Var for mutable references in Kotlin - Mini Project: Build & Apply

Using <code>var</code> for Mutable References in Kotlin

📖 Scenario: Imagine you are managing a simple inventory system for a small store. You want to keep track of the number of items available and update this number as items are sold or restocked.

🎯 Goal: You will create a mutable variable using var in Kotlin to hold the inventory count. Then, you will update this count to reflect sales and restocking.

📋 What You'll Learn

Create a mutable variable called inventoryCount with an initial value of 50.

Create a variable called soldItems with the value 5.

Update inventoryCount by subtracting soldItems.

Print the updated inventoryCount.

Create a variable called restockedItems with the value 10.

Update inventoryCount by adding restockedItems.

Print the final inventoryCount.

💡 Why This Matters

🌍 Real World

Managing inventory counts is a common task in stores, warehouses, and online shops. Using mutable variables helps keep track of changing stock levels.

💼 Career

Understanding mutable variables is essential for any Kotlin developer working on apps that handle data changes, such as inventory systems, games, or user settings.

1

Create a mutable variable for inventory count

Create a mutable variable called inventoryCount and set it to 50.

Kotlin

# Your code here

Hint

Use var to create a variable that can change later.

2

Create a variable for sold items

Create a variable called soldItems and set it to 5.

Kotlin

var inventoryCount = 50
# Your code here

Hint

Use val for values that do not change.

3

Update inventory after sales

Update inventoryCount by subtracting soldItems using inventoryCount = inventoryCount - soldItems.

Kotlin

var inventoryCount = 50
val soldItems = 5
# Your code here

Hint

Assign the new value back to inventoryCount after subtracting.

4

Print updated inventory and restock

Print the current inventoryCount. Then create a variable called restockedItems with value 10, update inventoryCount by adding restockedItems, and print the final inventoryCount.

Kotlin

var inventoryCount = 50
val soldItems = 5
inventoryCount = inventoryCount - soldItems
# Your code here

Hint

Use println() to show the values. Remember to update inventoryCount after restocking.

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