Control Flow - When Expression

Instead of writing many if..else expressions, you can use the when expression, which is much easier to read.

It is used to select one of many code blocks to be executed

val day = 4

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

// Outputs "Thursday" (day 4)

The when expression is similar to the switch statement in Java.

This is how it works:

• The when variable (day) is evaluated once
• The value of the day variable is compared with the values of each "branch"
• Each branch starts with a value, followed by an arrow (->) and a result
• If there is a match, the associated block of code is executed
• else is used to specify some code to run if there is no match
• In the example above, the value of day is 4, meaning "Thursday" will be printed

When as an expression 

Just like if-else, when can also be used as an expression by assigning it to a variable. Here is an example:

/**
var x : Any = 13.37
    when(x) {
        is Int -> println("$x is an Int")
        !is Double -> println("$x is not Double")
        is String -> println("$x is a String")
        else -> println("$x is none of the above")
    }
**/

val x : Any = 13.37
//assign when to a variable
  val result =  when(x) {
//let condition for each block be only a string
        is Int -> "is an Int"
        !is Double -> "is not Double"
        is String -> "is a String"
        else -> "is none of the above"
    }
//then print x with the result
print("$x $result")

Now we have simplified the first when statement to a shorter and cleaner code using the expression

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Here you can find the code of the lectures "If Statements" and "When Expressions" with some more details:

fun main(){
    // Control Flows
    // If Statements
    var age = 17
    if(age >= 21){
        print("now you may drink in the US")
    }
    // Else If Statement - only executed if the if statement is not true
    else if(age >= 18){
        print("now you may vote")
    }
    // Else If Statement - only executed if the foregoing else if statement is not true
    else if (age >= 16){
        print("you now may drive")
    }
    // else is only executed if all of the foregoing statements weren't true
    else{
        print("you're too young")
    }

    // Kotlin’s "when" expression is the replacement of the switch statement
    // from other languages like C, C++, and Java.
    // It is compact and more powerful than switch statements.
    var season = 3
    when(season) {
        1 -> println("Spring")
        2 -> println("Summer")
        3 -> println("Fall")
        4 -> println("Winter")
        else -> println("Invalid Season")
    }
    var month = 3
    when(month) {
        1,2,3 -> println("Spring")
        in 4..6 -> println("Summer")
        in 7..9 -> println("Fall")
        in 10..12 -> println("Winter")
        else -> println("Invalid Season")
    }

    // challenge - translate the if statement with the age to a when expression
    when(age){
        // with the !in it's the same as saying not in ...
        !in 0..20  -> print("now you may drink in the US")
        in 18..20  -> print("now you may vote")
        16,17 -> print("you now may drive")
        else -> print("you're too young")
    }

    var x : Any = 13.37
    when(x) {
        is Int -> println("$x is an Int")
        !is Double -> println("$x is not Double")
        is String -> println("$x is a String")
        else -> println("$x is none of the above")
    }
}

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