Computed properties in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a computed property that returns the double of the stored property.

Swift

struct Number {
    var value: Int
    var doubleValue: Int {
        return value [1] 2
    }
}

Drag options to blanks, or click blank then click option'

A/

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of * will add 2 instead of doubling.

Using / will divide instead of multiply.

The computed property doubleValue returns value multiplied by 2 using the * operator.

2fill in blank

medium

Complete the code to create a computed property that returns the full name by combining first and last names.

Swift

struct Person {
    var firstName: String
    var lastName: String
    var fullName: String {
        return firstName [1] " " [1] lastName
    }
}

Drag options to blanks, or click blank then click option'

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using * or - operators with strings causes errors.

Forgetting to add a space between names.

The computed property fullName concatenates firstName, a space, and lastName using the + operator.

3fill in blank

hard

Fix the error in the computed property to correctly calculate the area of a rectangle.

Swift

struct Rectangle {
    var width: Double
    var height: Double
    var area: Double {
        return width [1] height
    }
}

Drag options to blanks, or click blank then click option'

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using + adds dimensions instead of calculating area.

Using / or - gives wrong results.

The area of a rectangle is width multiplied by height, so the * operator is correct.

4fill in blank

hard

Fill both blanks to create a computed property that returns true if the number is even.

Swift

struct NumberChecker {
    var number: Int
    var isEven: Bool {
        return number [1] 2 [2] 0
    }
}

Drag options to blanks, or click blank then click option'

A%

B==

C!=

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using / instead of % to get remainder.

Using != instead of == causes wrong logic.

The computed property uses the modulus operator % to get the remainder when dividing by 2, then checks if it is equal to 0 with ==.

5fill in blank

hard

Fill all three blanks to create a computed property that returns the initials of a person in uppercase.

Swift

struct Person {
    var firstName: String
    var lastName: String
    var initials: String {
        return firstName[1].uppercased() + lastName[2].uppercased()
    }
}

Drag options to blanks, or click blank then click option'

Aprefix(1)

Bsuffix(1)

Cprefix(0)

DdropFirst()

Attempts:

3 left

💡 Hint

Common Mistakes

Using suffix(1) gets last letter, not initial.

Using prefix(0) returns empty string.

Using dropFirst() removes first letter instead of selecting it.

The computed property uses prefix(1) to get the first letter of each name, then converts it to uppercase.

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