Adding methods in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code with a method?

Consider this Swift struct with a method. What will be printed when the code runs?

Swift

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

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

A Compilation error: cannot use mutating method on variable

B 1

C 0

D Runtime error

💡 Hint

Remember that mutating methods can change properties of structs when called on variables.

The method increment() increases count by 1. Since myCounter is a variable (var), calling increment() works and count becomes 1.

❓ Predict Output

intermediate

What does this class method print?

Look at this Swift class with a method. What will be printed when the code runs?

Swift

class Greeter {
    func greet() {
        print("Hello, Swift!")
    }
}

let greeter = Greeter()
greeter.greet()

A Compilation error: cannot call method on constant

B No output

C Runtime error: method not found

D Hello, Swift!

💡 Hint

Classes allow methods to be called on instances even if the instance is a constant.

The greet() method prints "Hello, Swift!". Calling it on greeter prints this message.

🔧 Debug

advanced

Why does this Swift code cause a compile error?

This Swift struct has a method that tries to modify a property but causes a compile error. Why?

Swift

struct Point {
    var x = 0
    var y = 0
    mutating func moveBy(dx: Int, dy: Int) {
        x += dx
        y += dy
    }
}

A Because moveBy is not marked as mutating, so it cannot change properties

B Because x and y are constants and cannot be changed

C Because structs cannot have methods

D Because dx and dy parameters are constants

💡 Hint

In Swift, methods that change properties of structs must be marked mutating.

The method moveBy tries to change x and y but is not marked mutating, so the compiler disallows it.

❓ Predict Output

advanced

What is the output of this Swift code with a static method?

Look at this Swift class with a static method. What will be printed when the code runs?

Swift

class MathHelper {
    static func square(_ number: Int) -> Int {
        return number * number
    }
}

print(MathHelper.square(5))

A Compilation error: cannot call static method like this

B 5

C 25

D Runtime error

💡 Hint

Static methods belong to the type itself and can be called without creating an instance.

The static method square returns the number multiplied by itself. Calling square(5) returns 25.

🧠 Conceptual

expert

Which statement about methods in Swift is true?

Choose the correct statement about methods in Swift.

A Mutating methods can modify properties of value types like structs and enums

B Instance methods cannot access properties of the instance

C Static methods can only be called on instances, not on the type itself

D Only classes can have methods; structs and enums cannot

💡 Hint

Think about how value types like structs behave when methods change their properties.

Mutating methods allow structs and enums to modify their own properties. Classes do not need mutating because they are reference types.

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