Enums with data in Rust - Mini Project: Build & Apply

Enums with Data in Rust

📖 Scenario: You are building a simple program to represent different types of messages in a chat application. Each message can be a text message, an image with a URL, or a location with latitude and longitude.

🎯 Goal: Create an enum called Message with different variants holding data, then write code to handle each message type and print a description.

📋 What You'll Learn

Create an enum Message with variants Text, Image, and Location

The Text variant holds a String

The Image variant holds a String representing the URL

The Location variant holds two f64 values for latitude and longitude

Create a variable msg of type Message with the Image variant and URL "https://example.com/pic.png"

Use a match expression on msg to print a message describing the content

💡 Why This Matters

🌍 Real World

Enums with data are useful to represent different types of related information in one variable, like messages, events, or commands.

💼 Career

Understanding enums with data is important for Rust programming jobs, especially in systems programming, web backend, and applications requiring safe and clear data handling.

1

Create the Message enum with data

Create an enum called Message with three variants: Text(String), Image(String), and Location(f64, f64).

Rust

# Your code here

Hint

Use the enum keyword and define variants with data inside parentheses.

2

Create a variable msg with the Image variant

Create a variable called msg of type Message and assign it the Image variant with the URL "https://example.com/pic.png".

Rust

enum Message {
    Text(String),
    Image(String),
    Location(f64, f64),
}

# Your code here

Hint

Use let to create msg and Message::Image(String::from(...)) to assign the variant.

3

Use match to handle msg and print descriptions

Write a match expression on msg with arms for Text, Image, and Location. For each, bind the data to variables and print a descriptive message using println!. Use text for Text, url for Image, and lat, lon for Location.

Rust

enum Message {
    Text(String),
    Image(String),
    Location(f64, f64),
}

let msg = Message::Image(String::from("https://example.com/pic.png"));

# Your code here

Hint

Use match with pattern matching to extract data and print messages.

4

Print the message description

Run the program to print the description of the msg variable. The output should be: Image message with URL: https://example.com/pic.png

Rust

enum Message {
    Text(String),
    Image(String),
    Location(f64, f64),
}

let msg = Message::Image(String::from("https://example.com/pic.png"));

match msg {
    Message::Text(text) => println!("Text message: {}", text),
    Message::Image(url) => println!("Image message with URL: {}", url),
    Message::Location(lat, lon) => println!("Location message at latitude {}, longitude {}", lat, lon),
}

# Your code here

Hint

Make sure to wrap your code in a main function and run it to see the output.

Practice

1. What is the main purpose of enums with data in Rust?

easy

A. To group related values that can be different types under one name

B. To create multiple copies of the same data

C. To perform arithmetic operations on numbers

D. To store only strings in a list

Solution

1. Step 1: Understand what enums with data do

   Enums with data allow each variant to hold different types or amounts of data, grouping related but different information.

2. Step 2: Compare options with this understanding

   Only To group related values that can be different types under one name correctly describes grouping related values of different types under one name.

3. Final Answer:

   To group related values that can be different types under one name -> Option A

4. Quick Check:

   Enums with data = group different types [OK]

Hint: Enums group different data types under one name [OK]

Common Mistakes:

• Thinking enums only store one data type
• Confusing enums with arrays or vectors
• Assuming enums perform calculations

2. Which of the following is the correct syntax to define an enum variant with data in Rust?

easy

A. enum Color { Red(u8, u8, u8), Blue }

B. enum Color { Red = (u8, u8, u8), Blue }

C. enum Color { Red: (u8, u8, u8), Blue }

D. enum Color { Red -> (u8, u8, u8), Blue }

Solution

1. Step 1: Recall enum variant syntax with data

   In Rust, enum variants with data are defined by listing the variant name followed by parentheses containing the data types.

2. Step 2: Check each option

   enum Color { Red(u8, u8, u8), Blue } uses correct syntax: Red(u8, u8, u8). Options A, B, and C use invalid syntax with ->, =, or : which are incorrect for enum variant data.

3. Final Answer:

   enum Color { Red(u8, u8, u8), Blue } -> Option A

4. Quick Check:

   Variant with data uses parentheses () [OK]

Hint: Use parentheses () after variant name for data [OK]

Common Mistakes:

• Using = instead of () for variant data
• Using : or -> which are invalid here
• Forgetting parentheses for data

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

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
}

fn main() {
    let msg = Message::Move { x: 10, y: 20 };
    match msg {
        Message::Quit => println!("Quit"),
        Message::Move { x, y } => println!("Move to {}, {}", x, y),
        Message::Write(text) => println!("Write {}", text),
    }
}

medium

A. Compilation error

B. Quit

C. Write 10, 20

D. Move to 10, 20

Solution

1. Step 1: Identify the enum variant used

   The variable msg is assigned Message::Move with x=10 and y=20.

2. Step 2: Match arms and output

   The match statement checks msg. For Message::Move, it prints "Move to {}, {}" with x and y values, so output is "Move to 10, 20".

3. Final Answer:

   Move to 10, 20 -> Option D

4. Quick Check:

   Match Message::Move prints coordinates [OK]

Hint: Match variant and print its data fields [OK]

Common Mistakes:

• Confusing variant names
• Expecting Quit or Write output
• Thinking code won't compile

4. The following Rust code has an error. What is the problem?

enum Shape {
    Circle(f64),
    Rectangle { width: f64, height: f64 },
}

fn area(shape: Shape) -> f64 {
    match shape {
        Shape::Circle(radius) => 3.14 * radius * radius,
        Shape::Rectangle(width, height) => width * height,
    }
}

medium

A. Missing semicolon after match arms

B. Circle variant should have named fields

C. Incorrect pattern for Rectangle variant in match

D. Function return type should be i32

Solution

1. Step 1: Check enum variant definitions

   Circle has tuple data (f64), Rectangle has named fields { width, height }.

2. Step 2: Check match patterns

   Circle pattern is correct. Rectangle pattern uses (width, height) which is wrong; it should use { width, height } to match named fields.

3. Final Answer:

   Incorrect pattern for Rectangle variant in match -> Option C

4. Quick Check:

   Match named fields with braces {} [OK]

Hint: Match named fields with braces {}, tuple fields with () [OK]

Common Mistakes:

• Using parentheses for named fields
• Ignoring pattern mismatch errors
• Changing return type unnecessarily

5. Given this enum and function, what is the output?

enum Event {
    Click { x: i32, y: i32 },
    KeyPress(char),
    Resize(u32, u32),
}

fn describe(event: Event) -> String {
    match event {
        Event::Click { x, y } if x > 0 && y > 0 => format!("Clicked at positive coords: {}, {}", x, y),
        Event::Click { .. } => "Clicked at non-positive coords".to_string(),
        Event::KeyPress(c) if c.is_alphabetic() => format!("Key pressed: {}", c),
        Event::KeyPress(_) => "Non-alphabetic key pressed".to_string(),
        Event::Resize(w, h) if w == h => "Resized to square".to_string(),
        Event::Resize(_, _) => "Resized to rectangle".to_string(),
    }
}

fn main() {
    let e = Event::Resize(100, 100);
    println!("{}", describe(e));
}

hard

A. Clicked at positive coords: 100, 100

B. Resized to square

C. Resized to rectangle

D. Non-alphabetic key pressed

Solution

1. Step 1: Identify the enum variant and values

   The event is Event::Resize with width=100 and height=100.

2. Step 2: Match arms with conditions

   The Resize arm with condition w == h matches because 100 == 100, so it returns "Resized to square".

3. Final Answer:

   Resized to square -> Option B

4. Quick Check:

   Resize with equal sides = square [OK]

Hint: Check match guards (if conditions) carefully [OK]

Common Mistakes:

• Ignoring if conditions in match arms
• Confusing Resize with Click variant
• Assuming default arm without conditions