Match guards in Rust - Time & Space Complexity

Time Complexity: Match guards

O(n)

Understanding Time Complexity

We want to see how using match guards affects the time it takes for a program to run.

Specifically, how the number of checks grows as the input gets bigger.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fn count_even_greater_than_five(numbers: &[i32]) -> usize {
    let mut count = 0;
    for &num in numbers {
        match num {
            n if n % 2 == 0 && n > 5 => count += 1,
            _ => (),
        }
    }
    count
}
    

This code counts how many numbers in a list are even and greater than five using a match with a guard.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each number in the list once.
• How many times: Exactly once for each number in the input list.

How Execution Grows With Input

Each number is checked once with the match guard condition.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of checks grows directly with the input size.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line as the list gets bigger.

Common Mistake

[X] Wrong: "Match guards make the code slower by adding extra loops or checks for each condition."

[OK] Correct: The match guard only adds a simple condition check inside the existing loop, so it does not add extra loops or multiply the work.

Interview Connect

Understanding how match guards affect performance helps you write clear and efficient Rust code, a skill that shows you think about both correctness and speed.

Self-Check

"What if we added a nested loop inside the match guard to check something else? How would the time complexity change?"

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