Kotlin when Expression

In Kotlin, the when expression is a conditional expression that allows you to check a value against a set of possible values and execute code based on the matched value. The basic syntax of the when expression is as follows:

val result = when (value) {
    option1 -> {
        // code to execute when value matches option1
        value1
    }
    option2 -> {
        // code to execute when value matches option2
        value2
    }
    else -> {
        // code to execute when value doesn't match any option
        defaultValue
    }
}

In this syntax, value is the value to check, option1 and option2 are the possible values to match against, value1 and value2 are the values to return when value matches option1 or option2, and defaultValue is the value to return when value doesn't match any option. The when expression returns the value of the matched branch.

Here's an example that demonstrates the use of the when expression in Kotlin:

fun main() {
    val day = "Monday"
    val message = when (day) {
        "Monday" -> "Today is Monday"
        "Tuesday" -> "Today is Tuesday"
        "Wednesday" -> "Today is Wednesday"
        "Thursday" -> "Today is Thursday"
        "Friday" -> "Today is Friday"
        else -> "Today is a weekend day"
    }
    println(message)
}

In this example, the program checks the value of the day variable against a set of possible values using the when expression. If day is "Monday", the program sets message to the string "Today is Monday". If day is "Tuesday", the program sets message to the string "Today is Tuesday", and so on. If day doesn't match any of the options, the program sets message to the string "Today is a weekend day". Finally, the program prints the value of message to the console using the println() function.

It's important to note that in Kotlin, the when expression can be used to check any value, not just an integer value or an enum value like in some other programming languages. The when expression can also be used as a statement, in which case it executes the code in the matched branch without returning a value. For example:

when (value) {
    option1 -> {
        // code to execute when value matches option1
    }
    option2 -> {
        // code to execute when value matches option2
    }
    else -> {
        // code to execute when value doesn't match any option
    }
}

In this example, the when expression executes the code in the branch that matches the value of value, or the else branch if no other branch matches. The when expression doesn't return a value.