Operators and Expressions

# CHAPTER 5

Operators and Expressions #

1. Chapter Introduction #

2. Learning Objectives #

• Perform basic math using Arithmetic Operators.

• Compare values using Relational/Comparison Operators.

• Combine conditions using Logical Operators.

• Understand Assignment Operators.

• Recognize Kotlin's unique Null Safety operators.

3. Arithmetic Operators #

• Addition: +

• Subtraction: -

• Multiplication: *

• Division: /

• Modulus (Remainder): %

fun main() {
    val a = 10
    val b = 3
    
    println("Addition: " + (a + b))       // 13
    println("Division: " + (a / b))       // 3 (Because both are Ints, the decimal is dropped!)
    println("Modulus: " + (a % b))        // 1 (The remainder of 10 divided by 3)
}

4. Assignment Operators #

• Standard: =

• Add and Assign: +=

• Subtract and Assign: -=

• Multiply and Assign: *=

• Divide and Assign: /=

fun main() {
    var score = 10
    score += 5  // Equivalent to: score = score + 5
    println(score) // 15
}

*Note: You cannot use Assignment operators on val variables, only var!*

5. Comparison (Relational) Operators #

• Equal to: ==

• Not Equal to: !=

• Greater than: >

• Less than: <

• Greater than or equal: >=

• Less than or equal: <=

fun main() {
    val a = 5
    val b = 10
    
    println(a == b) // false
    println(a != b) // true
    println(a < b)  // true
}

*In Java, == compares memory references for Objects. In Kotlin, == automatically checks the actual data (structural equality).*

6. Logical Operators #

• AND: && (Returns true ONLY if BOTH sides are true)

• OR: || (Returns true if AT LEAST ONE side is true)

• NOT: ! (Reverses the boolean value)

fun main() {
    val hasTicket = true
    val isVip = false
    
    println(hasTicket && isVip) // false (Both must be true)
    println(hasTicket || isVip) // true (Only one needs to be true)
    println(!hasTicket)         // false (Reverses true to false)
}

7. Kotlin's Special Operators (Preview) #

#### The Safe Call Operator ?. If you try to access a property on an object that is null, Java crashes. Kotlin allows you to use ?.. It says: "If the variable is not null, execute this. If it is null, just return null safely and don't crash!"

// Assume 'name' might be null
var name: String? = null
println(name?.length) // Prints "null" instead of crashing!

#### The Elvis Operator ?: Named because it looks like Elvis Presley's hair emoji! It provides a "default" value if the variable on the left is null.

var name: String? = null
// If 'name' is null, use "Guest" instead
val displayName = name ?: "Guest" 
println(displayName) // Prints "Guest"

8. Increment and Decrement #

var i = 0
i++ // i is now 1
i-- // i is back to 0

9. Common Mistakes #

• Integer Division: Dividing 10 / 3 in Kotlin yields 3, not 3.333. Because both numbers are Integers, the result is forced to be an Integer. To get a decimal, at least one number must be a Float or Double: 10.0 / 3.

• Using = instead of ==: Writing if (a = b) is a syntax error in Kotlin. = is for assignment. == is for comparison.

10. Best Practices #

• Parentheses for Clarity: Even when order of operations (PEMDAS) dictates the math, use parentheses to make complex logical expressions readable for humans: if ((x > 5) && (y < 10))

11. Exercises #

1. 1. Declare two var variables, x = 15 and y = 4.

1. 2. Print the remainder of x divided by y.

1. 3. Use += to add 5 to x.

1. 4. Print the boolean result of whether x is now greater than 15.

12. MCQs with Answers #

Q10. Can you use x++ if x was declared using val? a) Yes b) No, because val is immutable and cannot be reassigned Answer: b) No.

13. Interview Questions #

• Q: Explain how the == operator differs in Kotlin compared to Java. (Answer: In Java, == checks for reference equality (if they point to the same memory). In Kotlin, == calls the .equals() method under the hood to check for structural/data equality).

• Q: Explain the purpose of the Elvis operator ?: with an example.

14. Summary #

15. Next Chapter Recommendation #