Methods in structs in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method modifying struct property

What is the output of this Swift code?

Swift

struct Counter {
    var count = 0
    mutating func increment() {
        count += 1
    }
}

var c = Counter()
c.increment()
print(c.count)

A Compilation error due to missing mutating keyword

B 1

C 0

D Runtime error

💡 Hint

Remember that methods that change properties in structs must be marked mutating.

The method increment() increases count by 1. Because it is marked mutating, it can modify the struct's property. So after calling increment(), count becomes 1.

❓ Predict Output

intermediate

Output of method returning computed value

What does this Swift code print?

Swift

struct Rectangle {
    var width: Int
    var height: Int
    func area() -> Int {
        return width * height
    }
}

let rect = Rectangle(width: 3, height: 4)
print(rect.area())

A Runtime error

B 7

C Compilation error: missing return statement

D 12

💡 Hint

The area is width multiplied by height.

The area() method returns width * height, which is 3 * 4 = 12.

🔧 Debug

advanced

Why does this method cause a compile error?

This Swift struct method tries to modify a property but causes a compile error. Why?

Swift

struct BankAccount {
    var balance: Double = 0.0
    func deposit(amount: Double) {
        balance += amount
    }
}

A deposit method must return a value

B balance is a constant and cannot be changed

C Method must be marked mutating to modify balance

D Structs cannot have methods that modify properties

💡 Hint

Think about how structs handle property changes inside methods.

In Swift, methods that modify properties of a struct must be marked with the mutating keyword. Without it, the compiler prevents modification.

❓ Predict Output

advanced

Output of method with self reassignment

What is the output of this Swift code?

Swift

struct Point {
    var x: Int
    var y: Int
    mutating func moveToOrigin() {
        self = Point(x: 0, y: 0)
    }
}

var p = Point(x: 5, y: 7)
p.moveToOrigin()
print(p.x, p.y)

A 0 0

B 5 7

C Compilation error: cannot assign to self

D Runtime error

💡 Hint

Struct methods can assign a new value to self if marked mutating.

The moveToOrigin() method replaces self with a new Point at (0,0). Because it is mutating, this is allowed. So p.x and p.y become 0.

🧠 Conceptual

expert

Why must methods modifying struct properties be marked mutating?

In Swift, why do methods that change properties of a struct need the mutating keyword?

A Because structs are value types and mutating methods signal that the method changes the instance itself

B Because all methods in structs must be mutating by default

C Because mutating methods run on a separate thread

D Because mutating methods are only allowed in classes, not structs

💡 Hint

Think about how value types behave when changed inside methods.

Structs are value types, so their instances are copied when assigned or passed. Marking a method mutating tells Swift the method will modify the instance, allowing changes to the copy.

Practice

1.

What is the main purpose of a method inside a Swift struct?

easy

A. To define actions the struct can perform

B. To store data values

C. To create a new struct type

D. To import external libraries

Solution

1. Step 1: Understand what methods do in structs

   Methods are functions inside structs that define behaviors or actions the struct can perform.

2. Step 2: Differentiate methods from data storage

   Properties store data, while methods perform actions using that data.

3. Final Answer:

   To define actions the struct can perform -> Option A

4. Quick Check:

   Methods = actions in structs [OK]

Hint: Methods = actions inside structs, properties = data [OK]

Common Mistakes:

• Confusing methods with properties
• Thinking methods create new types
• Mixing methods with import statements

2.

Which keyword must be used before a method in a Swift struct if it changes the struct's properties?

easy

A. final

B. override

C. static

D. mutating

Solution

1. Step 1: Recall Swift's rule for modifying struct properties

   Struct methods that change properties must be marked with mutating to allow modification.

2. Step 2: Identify the correct keyword

   Among the options, only mutating enables property changes inside struct methods.

3. Final Answer:

   mutating -> Option D

4. Quick Check:

   Change properties = mutating [OK]

Hint: Use mutating for methods that change struct properties [OK]

Common Mistakes:

• Using override in structs (only for classes)
• Confusing static with mutating
• Omitting mutating keyword causes errors

3.

What will be the output of this Swift code?

struct Counter {
    var count = 0
    mutating func increment() {
        count += 1
    }
}

var myCounter = Counter()
myCounter.increment()
print(myCounter.count)

medium

A. 0

B. 1

C. Error: Cannot modify property

D. nil

Solution

1. Step 1: Understand the mutating method increment()

   The method increases count by 1 and is marked mutating, so it can change the property.

2. Step 2: Trace the code execution

   myCounter starts with count = 0, increment() adds 1, so count becomes 1, then printed.

3. Final Answer:

   1 -> Option B

4. Quick Check:

   mutating method changes count to 1 [OK]

Hint: mutating methods can change properties, so count increments [OK]

Common Mistakes:

• Forgetting mutating causes error
• Expecting count to stay 0
• Thinking print outputs nil

4.

Identify the error in this Swift struct method and how to fix it:

struct Score {
    var points = 0
    func addPoint() {
        points += 1
    }
}

medium

A. Method should return points value

B. Property points must be let instead of var

C. Method must be marked mutating to change points

D. No error, code runs fine

Solution

1. Step 1: Check if method modifies a property

   addPoint() tries to increase points by 1, which modifies the struct's property.

2. Step 2: Recall Swift rule for modifying properties in structs

   Methods that modify properties must be marked mutating, otherwise compiler error occurs.

3. Final Answer:

   Method must be marked mutating to change points -> Option C

4. Quick Check:

   Modify property in struct method = mutating needed [OK]

Hint: Add mutating keyword to methods that change struct properties [OK]

Common Mistakes:

• Omitting mutating keyword
• Changing var to let (makes property immutable)
• Thinking method must return value

5.

Given this struct, which method correctly resets the level property to 1?

struct Game {
    var level = 5

    // Which method resets level to 1?
}

hard

A. mutating func reset() { level = 1 }

B. mutating func reset() -> Int { return 1 }

C. static func reset() { level = 1 }

D. func reset() { level = 1 }

Solution

1. Step 1: Identify method that changes a property

   Resetting level means changing the property, so method must be mutating.

2. Step 2: Check each option

   A lacks mutating, so cannot modify property. B is mutating but only returns 1 without changing level. C is static and cannot access instance property. D is mutating and correctly sets level to 1.

3. Final Answer:

   mutating func reset() { level = 1 } -> Option A

4. Quick Check:

   Change property in struct method = mutating func [OK]

Hint: Use mutating func to change struct properties inside methods [OK]

Common Mistakes:

• Forgetting mutating keyword
• Using static func to access instance properties
• Returning value instead of changing property