Enum variants in Rust

Introduction

Enums let you group related values under one name. Enum variants are the different options you can choose from inside that group.

When you want to represent a choice between several options, like days of the week.

When you need to handle different types of messages or commands in a program.

When you want to group related data that can be one of several types.

When you want to write clear code that handles different cases explicitly.

Syntax

Rust

enum Name {
    Variant1,
    Variant2,
    Variant3
}

Each variant is like a named option inside the enum.

Variants can hold data or be simple names.

Examples

Simple enum with four variants representing directions.

Rust

enum Direction {
    North,
    East,
    South,
    West
}

Enum variants can hold data: named fields, single values, or multiple values.

Rust

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

Sample Program

This program defines an enum with three variants. It prints a message depending on the variant.

Rust

enum TrafficLight {
    Red,
    Yellow,
    Green
}

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

fn main() {
    let light = TrafficLight::Yellow;
    print_light(light);
}

Important Notes

You use :: to access enum variants.

Enums help make your code safer by forcing you to handle all cases.

Summary

Enum variants are named options inside an enum.

Variants can be simple or hold data.

Use enums to clearly represent choices or states.

Practice

1. What is an enum variant in Rust?

easy

A. A function inside a struct

B. A named option inside an enum representing a possible value

C. A variable declared with let

D. A type of loop in Rust

Solution

1. Step 1: Understand enum structure

   An enum groups related named options called variants.

2. Step 2: Define variant role

   Each variant represents a possible value the enum can hold.

3. Final Answer:

   A named option inside an enum representing a possible value -> Option B

4. Quick Check:

   Enum variant = named option [OK]

Hint: Enum variants are named choices inside enums [OK]

Common Mistakes:

• Confusing variants with functions
• Thinking variants are variables
• Mixing variants with loops

2. Which of the following is the correct syntax to define an enum variant named Red inside an enum Color?

easy

A. enum Color { Red }

B. enum Color = Red;

C. struct Color { Red }

D. let Color = Red;

Solution

1. Step 1: Recall enum syntax

   Enums use the syntax enum Name { Variant1, Variant2 }.

2. Step 2: Match correct syntax

   enum Color { Red } correctly defines enum Color with variant Red.

3. Final Answer:

   enum Color { Red } -> Option A

4. Quick Check:

   Enum variant syntax = enum Name { Variant } [OK]

Hint: Enum variants go inside braces after enum name [OK]

Common Mistakes:

• Using '=' instead of braces
• Defining variants inside struct
• Using let instead of enum

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

enum Direction {
    North,
    East,
    South,
    West,
}

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

medium

A. Going North

B. Going South

C. Going East

D. Going West

Solution

1. Step 1: Identify enum variant assigned

   The variable dir is set to Direction::East.

2. Step 2: Match statement output

   The match arm for Direction::East prints "Going East".

3. Final Answer:

   Going East -> Option C

4. Quick Check:

   Match on Direction::East prints "Going East" [OK]

Hint: Match prints message for assigned enum variant [OK]

Common Mistakes:

• Confusing variant names
• Ignoring match arms
• Assuming default output

4. Identify the error in this enum variant usage:

enum Status {
    Active,
    Inactive,
}

fn main() {
    let s = Status::active;
}

medium

A. Enum variants must have data inside parentheses

B. Missing semicolon after enum definition

C. Cannot assign enum variant to variable

D. Enum variant names are case-sensitive; should be 'Active'

Solution

1. Step 1: Check variant usage

   The code uses Status::active but enum defines Active with uppercase A.

2. Step 2: Understand Rust case sensitivity

   Rust enum variants are case-sensitive; active is undefined.

3. Final Answer:

   Enum variant names are case-sensitive; should be 'Active' -> Option D

4. Quick Check:

   Variant names must match case exactly [OK]

Hint: Enum variant names are case-sensitive [OK]

Common Mistakes:

• Using wrong case for variants
• Forgetting semicolons (not error here)
• Thinking variants need data always

5. Given this enum with data:

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

What will this program print?

hard

A. Move to (10, 20)

B. Quit message

C. Write: Move to (10, 20)

D. Compilation error due to missing variant

Solution

1. Step 1: Identify enum variant with data

   The variable msg is Message::Move { x: 10, y: 20 }.

2. Step 2: Match arms handle each variant

   The match arm for Message::Move { x, y } prints coordinates.

3. Final Answer:

   Move to (10, 20) -> Option A

4. Quick Check:

   Match prints coordinates for Move variant [OK]

Hint: Match extracts data from enum variant fields [OK]

Common Mistakes:

• Confusing variant names
• Ignoring data inside variants
• Expecting compilation error incorrectly