Struct declaration syntax in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple struct instance

What is the output of this Swift code when run?

Swift

struct Person {
    var name: String
    var age: Int
}

let p = Person(name: "Anna", age: 30)
print(p.name)

A Anna

B 30

C Person(name: "Anna", age: 30)

D Error: Missing initializer

💡 Hint

Look at what property is printed from the struct instance.

The code creates a Person struct with name and age. Printing p.name outputs the string "Anna".

❓ Predict Output

intermediate

Value of a struct property after modification

What is the value of the property 'age' after running this Swift code?

Swift

struct Animal {
    var species: String
    var age: Int
}

var dog = Animal(species: "Dog", age: 5)
dog.age = 6
print(dog.age)

A 6

B 5

C Error: Cannot assign to property

D 0

💡 Hint

Check if the struct instance is declared as variable or constant.

Since 'dog' is declared with 'var', its properties can be changed. The age is updated to 6.

❓ Predict Output

advanced

Output of struct with method

What does this Swift code print?

Swift

struct Rectangle {
    var width: Int
    var height: Int
    func area() -> Int {
        return width * height
    }
}

let rect = Rectangle(width: 3, height: 4)
print(rect.area())

A 7

B Error: Missing return statement

C 12

D 0

💡 Hint

Multiply width and height to get the area.

The area method returns width * height, which is 3 * 4 = 12.

❓ Predict Output

advanced

Error raised by incorrect struct declaration

What error does this Swift code produce?

Swift

struct Car {
    var model: String
    var year: Int

    func description() -> String {
        return "Model: \(model), Year: \(year)"
    }
}

let myCar = Car(model: "Tesla")

A No error, compiles and runs fine

B Error: Missing argument for parameter 'year' in call

C Error: Cannot assign to property 'model' in initializer

D Error: Method description missing return type

💡 Hint

Check if all stored properties have values after init.

The struct has two stored properties but the init only sets 'model'. 'year' is not initialized, causing a compile error.

🧠 Conceptual

expert

Number of items in a struct array after filtering

Given this Swift code, how many items remain in the 'youngPeople' array after filtering?

Swift

struct Person {
    var name: String
    var age: Int
}

let people = [
    Person(name: "John", age: 20),
    Person(name: "Jane", age: 17),
    Person(name: "Bob", age: 25),
    Person(name: "Alice", age: 16)
]

let youngPeople = people.filter { $0.age < 18 }
print(youngPeople.count)

A 3

B 4

C 1

D 2

💡 Hint

Count how many people have age less than 18.

Jane (17) and Alice (16) are under 18, so 2 items remain after filtering.

Practice

1. What keyword is used to declare a struct in Swift?

easy

A. struct

B. class

C. function

D. var

Solution

1. Step 1: Identify the keyword for struct declaration

   In Swift, the keyword struct is used to declare a struct type.

2. Step 2: Differentiate from other keywords

   class declares classes, function is not a Swift keyword, and var declares variables.

3. Final Answer:

   struct -> Option A

4. Quick Check:

   Keyword for struct declaration = struct [OK]

Hint: Remember: struct keyword starts struct declarations [OK]

Common Mistakes:

• Using class instead of struct
• Confusing var with struct
• Trying function keyword
• Omitting the keyword

2. Which of the following is the correct syntax to declare a struct named Person with a property name of type String?

easy

A. struct Person = { var name: String }

B. struct Person (var name: String)

C. struct Person { var name: String }

D. struct Person: var name String

Solution

1. Step 1: Check struct declaration syntax

   Structs use curly braces {} to enclose properties. Properties are declared with var or let inside braces.

2. Step 2: Validate each option

   struct Person { var name: String } uses correct syntax: struct Person { var name: String }. Others misuse parentheses, equal sign, or colons incorrectly.

3. Final Answer:

   struct Person { var name: String } -> Option C

4. Quick Check:

   Struct syntax uses braces and var for properties [OK]

Hint: Use braces {} and var for properties inside struct [OK]

Common Mistakes:

• Using parentheses instead of braces
• Adding equal sign after struct name
• Incorrect colon usage
• Missing var keyword

3. What will be the output of this Swift code?

struct Car {
    var model: String
    func description() -> String {
        return "Car model is \(model)"
    }
}
let myCar = Car(model: "Tesla")
print(myCar.description())

medium

A. Car model is Tesla

B. Car model is model

C. Tesla

D. Error: Missing initializer

Solution

1. Step 1: Understand struct with property and method

   The struct Car has a property model and a method description() that returns a string including the model.

2. Step 2: Analyze code execution

   Creating myCar with model "Tesla" and calling description() returns "Car model is Tesla" which is printed.

3. Final Answer:

   Car model is Tesla -> Option A

4. Quick Check:

   Method returns string with model value [OK]

Hint: Methods can use properties with \(property) inside strings [OK]

Common Mistakes:

• Expecting property name printed literally
• Forgetting to call method
• Assuming error due to no explicit initializer
• Confusing output with just model string

4. Identify the error in this struct declaration:

struct Book {
    var title String
    var pages: Int
}

medium

A. Missing var keyword before pages

B. Curly braces missing

C. Struct keyword misspelled

D. Missing colon after title

Solution

1. Step 1: Check property declarations

   Each property must have a colon between name and type, e.g., var title: String.

2. Step 2: Identify the error

   The line var title String is missing the colon after title. Other parts are correct.

3. Final Answer:

   Missing colon after title -> Option D

4. Quick Check:

   Property syntax requires colon between name and type [OK]

Hint: Always put colon between property name and type [OK]

Common Mistakes:

• Omitting colon between name and type
• Forgetting var keyword
• Misspelling struct
• Missing braces

5. You want to create a struct Rectangle with properties width and height (both Int), and a method area() that returns the area as Int. Which is the correct complete declaration?

hard

A. struct Rectangle {
var width Int
var height Int
func area() -> Int {
return width + height
}
}

B. struct Rectangle {
var width: Int
var height: Int
func area() -> Int {
return width * height
}
}

C. struct Rectangle {
var width: Int
var height: Int
func area() {
width * height
}
}

D. struct Rectangle {
width: Int
height: Int
func area() -> Int {
return width * height
}
}

Solution

1. Step 1: Check property declarations

   Properties must have var keyword and colon between name and type. struct Rectangle { var width: Int var height: Int func area() -> Int { return width * height } } correctly uses var width: Int and var height: Int.

2. Step 2: Check method syntax and logic

   Method area() returns Int and correctly calculates width * height. struct Rectangle { var width: Int var height: Int func area() -> Int { return width * height } } matches this.

3. Final Answer:

   Complete struct with correct properties and method -> Option B

4. Quick Check:

   Properties need var and colon; method returns correct calculation [OK]

Hint: Use var with colon for properties; return correct calculation [OK]

Common Mistakes:

• Omitting var keyword
• Missing colon after property name
• Method missing return type
• Incorrect calculation in method