Var for variables (mutable) in Swift

Introduction

Use var to create a variable that can change its value later. It helps you store information that might update.

When you want to keep track of a score that changes during a game.

When you need to store a user's name that can be updated.

When you want to count how many times a button is pressed.

When you want to store the current temperature that updates regularly.

When you want to remember a list size that can grow or shrink.

Syntax

Swift

var variableName: Type = initialValue

You can omit the type if you provide an initial value, and Swift will infer the type.

Variables declared with var can be changed later in the code.

Examples

Creates an integer variable named age with the value 25.

Swift

var age: Int = 25

Creates a string variable named name with the value "Alice". Type is inferred.

Swift

var name = "Alice"

Creates a variable score starting at 0, then changes it to 10.

Swift

var score = 0
score = 10

Sample Program

This program shows how a variable counter starts at 1 and then changes to 5.

Swift

var counter = 1
print("Counter is \(counter)")
counter = 5
print("Counter changed to \(counter)")

Important Notes

Use var only when you need to change the value. Otherwise, use let for constants.

Changing a var variable is easy and helps keep your program flexible.

Summary

var creates a variable that can change.

Variables store information that can update during the program.

Use var when you expect the value to change later.

Practice

1. What does the var keyword do in Swift?

easy

A. It creates a variable whose value can change later.

B. It creates a constant value that cannot change.

C. It defines a function.

D. It declares a type alias.

Solution

1. Step 1: Understand the purpose of var

   The var keyword is used to declare variables that can be changed after they are set.

2. Step 2: Compare with constants

   Unlike let, which creates constants, var allows the value to be updated.

3. Final Answer:

   It creates a variable whose value can change later. -> Option A

4. Quick Check:

   var means mutable variable [OK]

Hint: Remember: var means variable, value can change [OK]

Common Mistakes:

• Confusing var with let (constant)
• Thinking var creates constants
• Mixing var with function declaration

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

easy

A. let score = 10

B. variable score = 10

C. const score = 10

D. var score = 10

Solution

1. Step 1: Identify the keyword for mutable variables

   In Swift, var declares a variable that can change, so use var score = 10.

2. Step 2: Check other options

   let creates a constant, variable and const are not valid Swift keywords.

3. Final Answer:

   var score = 10 -> Option D

4. Quick Check:

   Mutable variable uses var [OK]

Hint: Use var for variables, let for constants [OK]

Common Mistakes:

• Using let instead of var for mutable variables
• Using invalid keywords like variable or const
• Missing the equals sign

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

var count = 5
count = 10
print(count)

medium

A. 10

B. 5

C. Error: Cannot assign to constant

D. nil

Solution

1. Step 1: Understand variable assignment

   The variable count is declared with var and initially set to 5, so it can be changed.

2. Step 2: Update and print value

   The value is changed to 10 before printing, so print(count) outputs 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Mutable var updates value correctly [OK]

Hint: var allows value change, so print updated value [OK]

Common Mistakes:

• Thinking var acts like let (constant)
• Expecting original value instead of updated
• Confusing syntax causing errors

4. Identify the error in this Swift code:

var name = "Alice"
name = 42
print(name)

medium

A. Syntax error on variable declaration

B. No error, prints 42

C. Type mismatch error when assigning 42 to name

D. Runtime error when printing

Solution

1. Step 1: Check variable type inference

   Variable name is inferred as String because of initial value "Alice".

2. Step 2: Assign incompatible type

   Assigning an integer 42 to a String variable causes a type mismatch error.

3. Final Answer:

   Type mismatch error when assigning 42 to name -> Option C

4. Quick Check:

   Swift variables keep type, cannot assign different type [OK]

Hint: Variable type fixed by first value, cannot assign different type [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking var allows any type changes
• Confusing syntax error with type error

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

1. let score = 0
2. score = 10

3. var score = 0
4. score = 10

5. var score: String
6. score = 10

7. let score: Int
8. score = 10

hard

A. Lines 1 and 2

B. Lines 3 and 4

C. Lines 5 and 6

D. Lines 7 and 8

Solution

1. Step 1: Identify mutable variable declaration

   Lines 3 and 4 declare score with var and assign 0, then update to 10, which is correct.

2. Step 2: Check other options

   Lines 1 and 2 use let (constant), so line 2 causes error. Lines 5 and 6 declare score as String but assign integer 10, causing type mismatch error. Lines 7 and 8 use let constant and then try to assign, causing error.

3. Final Answer:

   Lines 3 and 4 -> Option B

4. Quick Check:

   Use var to declare and update variables [OK]

Hint: Declare with var and assign before update [OK]

Common Mistakes:

• Using let for variables that change
• Type mismatch with wrong type annotation
• Confusing var and let usage