Creating struct instances in Rust - Try It Yourself

Creating struct instances

📖 Scenario: You are building a simple program to store information about books in a library. Each book has a title, an author, and a number of pages.

🎯 Goal: Create a Rust struct to represent a book, then create instances of this struct with specific book details.

📋 What You'll Learn

Define a struct named Book with fields title, author, and pages

Create two instances of Book with exact given values

Print the details of the created books

💡 Why This Matters

🌍 Real World

Structs help organize data about real-world objects like books, users, or products in programs.

💼 Career

Understanding structs is essential for Rust programming jobs, especially in systems programming, web backend, and game development.

1

Define the Book struct

Define a struct called Book with three fields: title and author as String, and pages as u32.

Rust

# Your code here

Hint

Use struct Book { title: String, author: String, pages: u32 } syntax.

2

Create two Book instances

Create two variables named book1 and book2 as instances of Book. Set book1 with title "The Rust Book", author "Steve", and pages 550. Set book2 with title "Learning Rust", author "Anna", and pages 320.

Rust

struct Book {
    title: String,
    author: String,
    pages: u32,
}

// Your code here

Hint

Use let book1 = Book { title: String::from("..."), author: String::from("..."), pages: ... };

3

Add a function to display book info

Create a function named print_book_info that takes a reference to a Book and prints its title, author, and pages in this format: "Title: The Rust Book, Author: Steve, Pages: 550".

Rust

struct Book {
    title: String,
    author: String,
    pages: u32,
}

let book1 = Book {
    title: String::from("The Rust Book"),
    author: String::from("Steve"),
    pages: 550,
};

let book2 = Book {
    title: String::from("Learning Rust"),
    author: String::from("Anna"),
    pages: 320,
};

// Your code here

Hint

Define fn print_book_info(book: &Book) and use println! with placeholders.

4

Print the book details

Call the function print_book_info twice: once with book1 and once with book2 to display their details.

Rust

struct Book {
    title: String,
    author: String,
    pages: u32,
}

let book1 = Book {
    title: String::from("The Rust Book"),
    author: String::from("Steve"),
    pages: 550,
};

let book2 = Book {
    title: String::from("Learning Rust"),
    author: String::from("Anna"),
    pages: 320,
};

fn print_book_info(book: &Book) {
    println!("Title: {}, Author: {}, Pages: {}", book.title, book.author, book.pages);
}

// Your code here

Hint

Call print_book_info(&book1); and print_book_info(&book2);.

Practice

1. What is the correct way to create an instance of a struct Person with fields name and age in Rust?

easy

A. let p = new Person { name: "Alice", age: 30 };

B. let p = Person("Alice", 30);

C. let p = Person { name: String::from("Alice"), age: 30 };

D. let p = Person(name = "Alice", age: 30);

Solution

1. Step 1: Understand struct instance syntax

   In Rust, you create a struct instance by naming the struct and assigning values to each field inside curly braces.

2. Step 2: Match the correct syntax

   let p = Person { name: String::from("Alice"), age: 30 }; uses Person { name: String::from("Alice"), age: 30 }, which is the correct way. Other options use incorrect syntax like tuple style or assignment inside parentheses.

3. Final Answer:

   let p = Person { name: String::from("Alice"), age: 30 }; -> Option C

4. Quick Check:

   Struct instance = StructName { field: value } [OK]

Hint: Use StructName { field: value } to create instances [OK]

Common Mistakes:

• Using parentheses instead of curly braces
• Trying to assign fields with = inside parentheses
• Forgetting to convert string literals to String type

2. Which of the following is a syntax error when creating a struct instance Car with fields make and year?

easy

A. let c = Car { make: String::from("Toyota"), year: 2020 };

B. let c = Car { make: "Toyota", year: 2020 };

C. let c = Car { make: String::from("Toyota"), year: "2020" };

D. let c = Car { make: String::from("Toyota"), year: 2020 }

Solution

1. Step 1: Check for syntax errors

   let c = Car { make: String::from("Toyota"), year: 2020 } is missing the semicolon after the let statement, which causes a syntax error in Rust.

2. Step 2: Review other options

   Options A, B, and C have semicolons and valid struct syntax, though B and C may cause type errors if field types do not match.

3. Final Answer:

   let c = Car { make: String::from("Toyota"), year: 2020 } -> Option D

4. Quick Check:

   Semicolon required after let statements [OK]

Hint: Match field types exactly when creating struct instances [OK]

Common Mistakes:

• Assigning string literals where String is expected without conversion
• Using wrong data types for fields
• Omitting semicolons (sometimes allowed but risky)

3. Given the struct and instance creation below, what will be printed?

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 5, y: 10 };
    println!("({}, {})", p.x, p.y);
}

medium

A. (5, 10)

B. (10, 5)

C. (x, y)

D. Compilation error

Solution

1. Step 1: Understand struct fields and instance values

   The struct Point has fields x and y. The instance p assigns x = 5 and y = 10.

2. Step 2: Analyze the print statement

   The println! macro prints the values of p.x and p.y in order, so output is (5, 10).

3. Final Answer:

   (5, 10) -> Option A

4. Quick Check:

   Print fields in order = (5, 10) [OK]

Hint: Print fields in order they appear in code [OK]

Common Mistakes:

• Mixing up field order in print
• Expecting field names printed instead of values
• Assuming compilation error without reason

4. Identify the error in the following code that tries to create a struct instance:

struct Book {
    title: String,
    pages: u32,
}

fn main() {
    let b = Book { title: "Rust Book", pages: 300 };
}

medium

A. Missing semicolon after struct definition

B. String literal assigned directly to String field without conversion

C. Field name 'pages' is misspelled

D. Struct instance missing field values

Solution

1. Step 1: Check field types and assigned values

   The field title is of type String, but the code assigns a string literal "Rust Book" directly, which is a "str" type, causing a type mismatch.

2. Step 2: Confirm other parts are correct

   The field pages is correctly assigned an integer. The struct definition and instance have correct syntax otherwise.

3. Final Answer:

   String literal assigned directly to String field without conversion -> Option B

4. Quick Check:

   Convert "str" to String with String::from() [OK]

Hint: Convert string literals to String with String::from() [OK]

Common Mistakes:

• Assigning "str" directly to String fields
• Misspelling field names
• Omitting required fields in instance

5. You have a struct Employee with fields name: String, id: u32, and active: bool. How do you create a new instance with name as "John", id as 101, and active as true, using shorthand for the active field if you have a variable active set to true?

hard

A. let active = true;
let e = Employee { name: String::from("John"), id: 101, active };

B. let active = true;
let e = Employee { name: "John", id: 101, active: active };

C. let e = Employee { name: String::from("John"), id: 101, active: true };

D. let active = true;
let e = Employee { name: String::from("John"), id: 101, active = active };

Solution

1. Step 1: Understand field shorthand syntax

   If a variable name matches a struct field name, you can use shorthand by just writing the variable name without field: value.

2. Step 2: Identify correct usage

   let active = true; let e = Employee { name: String::from("John"), id: 101, active }; uses shorthand correctly: active variable is used directly as a field. let active = true; let e = Employee { name: "John", id: 101, active: active }; uses explicit active: active but has type mismatch on name. let e = Employee { name: String::from("John"), id: 101, active: true }; does not use the variable. let active = true; let e = Employee { name: String::from("John"), id: 101, active = active }; uses invalid syntax with = inside struct initialization.

3. Final Answer:

   let active = true; let e = Employee { name: String::from("John"), id: 101, active }; -> Option A

4. Quick Check:

   Use shorthand when variable name matches field name [OK]

Hint: Use field shorthand: just write variable name if it matches field [OK]

Common Mistakes:

• Using = instead of : in struct initialization
• Not converting string literals to String
• Not using shorthand when possible