Var for variables (mutable) in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does var mean in Swift?

var is used to declare a variable that can change its value later. It means the value is mutable.

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How do you declare a variable named score with the value 10 using var?

You write: var score = 10. This means score can be changed later.

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What happens if you try to change a variable declared with let instead of var?

let creates a constant, which means its value cannot change. Trying to change it causes an error.

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Why use var instead of let?

Use var when you want to change the value later, like a score in a game. Use let when the value stays the same.

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Example: What is the output of this code?

var count = 5
count = 10
print(count)

The output is 10 because count was changed from 5 to 10.

Which keyword declares a variable that can change its value in Swift?

A const

B let

C var

D static

var declares a mutable variable. let declares a constant.

What happens if you try to assign a new value to a let constant?

A It changes successfully

B It causes a compile-time error

C It prints a warning but runs

D It converts to <code>var</code> automatically

let constants cannot be changed. Trying to do so causes an error.

Which of these is a correct way to declare a mutable variable name with value "Anna"?

A static name = "Anna"

B let name = "Anna"

C const name = "Anna"

D var name = "Anna"

var name = "Anna" declares a variable that can be changed later.

If you want a value to never change, which keyword should you use?

A let

B change

C mutable

D var

let creates a constant that cannot be changed.

What will this code print?

var x = 3
x = 7
print(x)

A 7

B 3

C Error

D Nothing

The variable x is changed to 7, so it prints 7.

Explain the difference between var and let in Swift.

Think about whether the value can be changed after it is set.

Write a short Swift code snippet that shows how to declare a variable and change its value.

Start with var, then assign a new value, then print.

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