Var for mutable references in Kotlin - Step-by-Step Execution

Concept Flow - Var for mutable references

Declare var x = 5

↓

x holds value 5

↓

Change x = 10

↓

x now holds value 10

↓

Use x in expressions

↓

Program continues

This flow shows how a variable declared with var can be changed to hold different values during program execution.

Execution Sample

Kotlin

var x = 5
println(x)
x = 10
println(x)

This code declares a mutable variable x, prints its initial value, changes it, then prints the new value.

Execution Table

Step	Action	Variable x	Output
1	Declare var x = 5	5
2	Print x	5	5
3	Assign x = 10	10
4	Print x	10	10
5	End of program	10

💡 Program ends after printing updated value of x

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
x	undefined	5	10	10

Key Moments - 2 Insights

Why can we change the value of x after declaring it with var?

Because var declares a mutable variable, so its value can be updated as shown in steps 1 and 3 of the execution_table.

What would happen if x was declared with val instead of var?

Declaring x with val makes it immutable, so trying to assign a new value like in step 3 would cause a compile error.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x after step 3?

A 5

B undefined

C 10

D Error

💡 Hint

Check the 'Variable x' column at step 3 in the execution_table.

At which step is the first output produced?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Output' column in the execution_table to find when output first appears.

If we change var x = 5 to val x = 5, what happens at step 3?

A x changes to 10

B Compile error

C x remains 5 silently

D Runtime error

💡 Hint

Recall that val means immutable; reassigning causes a compile error.

Concept Snapshot

var declares a mutable variable in Kotlin
You can change its value anytime after declaration
Use var when you need to update the variable
val declares an immutable variable (cannot change)
Changing a var variable updates its stored value
Printing shows current value at that moment

Full Transcript

This example shows how to use var in Kotlin to create a mutable variable named x. Initially, x is set to 5. When we print x, it outputs 5. Then we change x to 10. Printing x again outputs 10. The key point is that var allows the variable to be changed after it is declared. If we used val instead, trying to change x would cause an error. The execution table tracks each step, showing the value of x and any output. The variable tracker shows how x changes from undefined to 5, then to 10. This helps beginners see how mutable variables work in Kotlin.

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