Adding computed properties in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code with a computed property?

Consider the following Swift struct with a computed property. What will be printed when the code runs?

Swift

struct Rectangle {
    var width: Double
    var height: Double
    var area: Double {
        return width * height
    }
}

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

A 7.0

B 0.0

C 12.0

D Error: Cannot access computed property

💡 Hint

Remember that computed properties calculate their value when accessed.

The computed property 'area' multiplies width and height. For width 3 and height 4, area is 12.0.

❓ Predict Output

intermediate

What is the value of the computed property after changing stored properties?

Given this Swift class with a computed property, what will be printed after modifying the stored properties?

Swift

class Circle {
    var radius: Double
    var diameter: Double {
        return radius * 2
    }
    init(radius: Double) {
        self.radius = radius
    }
}

let circle = Circle(radius: 5)
print(circle.diameter)
circle.radius = 7
print(circle.diameter)

A 10.0 and 14.0

B 10.0 and 10.0

C 5.0 and 7.0

D Error: Cannot modify radius

💡 Hint

Computed properties reflect the current state of stored properties.

The diameter is always twice the radius. Initially 5*2=10, after radius changes to 7, diameter is 14.

🔧 Debug

advanced

Why does this computed property cause a compile-time error?

Examine the Swift struct below. Why does accessing the computed property cause a compile-time error?

Swift

struct Counter {
    var count: Int = 0
    mutating var nextCount: Int {
        count += 1
        return count
    }
}

var counter = Counter()
print(counter.nextCount)

A Computed properties cannot return Int values

B Cannot modify 'count' inside a computed property without 'mutating' keyword

C Missing return statement in computed property

D Structs cannot have computed properties

💡 Hint

Think about mutability rules in Swift structs.

In Swift, modifying a struct's property inside a computed property requires the property to be marked 'mutating'.

📝 Syntax

advanced

Which option correctly defines a read-write computed property in Swift?

Choose the correct syntax for a computed property that can be read and written to.

A

var temperature: Double {
    get {
        return celsius
    }
}

B

var temperature: Double = {
    return celsius
}

C

var temperature: Double {
    return celsius
    set {
        celsius = newValue
    }
}

D

var temperature: Double {
    get {
        return celsius
    }
    set {
        celsius = newValue
    }
}

💡 Hint

A read-write computed property needs both get and set blocks properly defined.

Option D correctly uses get and set blocks. Option D is invalid syntax. Option D has set after return without get block. Option D is read-only.

🚀 Application

expert

How many times is the computed property evaluated in this Swift code?

Consider this Swift code. How many times will the computed property 'description' be evaluated during execution?

Swift

struct Person {
    var firstName: String
    var lastName: String
    var description: String {
        print("Computing description")
        return "\(firstName) \(lastName)"
    }
}

let person = Person(firstName: "Jane", lastName: "Doe")
print(person.description)
print(person.description)

A 2 times

B 1 time

C 0 times

D 3 times

💡 Hint

Each access to a computed property runs its code.

The computed property runs its code every time it is accessed. Here, it is accessed twice, so printed twice.