Match overview in Rust

Introduction

The match expression helps you check a value against many possibilities and run code based on which one fits. It makes your code clear and easy to read.

When you want to do different things based on different values of a variable.

When you have a few fixed options and want to handle each one clearly.

When you want to replace many <code>if-else</code> checks with cleaner code.

When you want to make sure you cover all possible cases of a value.

When you want to extract data from complex values and use it.

Syntax

Rust

match value {
    pattern1 => expression1,
    pattern2 => expression2,
    _ => default_expression,
}

The value is what you want to check.

Each pattern is a possible match. The arrow => points to the code to run if it matches.

Examples

This checks the number and prints its name or "Other number" if no match.

Rust

let number = 3;
match number {
    1 => println!("One"),
    2 => println!("Two"),
    3 => println!("Three"),
    _ => println!("Other number"),
}

This matches string values and prints a message for each color.

Rust

let color = "red";
match color {
    "red" => println!("Stop"),
    "green" => println!("Go"),
    "yellow" => println!("Slow down"),
    _ => println!("Unknown color"),
}

This matches enum variants and prints a direction message.

Rust

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

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

Sample Program

This program uses match to find the name of the day from its number and prints it.

Rust

fn main() {
    let day = 3;
    let day_name = match day {
        1 => "Monday",
        2 => "Tuesday",
        3 => "Wednesday",
        4 => "Thursday",
        5 => "Friday",
        6 => "Saturday",
        7 => "Sunday",
        _ => "Invalid day",
    };
    println!("Day {} is {}", day, day_name);
}

Important Notes

The _ pattern is a catch-all for any value not matched earlier.

Each match arm must return the same type if you use match as an expression.

You can match complex patterns like enums, tuples, and even use guards for extra checks.

Summary

Match checks a value against many patterns clearly.

It helps replace many if-else statements with cleaner code.

Always include a catch-all _ to handle unexpected cases.

Practice

1. What is the main purpose of the match statement in Rust?

easy

A. To check a value against multiple patterns clearly and concisely

B. To create loops that run a fixed number of times

C. To define new data types

D. To handle errors using exceptions

Solution

1. Step 1: Understand the role of match

   The match statement compares a value against many patterns, making code clearer than many if-else statements.

2. Step 2: Compare with other options

   Loops run repeatedly, data types define structures, and exceptions handle errors, but match is for pattern matching.

3. Final Answer:

   To check a value against multiple patterns clearly and concisely -> Option A

4. Quick Check:

   Match = pattern checking [OK]

Hint: Match replaces many if-else with clear pattern checks [OK]

Common Mistakes:

• Confusing match with loops
• Thinking match defines types
• Assuming match handles exceptions

2. Which of the following is the correct syntax for a match expression in Rust?

easy

A. match value { pattern1 => expr1, pattern2 => expr2, _ => default_expr }

B. match (value) { pattern1: expr1; pattern2: expr2; default: default_expr }

C. match value { pattern1 -> expr1, pattern2 -> expr2, _ -> default_expr }

D. match value { pattern1 => expr1; pattern2 => expr2; _ => default_expr }

Solution

1. Step 1: Recall Rust match syntax

   Rust uses curly braces with patterns followed by => and expressions separated by commas.

2. Step 2: Check each option

   match value { pattern1 => expr1, pattern2 => expr2, _ => default_expr } uses correct braces, arrows, and commas. Options B and D use semicolons incorrectly. match value { pattern1 -> expr1, pattern2 -> expr2, _ -> default_expr } uses -> instead of =>.

3. Final Answer:

   match value { pattern1 => expr1, pattern2 => expr2, _ => default_expr } -> Option A

4. Quick Check:

   Correct match syntax uses => and commas [OK]

Hint: Use => between pattern and expression, separate arms with commas [OK]

Common Mistakes:

• Using semicolons instead of commas
• Using -> instead of =>
• Missing the catch-all _ pattern

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

let number = 3;
match number {
    1 => println!("One"),
    2 => println!("Two"),
    3 => println!("Three"),
    _ => println!("Other"),
}

medium

A. One

B. Two

C. Other

D. Three

Solution

1. Step 1: Identify the matched value

   The variable number is 3, so the match will look for pattern 3.

2. Step 2: Find the matching arm

   The arm 3 => println!("Three") matches and prints "Three".

3. Final Answer:

   Three -> Option D

4. Quick Check:

   number=3 matches 3 => "Three" [OK]

Hint: Match value exactly to print correct output [OK]

Common Mistakes:

• Choosing default _ arm instead of exact match
• Confusing numbers with strings
• Ignoring the value of number

4. Identify the error in this Rust match code snippet:

let x = 5;
match x {
    1 => println!("One"),
    2 => println!("Two"),
    3 => println!("Three")
}

medium

A. Missing semicolon after println! macros

B. Using parentheses instead of curly braces in match

C. Missing catch-all _ pattern to handle other values

D. Incorrect variable declaration

Solution

1. Step 1: Check match arms completeness

   The match covers 1, 2, and 3 but does not handle other values like 5.

2. Step 2: Understand Rust match requirements

   Rust requires all possible values to be covered, usually by a catch-all _ pattern.

3. Final Answer:

   Missing catch-all _ pattern to handle other values -> Option C

4. Quick Check:

   Always include _ to cover unmatched cases [OK]

Hint: Always add _ arm to handle unexpected values [OK]

Common Mistakes:

• Forgetting the catch-all _ pattern
• Thinking semicolons are mandatory after println!
• Confusing braces with parentheses

5. Given this Rust code, what will be the output?

let grade = 'B';
match grade {
    'A' | 'B' | 'C' => println!("Pass"),
    'D' => println!("Conditional Pass"),
    _ => println!("Fail"),
}

hard

A. Conditional Pass

B. Pass

C. Fail

D. Compilation error

Solution

1. Step 1: Identify the value and patterns

   The variable grade is 'B'. The first arm matches 'A', 'B', or 'C'.

2. Step 2: Determine which arm runs

   Since 'B' matches the first arm, it prints "Pass".

3. Final Answer:

   Pass -> Option B

4. Quick Check:

   Multiple patterns with | match 'B' => "Pass" [OK]

Hint: Use | to match multiple patterns in one arm [OK]

Common Mistakes:

• Assuming only single pattern per arm
• Choosing wrong output for 'B'
• Thinking code causes compilation error