
• The branch corresponding to 3 is executed, printing "Wednesday" to the console.

Using Multiple Conditions

You can combine multiple conditions in a single branch by separating them with commas.

Syntax

when (expression) {
    value1, value2 -> {
        // Code to execute if expression == value1 or expression == value2
    }
    else -> {
        // Code to execute if expression does not match any value
    }
}

Example

fun main() {
    val number = 3

    when (number) {
        1, 3, 5, 7, 9 -> println("Odd number")
        2, 4, 6, 8, 10 -> println("Even number")
        else -> println("Number not in range 1-10")
    }
}

Explanation:

• The branch corresponding to 3 is executed, printing "Odd number" to the console.

Using when as an Expression

In Kotlin, when can also be used as an expression that returns a value. This is useful when you want to assign a value based on a condition.

Syntax

val result = when (expression) {
    value1 -> result1
    value2 -> result2
    else -> defaultResult
}

Example

fun main() {
    val number = 4

    val parity = when (number) {
        1, 3, 5, 7, 9 -> "Odd"
        2, 4, 6, 8, 10 -> "Even"
        else -> "Unknown"
    }

    println("Number $number is $parity")
}

Explanation:

• The expression evaluates to "Even" because number is 4.
• parity is assigned the value "Even".
• The output is "Number 4 is Even".

Checking Types with when

The when statement can be used to check the type of an expression.

Syntax

when (expression) {
    is Type1 -> {
        // Code to execute if expression is of Type1
    }
    is Type2 -> {
        // Code to execute if expression is of Type2
    }
    else -> {
        // Code to execute if expression is of neither Type1 nor Type2
    }
}

Example

fun main() {
    val obj: Any = "Hello"

    when (obj) {
        is String -> println("obj is a String of length ${obj.length}")
        is Int -> println("obj is an Int with value $obj")
        is Double -> println("obj is a Double with value $obj")
        else -> println("Unknown type")
    }
}

Explanation:

• The expression obj is evaluated.
• Since obj is a String, the branch for String is executed, printing "obj is a String of length 5".

Using when without an Argument

You can use when without an argument to evaluate multiple Boolean conditions.

Syntax

when {
    condition1 -> {
        // Code to execute if condition1 is true
    }
    condition2 -> {
        // Code to execute if condition2 is true
    }
    else -> {
        // Code to execute if none of the conditions are true
    }
}

Example

fun main() {
    val number = -10

    when {
        number > 0 -> println("Number is positive")
        number < 0 -> println("Number is negative")
        else -> println("Number is zero")
    }
}

Explanation:

• The first condition checks if number is greater than 0.
• The second condition checks if number is less than 0.
• Since number is -10, the second condition is true, and "Number is negative" is printed.

Example Program with when Statement

Here is an example program that demonstrates the use of various forms of the when statement in Kotlin:

fun main() {
    val day = 3

    // Basic when statement
    when (day) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        4 -> println("Thursday")
        5 -> println("Friday")
        6 -> println("Saturday")
        7 -> println("Sunday")
        else -> println("Invalid day")
    }

    val number = 4

    // when as an expression
    val parity = when (number) {
        1, 3, 5, 7, 9 -> "Odd"
        2, 4, 6, 8, 10 -> "Even"
        else -> "Unknown"
    }

    println("Number $number is $parity")

    val obj: Any = 123.45

    // Checking types with when
    when (obj) {
        is String -> println("obj is a String of length ${obj.length}")
        is Int -> println("obj is an Int with value $obj")
        is Double -> println("obj is a Double with value $obj")
        else -> println("Unknown type")
    }

    // when without argument
    val temperature = 30

    when {
        temperature < 0 -> println("It's freezing")
        temperature in 0..15 -> println("It's cold")
        temperature in 16..25 -> println("It's mild")
        temperature > 25 -> println("It's warm")
        else -> println("Unknown temperature")
    }
}

Conclusion

In this chapter, you learned about the when statement in Kotlin, including its syntax and usage with multiple conditions, as an expression, and for type checking. You also saw how to use when without an argument to evaluate multiple Boolean conditions. The when statement is a powerful and flexible tool for making decisions in your Kotlin programs.