Var for mutable references in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using var?

Consider the following Kotlin code snippet. What will be printed?

Kotlin

fun main() {
    var number = 10
    number = 20
    println(number)
}

A 20

B 10

C Compilation error

D Runtime exception

💡 Hint

Remember that var allows the variable to be changed.

The variable number is declared with var, so it can be reassigned. Initially it is 10, then changed to 20, so println outputs 20.

❓ Predict Output

intermediate

What happens when you reassign a val variable?

What will happen when running this Kotlin code?

Kotlin

fun main() {
    val text = "Hello"
    text = "World"
    println(text)
}

A World

B Runtime exception

C Hello

D Compilation error

💡 Hint

val means the variable cannot be reassigned.

The variable text is declared with val, which means it is read-only. Trying to assign a new value causes a compilation error.

❓ Predict Output

advanced

What is the output when modifying a mutable list declared with var?

Analyze this Kotlin code and determine the output.

Kotlin

fun main() {
    var list = mutableListOf(1, 2, 3)
    list.add(4)
    println(list)
}

A [1, 2, 3, 4]

B Runtime exception

C [1, 2, 3]

D Compilation error

💡 Hint

Mutable lists can be changed even if declared with var.

The variable list is mutable and declared with var. Adding 4 modifies the list, so printing it shows all four elements.

❓ Predict Output

advanced

What error occurs when trying to reassign a val mutable list?

What will happen when running this Kotlin code?

Kotlin

fun main() {
    val list = mutableListOf(1, 2, 3)
    list = mutableListOf(4, 5, 6)
    println(list)
}

A [4, 5, 6]

B [1, 2, 3]

C Compilation error

D Runtime exception

💡 Hint

val means the reference cannot be changed, even if the object is mutable.

The variable list is declared with val, so you cannot assign a new list to it. This causes a compilation error.

🧠 Conceptual

expert

Why use var for mutable references in Kotlin?

Which statement best explains why var is used for mutable references in Kotlin?

A <p>Because <code>var</code> makes the object itself immutable.</p>

B <p>Because <code>var</code> allows changing the reference to point to a different object or value.</p>

C <p>Because <code>var</code> prevents any changes to the variable after initialization.</p>

D <p>Because <code>var</code> automatically synchronizes access in multithreaded code.</p>

💡 Hint

Think about what var allows you to do with a variable.

var means the variable's reference can be changed to point to a different object or value. It does not make the object immutable or handle synchronization.

Practice

1. What does the var keyword do in Kotlin?

easy

A. Declares a class

B. Creates a constant value that cannot be changed

C. Creates a variable whose value can be changed later

D. Defines a function

Solution

1. Step 1: Understand the role of var

   The var keyword declares a variable that can be changed after it is created.

2. Step 2: Compare with other keywords

   val creates a constant (unchangeable) variable, so var is for mutable variables.

3. Final Answer:

   Creates a variable whose value can be changed later -> Option C

4. Quick Check:

   var means mutable variable [OK]

Hint: Remember: var = variable that can vary [OK]

Common Mistakes:

• Confusing var with val (immutable)
• Thinking var declares constants
• Mixing var with function or class declarations

2. Which of the following is the correct way to declare a mutable integer variable named count with initial value 10 in Kotlin?

easy

A. val count: Int = 10

B. val count = 10

C. var count: Int

D. var count = 10

Solution

1. Step 1: Identify mutable variable declaration

   To declare a mutable variable, use var with an initial value.

2. Step 2: Check syntax correctness

   var count = 10 correctly declares a mutable integer variable with initial value 10.

3. Final Answer:

   var count = 10 -> Option D

4. Quick Check:

   Mutable variable needs var and initial value [OK]

Hint: Use var with initial value for mutable variables [OK]

Common Mistakes:

• Using val instead of var for mutable variables
• Declaring var without initial value or type
• Confusing val and var syntax

3. What will be the output of this Kotlin code?

var x = 5
x = 10
println(x)

medium

A. 10

B. Compilation error

C. 5

D. Runtime error

Solution

1. Step 1: Understand variable assignment

   Variable x is declared mutable with initial value 5, then changed to 10.

2. Step 2: Print the current value of x

   The print statement outputs the updated value 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Mutable var allows reassignment, so output is 10 [OK]

Hint: var lets you change value; print latest value [OK]

Common Mistakes:

• Thinking var variables cannot be reassigned
• Expecting initial value to print
• Confusing compilation and runtime errors

4. Identify the error in this Kotlin code snippet:

var name: String
name = "Alice"
name = 123

medium

A. Variable name must be declared with val

B. Cannot assign a number to a String variable

C. Missing initial value for var

D. Syntax error in variable declaration

Solution

1. Step 1: Check variable type and assignments

   Variable name is declared as String, assigned "Alice" (valid), then assigned 123 (number).

2. Step 2: Identify type mismatch error

   Assigning an integer to a String variable causes a type error.

3. Final Answer:

   Cannot assign a number to a String variable -> Option B

4. Quick Check:

   Type mismatch causes error [OK]

Hint: Check variable type before assigning new value [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking var requires initial value always
• Confusing val and var usage

5. You want to keep track of a user's score that changes during the game. Which Kotlin code correctly declares and updates this score using var?

hard

A. var score = 0
score = score + 10
println(score)

B. val score = 0
score = score + 10
println(score)

C. var score: Int
println(score)

D. val score: Int = 0
score += 10
println(score)

Solution

1. Step 1: Declare a mutable variable with initial value

   Use var score = 0 to allow changes to score.

2. Step 2: Update the score correctly

   Adding 10 to score with score = score + 10 works because score is mutable.

3. Final Answer:

   var score = 0 score = score + 10 println(score) -> Option A

4. Quick Check:

   Mutable var allows updating score [OK]

Hint: Use var for values that change, val for constants [OK]

Common Mistakes:

• Using val for variables that need updating
• Declaring var without initial value and using before assignment
• Trying to update val variables causing errors