Var for mutable references in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a mutable variable named count initialized to 10.

Kotlin

var count: Int = [1]

Drag options to blanks, or click blank then click option'

Avar

Bval

C10

D"10"

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around 10, which makes it a string instead of an integer.

Using val instead of var for mutable variables.

The keyword var declares a mutable variable, and count is initialized to the integer 10.

2fill in blank

medium

Complete the code to change the value of the mutable variable score to 42.

Kotlin

score [1] 42

Drag options to blanks, or click blank then click option'

A->

B=

C==

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of = for assignment.

Using arrow -> or colon : which are not assignment operators.

The single equals sign = is used to assign a new value to a variable.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword to declare a mutable variable.

Kotlin

[1] name: String = "Alice"

Drag options to blanks, or click blank then click option'

Avar

Bconst

Cval

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using val when the variable needs to be mutable.

Using const or let which are not Kotlin keywords for variable declaration.

The keyword var declares a mutable variable in Kotlin. val declares an immutable variable.

4fill in blank

hard

Fill both blanks to declare a mutable variable temperature of type Double initialized to 36.6.

Kotlin

[1] temperature: [2] = 36.6

Drag options to blanks, or click blank then click option'

Avar

Bval

CDouble

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using val instead of var for mutability.

Using Int type for a decimal number.

Use var to declare a mutable variable and Double as the type for decimal numbers.

5fill in blank

hard

Fill all three blanks to declare a mutable variable message of type String and assign it the value "Hello".

Kotlin

[1] message: [2] [3] "Hello"

Drag options to blanks, or click blank then click option'

Avar

BString

C=

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using val instead of var for mutability.

Forgetting the assignment operator =.

Using wrong type or missing the type declaration.

Use var for a mutable variable, String as the type, and = to assign the value.

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