Match guards in Rust - Mini Project: Build & Apply

Match Guards in Rust

📖 Scenario: You are building a simple program that categorizes numbers based on their value. You want to use Rust's match statement with match guards to check extra conditions.

🎯 Goal: Create a Rust program that uses match with match guards to print different messages depending on whether a number is positive, negative, or zero, and if it is even or odd.

📋 What You'll Learn

Create a variable called number with the value 7.

Create a variable called threshold with the value 0.

Use a match statement on number with match guards to check if the number is greater than, less than, or equal to threshold, and whether it is even or odd.

Print the correct message for each case.

💡 Why This Matters

🌍 Real World

Match guards help you handle different cases with extra conditions, useful in input validation, parsing, and decision-making in programs.

💼 Career

Understanding match guards is important for Rust developers to write clear and efficient code that handles complex conditions safely.

1

Create the number variable

Create a variable called number and set it to 7.

Rust

# Create a variable called number and set it to 7
# Your code here

Hint

Use let number = 7; to create the variable.

2

Create the threshold variable

Add a variable called threshold and set it to 0 below the number variable.

Rust

fn main() {
    let number = 7;
    # Create a variable called threshold and set it to 0
    # Your code here
}

Hint

Use let threshold = 0; to create the variable.

3

Use match with match guards

Write a match statement on number using match guards to check these cases:
- If number is greater than threshold and even,
- If number is greater than threshold and odd,
- If number is less than threshold,
- If number is equal to threshold.
Inside each arm, assign a string message describing the case to a variable called message.

Rust

fn main() {
    let number = 7;
    let threshold = 0;

    // Use match with guards on number
    // Assign a message string to variable message
    // Your code here
}

Hint

Use match number { n if condition => ... } to add match guards.

4

Print the message

Add a println! statement to print the message variable.

Rust

fn main() {
    let number = 7;
    let threshold = 0;

    let message = match number {
        n if n > threshold && n % 2 == 0 => "Number is positive and even",
        n if n > threshold && n % 2 != 0 => "Number is positive and odd",
        n if n  "Number is negative",
        _ => "Number is zero",
    };

    // Print the message
    // Your code here
}

Hint

Use println!("{}", message); to print the message.

Practice

1.

What is the purpose of a match guard in Rust?

easy

A. To add an extra condition to a pattern in a match statement

B. To replace the entire match statement with an if statement

C. To declare a new variable inside a match arm

D. To loop over all patterns automatically

Solution

1. Step 1: Understand match guards

   Match guards add extra conditions to patterns in a match, allowing more precise checks.

2. Step 2: Compare options

   Only To add an extra condition to a pattern in a match statement correctly describes adding extra conditions to patterns.

3. Final Answer:

   To add an extra condition to a pattern in a match statement -> Option A

4. Quick Check:

   Match guard = extra condition [OK]

Hint: Match guards add conditions after patterns using if [OK]

Common Mistakes:

• Thinking match guards replace match statements
• Confusing match guards with variable declarations
• Assuming match guards loop over patterns

2.

Which of the following is the correct syntax for a match guard in Rust?

match x {
    5 if x > 3 => println!("Greater than 3"),
    _ => println!("Other")
}

easy

A. match x { if 5 > 3 => println!("Greater than 3"), _ => println!("Other") }

B. match x { 5 => if x > 3 { println!("Greater than 3") }, _ => println!("Other") }

C. match x { 5 if x > 3 => println!("Greater than 3"), _ => println!("Other") }

D. match x { 5 => println!("Greater than 3") if x > 3, _ => println!("Other") }

Solution

1. Step 1: Identify correct match guard syntax

   Match guards use pattern if condition => expression syntax.

2. Step 2: Check options

   match x { 5 if x > 3 => println!("Greater than 3"), _ => println!("Other") } correctly places if x > 3 after the pattern 5.

3. Final Answer:

   match x { 5 if x > 3 => println!("Greater than 3"), _ => println!("Other") } -> Option C

4. Quick Check:

   Pattern if condition => action [OK]

Hint: Use 'pattern if condition => action' for match guards [OK]

Common Mistakes:

• Placing if before the pattern
• Using if inside the match arm block
• Putting if after the expression

3.

What will be the output of the following Rust code?

let num = 7;
match num {
    n if n < 5 => println!("Less than 5"),
    n if n <= 10 => println!("Between 5 and 10"),
    _ => println!("Greater than 10"),
}

medium

A. Between 5 and 10

B. No output (compile error)

C. Greater than 10

D. Less than 5

Solution

1. Step 1: Evaluate match arms in order

   num is 7, first guard checks if 7 < 5 (false), so skip.

2. Step 2: Check second guard

   7 <= 10 is true, so this arm runs and prints "Between 5 and 10".

3. Final Answer:

   Between 5 and 10 -> Option A

4. Quick Check:

   7 ≤ 10 matches second guard [OK]

Hint: Match arms checked top to bottom; first true guard runs [OK]

Common Mistakes:

• Assuming first guard matches 7
• Thinking all guards run
• Confusing output with compile error

4.

Identify the error in this Rust code using match guards:

let x = 10;
match x {
    n if n > 5 => println!("Greater than 5"),
    n if n > 8 => println!("Greater than 8"),
    _ => println!("Other"),
}

medium

A. Syntax error: match guards cannot use variables

B. The wildcard pattern must come before guarded patterns

C. Missing semicolon after println! macros

D. The second guard is unreachable because the first guard covers all cases greater than 5

Solution

1. Step 1: Analyze guard order and coverage

   The first guard matches all n > 5, so it includes all n > 8 as well.

2. Step 2: Identify unreachable code

   The second guard is never reached because the first guard catches those values first.

3. Final Answer:

   The second guard is unreachable because the first guard covers all cases greater than 5 -> Option D

4. Quick Check:

   First guard shadows second guard [OK]

Hint: Order guards from most specific to least specific [OK]

Common Mistakes:

• Ignoring unreachable code warnings
• Thinking guards can overlap safely
• Misunderstanding guard evaluation order

5.

Given a list of integers, write a Rust match expression with match guards to print:

• "Small" if the number is less than 10
• "Medium" if the number is between 10 and 20 (inclusive)
• "Large" if the number is greater than 20

Which of the following match arms correctly implements this logic for a variable n?

hard

A. match n {
x if x <= 10 => println!("Small"),
x if x > 10 && x < 20 => println!("Medium"),
_ => println!("Large"),
}

B. match n {
x if x < 10 => println!("Small"),
x if x >= 10 && x <= 20 => println!("Medium"),
_ => println!("Large"),
}

C. match n {
x if x < 10 => println!("Small"),
x if x > 20 => println!("Medium"),
_ => println!("Large"),
}

D. match n {
x if x < 10 => println!("Small"),
x if x > 10 && x < 20 => println!("Medium"),
_ => println!("Large"),
}

Solution

1. Step 1: Check conditions for each size category

   "Small" is for n < 10, "Medium" for 10 ≤ n ≤ 20, "Large" for n > 20.

2. Step 2: Verify each match arm

   match n { x if x < 10 => println!("Small"), x if x >= 10 && x <= 20 => println!("Medium"), _ => println!("Large"), } correctly uses x < 10, x >= 10 && x <= 20, and wildcard for others.

3. Final Answer:

   match n { x if x < 10 => println!("Small"), x if x >= 10 && x <= 20 => println!("Medium"), _ => println!("Large"), } -> Option B

4. Quick Check:

   Guards match exact ranges [OK]

Hint: Use inclusive ranges with && for medium category [OK]

Common Mistakes:

• Using wrong comparison operators
• Overlapping ranges causing unreachable arms
• Missing inclusive bounds for medium range