Match guards in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print "small" when the number is less than 10.

Rust

let num = 5;
match num {
    x if x [1] 10 => println!("small"),
    _ => println!("big"),
}

Drag options to blanks, or click blank then click option'

A<

B>=

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in the guard.

Using '==' which only matches exactly 10.

The match guard uses '<' to check if the number is less than 10.

2fill in blank

medium

Complete the code to print "even" when the number is even using a match guard.

Rust

let num = 8;
match num {
    x if x [1] 2 == 0 => println!("even"),
    _ => println!("odd"),
}

Drag options to blanks, or click blank then click option'

A/

B%

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/' instead of '%' which does division, not remainder.

Using '+' or '-' which are arithmetic operators but not for checking evenness.

The '%' operator is used to find the remainder. If remainder is 0 when divided by 2, the number is even.

3fill in blank

hard

Fix the error in the match guard to correctly check if the character is a vowel.

Rust

let ch = 'a';
match ch {
    c if c == 'a' || c == 'e' || c == 'i' || c == 'o' || c [1] 'u' => println!("vowel"),
    _ => println!("consonant"),
}

Drag options to blanks, or click blank then click option'

A==

B!=

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '==' in the condition.

Using '!=' which checks for inequality.

The equality operator '==' is needed to compare c with 'u'. Using '=' causes a syntax error.

4fill in blank

hard

Fill both blanks to create a match guard that matches numbers between 1 and 5 inclusive.

Rust

let num = 3;
match num {
    x if x [1] 1 && x [2] 5 => println!("between 1 and 5"),
    _ => println!("out of range"),
}

Drag options to blanks, or click blank then click option'

A>=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '<=' excludes 5.

Using '>' instead of '>=' excludes 1.

The guard checks if x is greater or equal to 1 and less or equal to 5.

5fill in blank

hard

Fill all three blanks to create a match guard that matches even numbers greater than 10.

Rust

let num = 12;
match num {
    x if x [1] 2 [2] 0 && x [3] 10 => println!("even and greater than 10"),
    _ => println!("other"),
}

Drag options to blanks, or click blank then click option'

A%

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/' instead of '%' for even check.

Using '<' instead of '>' for greater than check.

Using '=' instead of '==' for comparison.

The guard uses '%' to check evenness, '>' to check greater than 10, and '==' to compare remainder to 0.

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