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.