Adding methods in Swift - Mini Project: Build & Apply

Adding methods

📖 Scenario: You are creating a simple program to manage a car's information and actions.

🎯 Goal: Build a Swift program that defines a Car struct with properties and adds methods to display details and update the car's mileage.

📋 What You'll Learn

Create a struct named Car with properties make, model, and mileage

Add a method displayDetails() that prints the car's make, model, and mileage

Add a method drive(distance: Int) that increases the mileage by the given distance

Create an instance of Car with specific values

Call the methods to show the car details and update mileage

💡 Why This Matters

🌍 Real World

Managing objects like cars with properties and actions is common in apps for car rentals, maintenance tracking, or sales.

💼 Career

Understanding how to add methods to data structures is essential for building interactive and functional software in Swift development.

1

Create the Car struct with properties

Create a struct called Car with three properties: make and model of type String, and mileage of type Int. Then create a variable myCar of type Car with make set to "Toyota", model set to "Corolla", and mileage set to 50000.

Swift

// Your code here

Hint

Use struct Car { ... } to define the structure. Add properties inside with var. Create myCar using the struct's initializer.

2

Add the displayDetails() method

Inside the Car struct, add a method called displayDetails() that prints the car's make, model, and mileage in this format: "Car: Toyota Corolla, Mileage: 50000".

Swift

struct Car {
    var make: String
    var model: String
    var mileage: Int

    // Add displayDetails() method here
}

var myCar = Car(make: "Toyota", model: "Corolla", mileage: 50000)

Hint

Define a func displayDetails() inside Car. Use print with string interpolation to show the details.

3

Add the drive(distance:) method

Inside the Car struct, add a method called drive(distance: Int) that increases the mileage property by the given distance. Remember to mark the method as mutating because it changes the struct's property.

Swift

struct Car {
    var make: String
    var model: String
    var mileage: Int

    func displayDetails() {
        print("Car: \(make) \(model), Mileage: \(mileage)")
    }

    // Add mutating drive(distance:) method here
}

var myCar = Car(make: "Toyota", model: "Corolla", mileage: 50000)

Hint

Use mutating func drive(distance: Int) to change mileage. Add the distance to mileage inside the method.

4

Use the methods and print the output

Call myCar.displayDetails() to print the initial details. Then call myCar.drive(distance: 150) to add 150 to the mileage. Finally, call myCar.displayDetails() again to print the updated details.

Swift

struct Car {
    var make: String
    var model: String
    var mileage: Int

    func displayDetails() {
        print("Car: \(make) \(model), Mileage: \(mileage)")
    }

    mutating func drive(distance: Int) {
        mileage += distance
    }
}

var myCar = Car(make: "Toyota", model: "Corolla", mileage: 50000)

// Call methods below

Hint

Call displayDetails() before and after calling drive(distance: 150) on myCar.

Practice

1.

What is the main purpose of adding methods to a struct or class in Swift?

easy

A. To store more data inside the type

B. To make the type immutable

C. To change the type to a different type

D. To add actions or behaviors related to the data inside the type

Solution

1. Step 1: Understand what methods do

   Methods are functions inside a struct or class that perform actions related to that type's data.

2. Step 2: Identify the purpose of methods

   Methods organize code by grouping behaviors with the data they act on, making code reusable and clear.

3. Final Answer:

   To add actions or behaviors related to the data inside the type -> Option D

4. Quick Check:

   Methods = actions on data [OK]

Hint: Methods are functions inside types that do things with data [OK]

Common Mistakes:

• Thinking methods only store data
• Confusing methods with properties
• Believing methods change the type itself

2.

Which of the following is the correct way to define a method named greet inside a Swift struct?

struct Person {
    // Your method here
}

easy

A. function greet() { print("Hello") }

B. method greet() { print("Hello") }

C. func greet() { print("Hello") }

D. def greet() { print("Hello") }

Solution

1. Step 1: Recall Swift method syntax

   In Swift, methods inside structs or classes use the keyword func followed by the method name and parentheses.

2. Step 2: Check each option

   func greet() { print("Hello") } uses func greet(), which is correct. The other options use incorrect keywords not valid in Swift.

3. Final Answer:

   func greet() { print("Hello") } -> Option C

4. Quick Check:

   Swift methods start with func [OK]

Hint: Use 'func' keyword to define methods in Swift [OK]

Common Mistakes:

• Using 'method', 'def', or 'function' instead of 'func'
• Missing parentheses after method name
• Not placing method inside struct or class

3.

What will be the output of the following Swift code?

struct Counter {
    var count = 0
    mutating func increment() {
        count += 1
    }
}

var c = Counter()
c.increment()
print(c.count)

medium

A. 1

B. Error: Cannot mutate property in method

C. 0

D. nil

Solution

1. Step 1: Understand the struct and method

   The struct has a variable count starting at 0. The method increment adds 1 to count. It is marked mutating to allow changing the struct's data.

2. Step 2: Trace the code execution

   We create c as a Counter instance with count = 0. Calling c.increment() increases count to 1. Printing c.count outputs 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Mutating method changes count from 0 to 1 [OK]

Hint: Mutating methods can change struct properties [OK]

Common Mistakes:

• Forgetting 'mutating' keyword causes error
• Expecting count to stay 0 after increment
• Confusing structs with classes (no mutating needed)

4.

Identify the error in this Swift code and choose the fix:

struct BankAccount {
    var balance: Double = 0.0
    func deposit(amount: Double) {
        balance += amount
    }
}

medium

A. Add mutating before func deposit

B. Change var balance to let balance

C. Remove amount parameter from deposit method

D. Change func to static func

Solution

1. Step 1: Understand the error cause

   The method deposit tries to change balance, a property of the struct. In Swift, methods that modify struct properties must be marked mutating.

2. Step 2: Identify the fix

   Adding mutating before func deposit allows the method to modify balance. Other options do not fix the mutation error.

3. Final Answer:

   Add mutating before func deposit -> Option A

4. Quick Check:

   Mutating keyword needed to change struct properties [OK]

Hint: Use 'mutating' for methods that change struct properties [OK]

Common Mistakes:

• Omitting 'mutating' keyword causes compile error
• Changing var to let prevents modification
• Misusing static keyword for instance methods

5.

You want to add a method reset() to this Swift struct that sets count back to zero. Which is the correct way to do this?

struct Counter {
    var count: Int
    // Add reset method here
}

hard

A. func reset() { count = 0 }

B. mutating func reset() { count = 0 }

C. static func reset() { count = 0 }

D. func reset() -> Int { return 0 }

Solution

1. Step 1: Understand method purpose and struct rules

   The method reset() must change the property count. Since Counter is a struct, methods that modify properties must be marked mutating.

2. Step 2: Evaluate options

   mutating func reset() { count = 0 } correctly uses mutating func reset() and sets count = 0. func reset() { count = 0 } lacks mutating, causing error. static func reset() { count = 0 } is static and cannot access instance property. func reset() -> Int { return 0 } returns 0 but does not change count.

3. Final Answer:

   mutating func reset() { count = 0 } -> Option B

4. Quick Check:

   Mutating method needed to reset struct property [OK]

Hint: Mutating methods change struct properties like resetting count [OK]

Common Mistakes:

• Forgetting 'mutating' keyword causes error
• Using static method cannot access instance data
• Returning value instead of changing property