Creating struct instances in Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new instance of the struct Point with x set to 5.

Rust

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

fn main() {
    let p = Point { x: [1], y: 0 };
    println!("({}, {})", p.x, p.y);
}

Drag options to blanks, or click blank then click option'

Ay

B5

Cx

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the field name instead of a value.

Setting x to zero instead of 5.

To create a struct instance, you assign values to its fields. Here, x should be 5.

2fill in blank

medium

Complete the code to create a Rectangle instance with width 10 and height 20.

Rust

struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect = Rectangle { width: [1], height: 20 };
    println!("{} x {}", rect.width, rect.height);
}

Drag options to blanks, or click blank then click option'

A10

B20

Cwidth

Dheight

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing width and height values.

Using the field name instead of a number.

To create the rectangle, set width to 10 as required.

3fill in blank

hard

Fix the error in creating a User instance by completing the missing field value.

Rust

struct User {
    username: String,
    active: bool,
}

fn main() {
    let user = User { username: String::from("alice"), active: [1] };
    println!("User active: {}", user.active);
}

Drag options to blanks, or click blank then click option'

Atrue

B"true"

CTrue

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around boolean values.

Using capitalized True which is invalid in Rust.

The active field is a boolean and should be set to true without quotes.

4fill in blank

hard

Fill both blanks to create a Book instance with title "Rust" and pages 300.

Rust

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

fn main() {
    let book = Book { title: String::from([1]), pages: [2] };
    println!("{} has {} pages", book.title, book.pages);
}

Drag options to blanks, or click blank then click option'

A"Rust"

B300

C"rust"

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using numbers as strings or strings without quotes.

Mixing up the number of pages.

The title is a string so it needs quotes, and pages is a number without quotes.

5fill in blank

hard

Fill all three blanks to create a Car instance with brand "Tesla", year 2022, and electric true.

Rust

struct Car {
    brand: String,
    year: u32,
    electric: bool,
}

fn main() {
    let car = Car {
        brand: String::from([1]),
        year: [2],
        electric: [3],
    };
    println!("{} {} electric: {}", car.brand, car.year, car.electric);
}

Drag options to blanks, or click blank then click option'

A"Tesla"

B2022

Ctrue

D"tesla"

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around boolean values.

Using lowercase brand without quotes.

Mixing up the year value.

Brand is a string needing quotes, year is a number, and electric is a boolean true without quotes.

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