Conditional Statements

# CHAPTER 7

Conditional Statements #

1. Chapter Introduction #

2. Learning Objectives #

• Use standard if, else if, and else statements.

• Assign values dynamically using if as an Expression.

• Use the when statement for multi-branch logic.

• Replace Java's switch statement with when.

• Build a Mini Project: Grade Calculator.

3. The if Statement #

fun main() {
    val isRaining = true

    if (isRaining) {
        println("Take an umbrella!")
    } else {
        println("Wear sunglasses!")
    }
}

For multiple conditions, use else if:

val score = 85

if (score >= 90) {
    println("Grade A")
} else if (score >= 80) {
    println("Grade B")
} else {
    println("Grade C or below")
}

4. if as an Expression (Kotlin Superpower) #

Because of this, Kotlin does not have a Ternary Operator (condition ? true : false). It doesn't need one!

fun main() {
    val a = 10
    val b = 20
    
    // Assigning the result of the if/else directly to the variable 'max'!
    val max = if (a > b) {
        a // Returns a
    } else {
        b // Returns b
    }

    println("The maximum value is $max")
    
    // Short, one-line version (Ternary equivalent):
    val min = if (a < b) a else b
}

*Note: If you use if as an expression, it MUST have an else branch.*

5. The when Expression #

fun main() {
    val dayNumber = 3

    when (dayNumber) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        4 -> println("Thursday")
        5 -> println("Friday")
        else -> println("Weekend or Invalid") // The 'else' acts like 'default'
    }
}

#### Advanced when Usage when can check multiple conditions, ranges, and even types!

val age = 15

// Using when as an Expression (assigning result to a variable)
val status = when (age) {
    in 0..12 -> "Child"         // Check if age is in a range (0 to 12)
    13, 14, 15 -> "Young Teen"  // Match multiple specific values
    in 16..19 -> "Teenager"
    else -> "Adult"
}

println("Status: $status")

6. Mini Project: Grade Calculator #

fun main() {
    print("Enter student score (0-100): ")
    val score = readln().toIntOrNull() ?: 0

    val grade = when (score) {
        in 90..100 -> "A"
        in 80..89  -> "B"
        in 70..79  -> "C"
        in 60..69  -> "D"
        in 0..59   -> "F"
        else -> "Invalid Score"
    }

    println("Final Grade: $grade")
}

7. Common Mistakes #

• Forgetting else in Expressions: If you assign the result of an if or when to a variable, you MUST provide an else branch. The compiler needs a guarantee that the variable will definitely receive *some* value.

• Using Java's switch: Kotlin does not have the switch keyword. Attempting to use it will result in a syntax error.

8. Best Practices #

• Prefer when over else if: If you have more than two branches of logic checking the same variable, always use when. It is vastly more readable.

• Single-line logic: Use the inline if expression (val res = if (x) a else b) instead of creating a mutable var and assigning it inside standard if/else blocks.

9. Exercises #

1. 1. Write a program that asks for the user's current temperature.

1. 2. Use an if expression to assign a String variable clothing.

1. 3. If temp > 25, clothing = "T-Shirt". Else, clothing = "Jacket".

1. 4. Print the result.

10. MCQs with Answers #

Q8. Can a single branch in a when expression match multiple values? a) Yes, by separating them with commas (e.g., 1, 2, 3 ->) b) No Answer: a) Yes.

Q9. Like if, can when be used as an expression to assign a variable? a) Yes b) No Answer: a) Yes.

11. Interview Questions #

• Q: Explain how Kotlin's if expression eliminates the need for a ternary operator. Provide a code example.

• Q: Compare Java's switch statement with Kotlin's when expression. Why is when considered superior? (Answer: when doesn't require break statements, prevents fall-through bugs, can be used as an expression, and supports complex range/type checking).

12. Summary #

13. Next Chapter Recommendation #