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.

Practice

1. What is a computed property in Swift?

easy

A. A property that can only be set but not read

B. A property that stores a fixed value permanently

C. A property that calculates its value dynamically when accessed

D. A property that is declared outside a class or struct

Solution

1. Step 1: Understand property types in Swift

   Stored properties hold fixed values, while computed properties calculate values dynamically.

2. Step 2: Identify computed property behavior

   Computed properties use code to return a value each time they are accessed instead of storing it.

3. Final Answer:

   A property that calculates its value dynamically when accessed -> Option C

4. Quick Check:

   Computed property = dynamic value calculation [OK]

Hint: Computed properties calculate values on demand, not stored [OK]

Common Mistakes:

• Confusing computed properties with stored properties
• Thinking computed properties store values permanently
• Assuming computed properties cannot have setters

2. Which of the following is the correct syntax for a computed property with a getter in Swift?

easy

A. var area: Double get { width * height }

B. var area: Double = width * height

C. var area: Double { get { width * height } }

D. var area: Double { return width * height }

Solution

1. Step 1: Recall computed property syntax

   Computed properties use curly braces with a getter that returns a value.

2. Step 2: Check each option

   var area: Double { return width * height } correctly uses braces and return keyword. var area: Double = width * height is stored property syntax. var area: Double { width * height } misses return keyword. var area: Double get { width * height } has incorrect getter syntax.

3. Final Answer:

   var area: Double { return width * height } -> Option D

4. Quick Check:

   Computed property getter uses braces and return [OK]

Hint: Use braces and return keyword for computed property getter [OK]

Common Mistakes:

• Omitting the return keyword inside getter
• Using equals sign instead of braces
• Incorrect getter syntax without braces

3. Consider this Swift struct:

struct Rectangle {
  var width: Double
  var height: Double
  var area: Double {
    return width * height
  }
}
let rect = Rectangle(width: 3, height: 4)
print(rect.area)

What is the output?

medium

A. Compilation error

B. 12.0

C. 0.0

D. 7.0

Solution

1. Step 1: Understand the struct and computed property

   Rectangle has width=3 and height=4. The computed property area returns width * height.

2. Step 2: Calculate the area value

   3 * 4 = 12.0, so rect.area returns 12.0.

3. Final Answer:

   12.0 -> Option B

4. Quick Check:

   3 * 4 = 12.0 [OK]

Hint: Multiply width and height to get area from computed property [OK]

Common Mistakes:

• Confusing stored and computed properties
• Expecting area to be zero by default
• Thinking code causes compilation error

4. Identify the error in this Swift code using a computed property:

struct Circle {
  var radius: Double
  var diameter: Double {
    get {
      return radius * 2
    }
    set {
      radius = newValue / 2
    }
  }
}
let c = Circle(radius: 5)
c.diameter = 20
print(c.radius)

medium

A. Cannot assign to property 'radius' because 'c' is a constant

B. Missing return keyword in getter

C. Setter syntax is incorrect

D. No error, code runs and prints 10

Solution

1. Step 1: Analyze variable mutability

   Variable 'c' is declared with 'let', making it immutable (constant).

2. Step 2: Understand setter behavior

   Setter tries to modify 'radius' property, but 'c' is constant, so assignment is not allowed.

3. Final Answer:

   Cannot assign to property 'radius' because 'c' is a constant -> Option A

4. Quick Check:

   Constant instance cannot change properties via setter [OK]

Hint: Use var, not let, to allow property setters to modify instance [OK]

Common Mistakes:

• Ignoring that 'let' makes instance immutable
• Thinking setter syntax is wrong
• Missing return keyword confusion

5. You want to create a Swift struct Temperature that stores a temperature in Celsius and has a computed property fahrenheit to get and set the temperature in Fahrenheit. Which implementation correctly updates Celsius when Fahrenheit is set?

hard

A. struct Temperature {
var celsius: Double
var fahrenheit: Double {
get { return celsius * 9 / 5 + 32 }
set { celsius = (newValue - 32) * 5 / 9 }
}
}

B. struct Temperature {
var celsius: Double
var fahrenheit: Double {
get { return celsius + 32 }
set { celsius = newValue - 32 }
}
}

C. struct Temperature {
var celsius: Double
var fahrenheit: Double {
get { return celsius * 2 + 30 }
set { celsius = (newValue - 30) / 2 }
}
}

D. struct Temperature {
var celsius: Double
var fahrenheit: Double {
get { return celsius * 5 / 9 + 32 }
set { celsius = (newValue - 32) * 9 / 5 }
}
}

Solution

1. Step 1: Recall temperature conversion formulas

   Fahrenheit = Celsius * 9/5 + 32; Celsius = (Fahrenheit - 32) * 5/9.

2. Step 2: Check each option's formulas

   struct Temperature { var celsius: Double var fahrenheit: Double { get { return celsius * 9 / 5 + 32 } set { celsius = (newValue - 32) * 5 / 9 } } } uses correct formulas in getter and setter. The other options use incorrect formulas.

3. Final Answer:

   The implementation with correct conversion formulas -> Option A

4. Quick Check:

   Correct formulas = struct Temperature { var celsius: Double var fahrenheit: Double { get { return celsius * 9 / 5 + 32 } set { celsius = (newValue - 32) * 5 / 9 } } } [OK]

Hint: Use correct math formulas for temperature conversion in getter/setter [OK]

Common Mistakes:

• Mixing up Celsius and Fahrenheit formulas
• Using addition instead of multiplication
• Forgetting to update Celsius in setter