Var for variables (mutable) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using var?

Consider this Swift code snippet. What will it print?

Swift

var count = 5
count = 10
print(count)

A 10

B 5

C Compilation error

D Runtime error

💡 Hint

Remember, var allows the value to change.

The variable count is declared with var, so it can be changed from 5 to 10. The print statement outputs 10.

❓ Predict Output

intermediate

What happens if you try to change a let constant?

What error will this Swift code produce?

Swift

let name = "Alice"
name = "Bob"
print(name)

A Runtime error

B Compile-time error: Cannot assign to value: 'name' is a 'let' constant

C Prints Alice

D Prints Bob

💡 Hint

let means the value cannot be changed after assignment.

Since name is declared with let, trying to assign a new value causes a compile-time error.

🔧 Debug

advanced

Why does this code cause an error?

Examine this Swift code. Why does it fail to compile?

Swift

var number = 10
number = "ten"
print(number)

A Prints 10

B Runtime error: Invalid type conversion

C Prints "ten"

D Type mismatch error: Cannot assign String to Int variable

💡 Hint

Swift variables have fixed types once declared.

The variable number is declared as an Int. Assigning a String causes a type mismatch compile error.

🧠 Conceptual

advanced

What is the difference between var and let in Swift?

Choose the best explanation of the difference between var and let in Swift.

A <code>var</code> and <code>let</code> are interchangeable and have no difference.

B <code>var</code> declares a constant; <code>let</code> declares a mutable variable.

C <code>var</code> declares a mutable variable; <code>let</code> declares an immutable constant.

D <code>var</code> is used only for integers; <code>let</code> is used only for strings.

💡 Hint

Think about whether the value can change after assignment.

var allows the value to be changed later. let means the value cannot be changed once set.

❓ Predict Output

expert

What is the output of this Swift code with var and scope?

What will this Swift code print?

Swift

var x = 5
if true {
    var x = 10
    print(x)
}
print(x)

A

10
5

B

5
10

C

10
10

D

5
5

💡 Hint

Remember that variables declared inside a block have their own scope.

The inner var x = 10 shadows the outer x inside the if block. So first print outputs 10, then outside the block it prints 5.

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