Computed properties in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a computed property in Swift?

A computed property does not store a value directly. Instead, it provides a getter and optionally a setter to calculate a value when accessed or set.

beginner

How do you define a read-only computed property in Swift?

You define a computed property with only a getter and no setter. For example:

var area: Double {
  return width * height
}

intermediate

Can computed properties have side effects in Swift?

Yes, computed properties can run code inside their getter or setter, so they can have side effects like logging or updating other values.

beginner

What is the difference between stored and computed properties?

Stored properties hold actual values in memory. Computed properties calculate values on demand without storing them.

intermediate

Show an example of a computed property with both getter and setter in Swift.

Example:

var temperatureCelsius: Double {
  get { return (temperatureFahrenheit - 32) * 5 / 9 }
  set { temperatureFahrenheit = newValue * 9 / 5 + 32 }
}

What does a computed property in Swift do?

A Cannot have a setter

B Stores a fixed value in memory

C Calculates a value when accessed

D Only stores strings

Computed properties calculate and return a value when accessed, instead of storing it.

Which keyword is used to define a computed property in Swift?

A let

B var

C func

D class

Computed properties are defined using 'var' because their value can change.

Can a computed property have a setter in Swift?

A Yes, to allow setting a new value

B No, computed properties are read-only

C Only if marked with 'let'

D Only inside structs

Computed properties can have a setter to update related values.

What happens if a computed property has only a getter?

A It causes a compile error

B It can be set

C It stores a value

D It is read-only

A computed property with only a getter is read-only.

Which of these is NOT true about computed properties?

A They store values directly

B They can have side effects

C They use get and set blocks

D They calculate values on demand

Computed properties do not store values directly; they calculate them.

Explain what a computed property is and how it differs from a stored property in Swift.

Think about how values are stored or calculated.

Write a simple Swift computed property that calculates the area of a rectangle given width and height stored properties.

Use a getter to calculate 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