Var for mutable references in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does var mean in Kotlin?

var declares a variable whose value can change (mutable).

beginner

How do you declare a variable that cannot change in Kotlin?

Use val to declare an immutable (read-only) variable.

beginner

What happens if you try to reassign a val variable?

Kotlin will give a compile error because val variables cannot be reassigned.

beginner

Example:
var count = 5
count = 10
What is the value of count after reassignment?

The value of count is 10 because var allows changing the value.

beginner

Why use var instead of val?

Use var when you need to change the value later, like a counter or user input.

Which keyword declares a mutable variable in Kotlin?

A const

B val

C var

D let

var declares a mutable variable that can be reassigned.

What happens if you try to assign a new value to a val variable?

A Compile-time error

B It works fine

C Runtime error

D Value changes silently

val variables cannot be reassigned; Kotlin gives a compile-time error.

Which of these is a correct way to declare a mutable integer variable with initial value 3?

A const number = 3

B val number = 3

C let number = 3

D var number = 3

var number = 3 declares a mutable variable with value 3.

If you want a variable that never changes, which keyword do you use?

A var

B val

C mutable

D const

val declares an immutable variable that cannot be reassigned.

What is the main difference between var and val?

A <code>var</code> is mutable; <code>val</code> is immutable

B <code>var</code> is immutable; <code>val</code> is mutable

C Both are mutable

D Both are immutable

var allows changing the value; val does not.

Explain the difference between var and val in Kotlin.

Think about whether the variable's value can change after you set it.

Give an example where you would use var instead of val.

Think about situations where the value needs to update.

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