Match with enums in Rust

Introduction

Matching with enums helps you choose what to do based on different named options. It makes your code clear and easy to follow.

When you have a variable that can be one of several fixed options.

When you want to run different code depending on the option chosen.

When you want to handle all possible cases safely without missing any.

When you want to write clear and readable code for decision making.

Syntax

Rust

enum Name {
    Option1,
    Option2,
    Option3,
}

match value {
    Name::Option1 => { /* code for Option1 */ },
    Name::Option2 => { /* code for Option2 */ },
    Name::Option3 => { /* code for Option3 */ },
}

The match keyword checks the value against each enum option.

Each arm ends with a comma and can run any code you want.

Examples

This example prints a message depending on the direction chosen.

Rust

enum Direction {
    Up,
    Down,
    Left,
    Right,
}

let dir = Direction::Left;

match dir {
    Direction::Up => println!("Going up!"),
    Direction::Down => println!("Going down!"),
    Direction::Left => println!("Going left!"),
    Direction::Right => println!("Going right!"),
}

This example shows how to react to traffic light colors.

Rust

enum Light {
    Red,
    Yellow,
    Green,
}

let light = Light::Green;

match light {
    Light::Red => println!("Stop"),
    Light::Yellow => println!("Get ready"),
    Light::Green => println!("Go"),
}

This example matches enum variants with data inside and uses that data.

Rust

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

let msg = Message::Move { x: 10, y: 20 };

match msg {
    Message::Quit => println!("Quit message"),
    Message::Move { x, y } => println!("Move to x: {}, y: {}", x, y),
    Message::Write(text) => println!("Write message: {}", text),
}

Sample Program

This program checks the weather and prints a message for each type.

Rust

enum Weather {
    Sunny,
    Rainy,
    Windy,
}

fn main() {
    let today = Weather::Rainy;

    match today {
        Weather::Sunny => println!("It's sunny outside!"),
        Weather::Rainy => println!("Don't forget your umbrella."),
        Weather::Windy => println!("Hold onto your hat!"),
    }
}

Important Notes

Every possible enum variant must be handled in the match to avoid errors.

You can use _ as a catch-all pattern if you don't want to list all variants.

Matching enums helps keep your code safe and clear by forcing you to think about all cases.

Summary

Use match to run code based on enum options.

Each enum variant is a case in the match arms.

This makes your code easy to read and safe from missing cases.

Practice

1.

What does the match statement do when used with enums in Rust?

easy

A. It checks the enum variant and runs the matching code block.

B. It converts the enum into a string automatically.

C. It creates a new enum variant.

D. It deletes the enum variant from memory.

Solution

1. Step 1: Understand the purpose of match with enums

   The match statement compares the enum value against each variant.

2. Step 2: Identify what happens on a match

   When a variant matches, the corresponding code block runs.

3. Final Answer:

   It checks the enum variant and runs the matching code block. -> Option A

4. Quick Check:

   Match runs code by enum variant [OK]

Hint: Match arms correspond to enum variants directly [OK]

Common Mistakes:

• Thinking match converts enums to strings
• Assuming match creates or deletes variants
• Confusing match with if-else statements

2.

Which of the following is the correct syntax to match an enum Color with variants Red, Green, and Blue?

enum Color { Red, Green, Blue }

easy

A. match color { Color.Red => {}, Color.Green => {}, Color.Blue => {} }

B. match color { Red => {}, Green => {}, Blue => {} }

C. match color { Color::Red => {}, Color::Green => {}, Color::Blue => {} }

D. match color { 'Red' => {}, 'Green' => {}, 'Blue' => {} }

Solution

1. Step 1: Recall enum variant syntax in match

   Enum variants must be prefixed with the enum name and double colon, like Color::Red.

2. Step 2: Check each option's syntax

   Only match color { Color::Red => {}, Color::Green => {}, Color::Blue => {} } uses the correct Color::Variant syntax.

3. Final Answer:

   match color { Color::Red => {}, Color::Green => {}, Color::Blue => {} } -> Option C

4. Quick Check:

   Enum variants use EnumName::Variant [OK]

Hint: Use EnumName::Variant in match arms [OK]

Common Mistakes:

• Omitting enum name prefix
• Using dot instead of double colon
• Using strings instead of variants

3.

What will be the output of this Rust code?

enum Direction { North, South, East, West }

fn main() {
    let dir = Direction::East;
    let message = match dir {
        Direction::North => "Going up",
        Direction::South => "Going down",
        Direction::East => "Going right",
        Direction::West => "Going left",
    };
    println!("{}", message);
}

medium

A. Going right

B. Going up

C. Going left

D. Going down

Solution

1. Step 1: Identify the enum variant assigned

   The variable dir is set to Direction::East.

2. Step 2: Match dir to the correct message

   The match arm for Direction::East returns "Going right".

3. Final Answer:

   Going right -> Option A

4. Quick Check:

   Direction::East matches "Going right" [OK]

Hint: Match returns value for the exact enum variant [OK]

Common Mistakes:

• Mixing up enum variants and their messages
• Forgetting to use EnumName::Variant in match
• Assuming default or fallback arm runs

4.

Find the error in this Rust code snippet using match with enums:

enum Status { Success, Failure }

fn check(status: Status) {
    match status {
        Status::Success => println!("Success"),
        // Missing Failure arm
    }
}

medium

A. Missing semicolon after println! macro

B. Function check should return a value

C. Incorrect enum variant syntax

D. Missing match arm for Status::Failure

Solution

1. Step 1: Check completeness of match arms

   The match statement covers Status::Success but misses Status::Failure.

2. Step 2: Understand Rust's exhaustiveness requirement

   Rust requires all enum variants to be matched or a catch-all arm (_) to avoid compile errors.

3. Final Answer:

   Missing match arm for Status::Failure -> Option D

4. Quick Check:

   All enum variants must be matched [OK]

Hint: Match must cover all enum variants or use _ arm [OK]

Common Mistakes:

• Leaving out enum variants in match arms
• Assuming partial match compiles
• Confusing semicolon placement with match completeness

5.

Given this enum with data:

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

Which match arm correctly extracts the x and y values from Message::Move?

hard

A. Message::Move(x, y) => println!("x: {}, y: {}", x, y),

B. Message::Move { x, y } => println!("x: {}, y: {}", x, y),

C. Message::Move(x: x, y: y) => println!("x: {}, y: {}", x, y),

D. Message::Move(x: x, y) => println!("x: {}, y: {}", x, y),

Solution

1. Step 1: Identify the enum variant with named fields

   Move uses named fields { x: i32, y: i32 }.

2. Step 2: Match syntax for named fields

   Use Message::Move { x, y } to destructure named fields directly.

3. Final Answer:

   Message::Move { x, y } => println!("x: {}, y: {}", x, y), -> Option B

4. Quick Check:

   Named fields use braces and field names [OK]

Hint: Use braces and field names to destructure named fields [OK]

Common Mistakes:

• Using tuple pattern for struct-like variant
• Mixing colon syntax incorrectly
• Omitting braces for named fields