Computed properties in Swift - Mini Project: Build & Apply

Computed Properties in Swift

📖 Scenario: You are creating a simple app to manage rectangles. You want to store the width and height, and also calculate the area automatically whenever needed.

🎯 Goal: Build a Swift struct called Rectangle with stored properties for width and height, and a computed property for area.

📋 What You'll Learn

Create a struct named Rectangle

Add stored properties width and height of type Double

Add a computed property area that returns the product of width and height

Create an instance of Rectangle with width 5.0 and height 3.0

Print the area of the rectangle

💡 Why This Matters

🌍 Real World

Computed properties help keep data consistent and up-to-date without storing extra values. For example, calculating area or volume on demand.

💼 Career

Understanding computed properties is essential for Swift developers building apps that manage dynamic data efficiently.

1

Create the Rectangle struct with width and height

Create a struct called Rectangle with two stored properties: width and height, both of type Double. Set no initial values.

Swift

struct Rectangle {
    // Your code here
}

Hint

Use struct Rectangle { var width: Double; var height: Double } to define the shape.

2

Add a computed property for area

Inside the Rectangle struct, add a computed property called area of type Double that returns the product of width and height.

Swift

struct Rectangle {
    var width: Double
    var height: Double
    // Your code here
}

Hint

Computed properties use { return ... } to calculate values on the fly.

3

Create an instance of Rectangle

Create a constant called myRect and assign it a Rectangle instance with width 5.0 and height 3.0.

Swift

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

// Your code here

Hint

Use let myRect = Rectangle(width: 5.0, height: 3.0) to create the rectangle.

4

Print the area of the rectangle

Write a print statement to display the area of myRect.

Swift

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

let myRect = Rectangle(width: 5.0, height: 3.0)

// Your code here

Hint

Use print(myRect.area) to show the area.

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