Mutating methods for value types in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the mutating keyword do in Swift?

It allows a method in a value type (like a struct or enum) to modify (change) the instance's properties.

beginner

Why do value types need mutating methods to change their properties?

Because value types are copied when assigned or passed, Swift requires explicit permission (mutating) to change their own data inside methods.

beginner

Can classes have mutating methods in Swift?

No. Classes are reference types and can always modify their properties inside methods without mutating.

beginner

Example: What will this code print?

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

It will print 1 because the increment() method changes the count property using mutating.

beginner

What happens if you remove mutating from a method that changes a property in a struct?

The Swift compiler will give an error because the method tries to change a property without permission to mutate the value type.

What keyword must you add to a method in a struct to modify its properties?

A override

B static

C final

D mutating

The mutating keyword allows methods in value types like structs to modify their own properties.

Which type requires mutating methods to change properties?

A Struct

B Protocol

C Class

D Enum without cases

Structs are value types and need mutating methods to change their properties.

What happens if you call a mutating method on a constant struct instance?

A The struct becomes mutable

B It works normally

C Compiler error: cannot use mutating method on let constant

D The method is ignored

You cannot call mutating methods on a struct instance declared with let because it is immutable.

Can enums have mutating methods in Swift?

A Only in classes

B Yes, to change their cases

C Only if they have raw values

D No, enums are immutable

Enums can have mutating methods to change their current case value.

Why don't classes need the mutating keyword?

A Because classes are reference types and can always modify their properties

B Because classes cannot change properties

C Because classes are immutable

D Because classes use <code>final</code> instead

Classes are reference types, so their methods can modify properties without mutating.

Explain why the mutating keyword is necessary for methods in Swift structs.

Think about how value types behave differently from classes.

Describe what happens if you try to modify a struct property inside a method without using mutating.

Consider Swift's safety rules for value types.

Practice

1. What is the purpose of the mutating keyword in Swift when used with methods inside a struct?

easy

A. It prevents the method from changing any properties.

B. It allows the method to modify properties of the struct.

C. It makes the method static and callable without an instance.

D. It marks the method as private and inaccessible outside the struct.

Solution

1. Step 1: Understand the role of mutating in structs

   In Swift, structs are value types and their properties cannot be changed inside instance methods unless the method is marked mutating.

2. Step 2: Effect of mutating keyword

   The mutating keyword allows the method to modify the struct's properties safely.

3. Final Answer:

   It allows the method to modify properties of the struct. -> Option B

4. Quick Check:

   mutating lets methods change struct data [OK]

Hint: Mutating means method can change struct's own properties [OK]

Common Mistakes:

• Thinking mutating makes method static
• Assuming mutating prevents changes
• Confusing mutating with access control

2. Which of the following is the correct syntax to declare a mutating method named updateValue inside a struct in Swift?

easy

A. mutating func updateValue() { }

B. func mutating updateValue() { }

C. func updateValue() mutating { }

D. mutate func updateValue() { }

Solution

1. Step 1: Recall correct order of keywords in method declaration

   In Swift, the mutating keyword comes before func when declaring a mutating method.

2. Step 2: Match the syntax

   mutating func updateValue() { } correctly shows mutating func updateValue() { }, which is the proper syntax.

3. Final Answer:

   mutating func updateValue() { } -> Option A

4. Quick Check:

   Keyword order: mutating before func [OK]

Hint: Put 'mutating' before 'func' in method declaration [OK]

Common Mistakes:

• Placing mutating after func
• Using 'mutate' instead of 'mutating'
• Adding mutating inside method body

3. Consider this Swift struct and code:

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

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

What will be printed?

medium

A. 0

B. Runtime error

C. Compilation error

D. 1

Solution

1. Step 1: Understand the mutating method effect

   The increment() method is marked mutating, so it can increase the count property by 1.

2. Step 2: Trace the code execution

   We create myCounter as a variable instance, call increment(), so count changes from 0 to 1, then print outputs 1.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Mutating method changes count from 0 to 1 [OK]

Hint: Mutating methods change struct vars if instance is variable [OK]

Common Mistakes:

• Expecting 0 because method looks like a function
• Thinking mutating methods cause errors always
• Using constant instance instead of variable

4. Given this Swift code:

struct Point {
    var x = 0
    var y = 0
    mutating func moveTo(newX: Int, newY: Int) {
        x = newX
        y = newY
    }
}

let p = Point()
p.moveTo(newX: 10, newY: 20)

What is the problem here?

medium

A. The method moveTo is missing the mutating keyword.

B. Structs cannot have methods that change properties.

C. Cannot call mutating method on a constant instance.

D. The properties x and y must be declared as constants.

Solution

1. Step 1: Identify instance mutability

   The instance p is declared with let, making it a constant.

2. Step 2: Understand mutating method call on constant

   Mutating methods cannot be called on constant instances because they change properties, which is not allowed.

3. Final Answer:

   Cannot call mutating method on a constant instance. -> Option C

4. Quick Check:

   Mutating methods need variable instances [OK]

Hint: Mutating methods require variable (var) instances, not constants [OK]

Common Mistakes:

• Thinking mutating keyword is missing
• Believing structs can't have mutating methods
• Assuming properties must be constants

5. You want to create a Swift struct BankAccount with a balance property. You need a mutating method deposit(amount:) that adds money to the balance only if the amount is positive. Which implementation correctly uses a mutating method to safely update the balance?

hard

A. struct BankAccount {
var balance: Double = 0
mutating func deposit(amount: Double) {
if amount > 0 {
balance += amount
}
}
}

B. struct BankAccount {
var balance: Double = 0
func deposit(amount: Double) {
if amount > 0 {
balance += amount
}
}
}

C. struct BankAccount {
let balance: Double = 0
mutating func deposit(amount: Double) {
balance += amount
}
}

D. struct BankAccount {
var balance: Double = 0
mutating func deposit(amount: Double) {
balance = amount
}
}

Solution

1. Step 1: Check mutating method and property mutability

   The method must be marked mutating to change balance, which must be a variable (var), not constant.

2. Step 2: Verify correct logic for deposit

   struct BankAccount { var balance: Double = 0 mutating func deposit(amount: Double) { if amount > 0 { balance += amount } } } adds amount only if positive, correctly updating balance. struct BankAccount { var balance: Double = 0 func deposit(amount: Double) { if amount > 0 { balance += amount } } } lacks mutating, so it cannot change balance. struct BankAccount { let balance: Double = 0 mutating func deposit(amount: Double) { balance += amount } } uses let for balance, so it cannot be changed. struct BankAccount { var balance: Double = 0 mutating func deposit(amount: Double) { balance = amount } } replaces balance with amount without checking positivity.

3. Final Answer:

   struct BankAccount { var balance: Double = 0 mutating func deposit(amount: Double) { if amount > 0 { balance += amount } } } -> Option A

4. Quick Check:

   Mutating method + var property + condition = correct [OK]

Hint: Mutating method must change var property with proper checks [OK]

Common Mistakes:

• Forgetting mutating keyword
• Using let instead of var for properties
• Not checking if amount is positive