The **`when` expression** in Kotlin takes a value as input and checks the value against the condition provided. 

If there is a match present for the value passed to the `when` expression, then the matched condition block will be executed. We should add the `else` branch to the `when` expression. The `else` branch will be executed when no condition is matched (like the default case in the switch statement). If the `else` condition is not included in the code and the statement does not match any `when` condition, then we will get an #key# error: error: 'when' expression must be exhaustive, add necessary 'else' branch #key#.	


> It is like a modified version of the switch statement with a return value. 

# Example

fun main(args : Array<String>) {
    val day = "Sunday"
    val dayType = when(day) {
        "Sunday" -> "Weekend"
        "Saturday" -> "Weekend"
        else -> "Working days"
    }
    println("$day is $dayType")
}

In the code above, we have created a `day` variable with the `Sunday` value, and passed it to the `when` expression. Inside the `when` expression, the value is matched with the available conditions. The `day` variable matches with the "Sunday" value, so it will return the `Weekend` as the return value of the `when` expression.

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# Example of handling multiple conditions

We can combine multiple conditions into a single block.   

fun main(args : Array<String>) {
    val day = "Sunday"
    val dayType = when(day) {
        "Monday", "Tuesday", "Wednesday" -> "First half"
        "Thursday", "Friday" -> "Second half"
        else -> "Week end"
    }
    println("$day is $dayType")
}

In the code above, we have added multiple conditions to a single block. If one of the conditions satisfies the value passed to the `when` expression, then that block will be executed. 

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# Using an expression as a condition
We can use an expression as a condition. 

fun main(args : Array<String>) {
    val char1 = "A"
    val char2 = "a"
    when(char1) {
        char2.toUpperCase() ->{
            println(char2)
        }
        else -> {
            println("Not equal")
        }
    }
}

In the code above, we have used the `char2.toUpperCase()` expression as a condition to the `when`  expression. We also skipped the return value and handled inside the condition block.

---

# Using the `in` operator as a condition


fun main(args : Array<String>) {
    val experienceInYears = 5
    val status = when(experienceInYears) {
        0 -> "Trainee"
        in 1..2 -> "Junior Deveoper"
        in 3..5 -> "Senior Developer"
        in 6..10 -> "Product Manager"
        in 11..20 -> "Manager"
        else -> "Fire him/her"
    }
    println(status);
}

In the code above, we used the `in` operator inside of the `when` block to check if the value was present in the range.

When to use the Kotlin "when" expression

Use Kotlin's when expression for condition-checking with an optional else branch, combining conditions, or using expressions and range checks.

When to use the Kotlin "when" expression

Example

Example of handling multiple conditions

Using an expression as a condition

Using the in operator as a condition

Using the `in` operator as a condition