Computed properties in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using computed properties?

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 12.0

B 7.0

C 0.0

D Error: Cannot access 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 value does the computed property return after mutation?

Given this Swift class with a computed property, what will be printed after changing the base value?

Swift

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

let c = Circle(radius: 5)
c.diameter = 20
print(c.radius)

A 10.0

B 5.0

C 20.0

D Error: Cannot assign to computed property

💡 Hint

Setting diameter changes radius because of the setter.

The setter divides the new diameter by 2 to update radius. Setting diameter to 20 sets radius to 10.

❓ Predict Output

advanced

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

Analyze the following Swift struct. What will be printed when accessing the computed property 'fullName'?

Swift

struct Person {
    private var firstName: String
    private var lastName: String
    var fullName: String {
        return "\(firstName) \(lastName)"
    }
    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
}

let p = Person(firstName: "Jane", lastName: "Doe")
print(p.fullName)

A Jane

B firstName lastName

C Jane Doe

D Error: Cannot access private property

💡 Hint

Computed properties can access private stored properties within the same struct.

The computed property 'fullName' returns the concatenation of firstName and lastName, which are accessible inside the struct.

❓ Predict Output

advanced

What error does this Swift code produce when using a computed property incorrectly?

Examine the following Swift code. What error will occur when compiling?

Swift

struct Square {
    var side: Double
    var area: Double {
        side * side
    }
    var perimeter: Double {
        return side * 4
    }
    var invalidComputed: Double {
        side * 2
        side * 2
    }
}

A Error: Cannot use expression without return in computed property

B Error: Duplicate computed property name

C No error, code compiles successfully

D Error: Missing return in computed property 'invalidComputed'

💡 Hint

Computed properties without explicit return require single expression bodies.

The computed property 'invalidComputed' lacks the 'return' keyword and uses a multi-line body without it, causing a compile error.

🧠 Conceptual

expert

How many times is the computed property 'value' accessed in this Swift code?

Consider this Swift struct with a computed property. How many times will the computed property 'value' be evaluated during the execution of the code below?

Swift

struct Counter {
    var base: Int
    var value: Int {
        print("Computing value")
        return base * 2
    }
}

let c = Counter(base: 3)
let sum = c.value + c.value + c.value
print(sum)

A 1 time

B 3 times

C 0 times

D 2 times

💡 Hint

Each access to a computed property runs its code anew unless cached.

The computed property 'value' is accessed three times in the sum expression, so 'Computing value' prints three times.