Conditionals

Conditionals are commands in Kotlin that handle decisions. With conditionals, code is dynamic, which means that it can behave differently given a different condition. In conditional statements different code blocks can be written which can execute when the condition is met.

If Statement

The if statement is used to invoke conditional code. In other words, the code will only run if a predicate is fulfilled.

if(condition){
    // Code block
}

For example, suppose a bot, Qbot, can go watch a movie if it finishes its homework. To accomplish this, code can be written that looks like:

fun main(){
   val FinishedHomeWork = true
   if (FinishedHomeWork){
   print("Can go out to watch the movie")
}

The output for the above code will be:

Can go out to watch the movie

Else Statement

The additional else block is used to provide an alternative for the cases when a predicate is not fulfilled.

if(condition){
    // Code block
}else{
    // Code block
}

In this example the value of FinishedHomework is given as false resulting in execution of the else statement:

fun main(){
    val FinishedHomeWork = false
    if (FinishedHomeWork){
        print("Can go to watch the movie")
    }else{
        print("Cannot go out to watch movie")
    }
}

The output for the above code will be:

Cannot go out to watch movie

If else-if Statement

By placing one if-else block after another, a structure is formed known as if else-if. It is a structure that checks conditions one after another until it finds the first one that is fulfilled, and the code within that block is executed. If all the conditions return false, the else block is called.

if (condition1) {
    // Code to execute if condition1 is true
} else if (condition2) {
    // Code to execute if condition1 is false and condition2 is true
} else {
    // Code to execute if both condition1 and condition2 are false
}

In the examples below, if probability is smaller or equal to 40, then “Unlikely” will be printed. If probability is over 40 but smaller or equal to 80, then “Likely” will be printed. And if probability is over 80 but smaller or equal to 100, then “Yes” will be printed. This is done by using the if else-if control structure:

fun main() {
    println("Is it going to rain?")
    val probability = 70
    if (probability <= 40) {
        println("Unlikely")
    } else if (probability <= 80) {
        println("Likely")
    } else if (probability < 100) {
        println("Yes")
    } else {
        println("What?")
    }
}

The output for the above code will be:

Is it going to rain?
Likely

When Statement

The when statement is an alternative to if else-if. On every branch of the when statement, a predicate and a body is specified. The body will be executed only for the first predicate that returns true. So it works just like if else-if, but it’s preferred because its syntax is better suited for multiple conditions. And thanks to the fact that it’s a single statement, Kotlin can run this code faster than checking multiple if else-if statements.

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 of the above values
    }
}

In the code below, the conditional statements of the previous example are implemented with a when statement in lieu of multiple if-else blocks:

fun main() {
println("Is it going to rain?")
val probability = 70
    when {
        probability <= 40 -> {
        println("Unlikely")
        }
        probability <= 80 -> {
        println("Likely")
        }
        probability < 100 -> {
        println("Yes")
        }
        else ->
        println("What?")
        }
    }
}

The output for the above code will be:

Is it going to rain?
Likely