Kotlin Control Flow: Mastering if, else, and Conditional Expressions

In programming, control flow determines the order in which instructions are executed. Kotlin provides a variety of control flow structures, such as if-else statements, loops, when expressions, and more, to help you make decisions, repeat actions, and break out of loops. In this first article of the control flow series, we’ll explore if, else, and conditional expressions in Kotlin.

By the end of this article, you’ll understand how to make decisions in your Kotlin programs and use if statements effectively.

The Basics of if and else in Kotlin

In Kotlin, if statements allow you to make decisions based on conditions. An if statement evaluates a Boolean expression, and if the expression is true, the block of code inside the if statement is executed. If the expression is false, the else block (if present) will be executed.

Example:

fun main() {
    val age = 18

    if (age >= 18) {
        println("You are an adult.")
    } else {
        println("You are not an adult.")
    }
}

In this example, the condition age >= 18 evaluates to true, so the if block executes, printing "You are an adult." If age had been less than 18, the else block would have executed.

if-else if Ladder

Kotlin allows you to chain multiple conditions together using else if blocks. This is useful when you need to check several conditions sequentially.

Example:

fun main() {
    val age = 16

    if (age >= 18) {
        println("You are an adult.")
    } else if (age >= 13) {
        println("You are a teenager.")
    } else {
        println("You are a child.")
    }
}

In this example, multiple conditions are checked in sequence. Since age = 16, the second condition (age >= 13) is true, so the corresponding block is executed, printing "You are a teenager."

Using if as an Expression

In Kotlin, if can be used as an expression, meaning it can return a value. This makes Kotlin’s if more powerful than in some other languages, where if is just a statement. When used as an expression, if returns the value of the branch that is executed.

Example:

fun main() {
    val age = 25

    val status = if (age >= 18) {
        "Adult"
    } else {
        "Minor"
    }

    println("Status: $status")  // Output: Status: Adult
}

Here, the if-else expression evaluates the condition and assigns either "Adult" or "Minor" to the status variable.

Ternary-Like Behavior

While Kotlin doesn’t have a built-in ternary operator (like in Java or JavaScript), you can achieve the same behavior using if expressions. The ternary operator is often used to condense if-else statements into a single line.

Ternary-like Example:

fun main() {
    val age = 17
    val status = if (age >= 18) "Adult" else "Minor"
    println(status)  // Output: Minor
}

In this example, we achieve a ternary-like operation using an if expression, which condenses the decision-making process into a single line.

Nested if-else Statements

You can nest if-else statements within other if-else statements to handle more complex decision-making. While this can be useful, you should be cautious as it can lead to less readable code if overused.

Example:

fun main() {
    val age = 21
    val hasID = true

    if (age >= 18) {
        if (hasID) {
            println("You can enter.")
        } else {
            println("You need an ID to enter.")
        }
    } else {
        println("You are not allowed to enter.")
    }
}

In this example, the outer if checks the age, and the inner if checks whether the person has an ID. This structure allows for more granular decision-making.

Multi-Branch if Expressions

Kotlin’s if expression can have multiple branches, making it easier to handle different outcomes without using a long chain of else if statements.

Example:

fun main() {
    val temperature = 30

    val weather = if (temperature > 30) {
        "Hot"
    } else if (temperature > 20) {
        "Warm"
    } else {
        "Cold"
    }

    println("The weather is $weather.")  // Output: The weather is Warm.
}

In this example, we handle different temperature ranges in a clean, multi-branch if expression.

Practical Use Case: Grading System

Let’s create a simple grading system where the grade is determined based on a score. This example uses if-else statements to evaluate the score and assign a grade.

Example:

fun main() {
    val score = 85

    val grade = if (score >= 90) {
        "A"
    } else if (score >= 80) {
        "B"
    } else if (score >= 70) {
        "C"
    } else if (score >= 60) {
        "D"
    } else {
        "F"
    }

    println("Your grade is: $grade")  // Output: Your grade is: B
}

Here, the if-else ladder assigns a grade based on the score.

Conclusion

The if-else construct in Kotlin is a versatile and powerful tool for controlling the flow of your program based on conditions. It allows you to:

• Use if and else for basic decision-making.
• Chain multiple conditions with else if.
• Use if as an expression to return values, offering a more concise alternative to traditional if-else statements.
• Nest if-else blocks for more complex conditions.
• Replace the ternary operator with concise if expressions.

Next up, we’ll dive into loops in Kotlin, where we’ll explore how to repeat actions using for, while, and do-while loops. Stay tuned for more control flow insights!