Mutating methods for value types in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

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

Consider this Swift struct with a mutating method that changes a property. What will be printed?

Swift

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

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

A 1

B 0

C Compilation error: mutating method called on a constant

D Runtime error

💡 Hint

Remember that mutating methods can change properties only if the instance is a variable.

The increment() method is marked mutating, so it can modify count. Since myCounter is a variable (var), calling increment() increases count from 0 to 1. Thus, printing myCounter.count outputs 1.

❓ Predict Output

intermediate

What error occurs when calling a mutating method on a constant struct?

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

Swift

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

let fixedPoint = Point()
fixedPoint.moveBy(dx: 5, dy: 5)

A No error, point moves to (5,5)

B Compilation error: Cannot use mutating method on a let constant

C Runtime error: Cannot modify constant

D Compilation error: Missing mutating keyword

💡 Hint

Think about whether a constant struct instance can be changed.

In Swift, structs are value types. A let constant instance cannot be modified. Calling a mutating method tries to change the instance, which is not allowed on constants. This causes a compile-time error.

🔧 Debug

advanced

Why does this mutating method cause a compile error?

Identify the reason for the compile error in this Swift code.

Swift

struct Rectangle {
    var width: Int
    var height: Int

    func doubleSize() {
        width *= 2
        height *= 2
    }
}

var rect = Rectangle(width: 3, height: 4)
rect.doubleSize()

A No error, code runs and doubles size

B Error because width and height are constants

C Error because doubleSize is not marked mutating but modifies properties

D Error because Rectangle is a class, not a struct

💡 Hint

Check if the method that changes properties is marked correctly.

The method doubleSize() tries to modify width and height, but it is not marked mutating. In Swift, only mutating methods can change properties of value types like structs. This causes a compile error.

📝 Syntax

advanced

Which option correctly declares a mutating method in Swift?

Choose the correct syntax for a mutating method that increments a value.

A mutate func increment() { value += 1 }

B func mutating increment() { value += 1 }

C func increment() mutating { value += 1 }

D mutating func increment() { value += 1 }

💡 Hint

The mutating keyword goes before func.

In Swift, the correct syntax to declare a mutating method is mutating func methodName() { }. Option D follows this syntax. Other options have incorrect keyword order or wrong keywords.

🚀 Application

expert

What is the final value of 'score' after calling the mutating method twice?

Given this Swift struct and code, what is the value of game.score after calling increaseScore() twice?

Swift

struct Game {
    var score = 0
    mutating func increaseScore() {
        score += 10
    }
}

var game = Game()
game.increaseScore()
game.increaseScore()

A 20

B 10

C 0

D Compilation error

💡 Hint

Each call adds 10 to score. How many calls are made?

The increaseScore() method adds 10 to score. It is called twice on a variable instance game. So, score becomes 0 + 10 + 10 = 20.