Match guards in Rust - Step-by-Step Execution

Concept Flow - Match guards

Start with value

↓

Match arm 1?

No→Match arm 2?

↓

Execute arm code

↓

End

The program checks each match arm in order. If the pattern matches and the guard condition is true, it runs that arm's code. Otherwise, it moves to the next arm.

Execution Sample

Rust

let x = 5;
match x {
  n if n < 3 => "small",
  n if n < 7 => "medium",
  _ => "large",
}

Match the value x with conditions using guards to decide if it's small, medium, or large.

Execution Table

Step	Match Arm Pattern	Guard Condition	Guard Result	Action Taken	Output
1	n if n < 3	5 < 3	false	Skip arm
2	n if n < 7	5 < 7	true	Execute arm	"medium"
3	_	N/A	N/A	Not reached

💡 Match arm 2 guard is true, so execution stops after returning "medium".

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	5	5	5	5
n	N/A	5	5	5

Key Moments - 2 Insights

Why does the first match arm get skipped even though the pattern matches?

Because the guard condition (5 < 3) is false, the arm is skipped as shown in execution_table step 1.

What happens if no guard condition is true?

The default arm (_) runs, as a fallback. Here, it is not reached because step 2's guard is true.

Visual Quiz - 3 Questions

Look at the execution table, what is the guard result at step 1?

A false

B true

C N/A

D error

💡 Hint

Check the 'Guard Result' column in execution_table row 1.

At which step does the match stop and return a value?

A Step 3

B Step 1

C Step 2

D Never stops

💡 Hint

Look at the 'Action Taken' and 'Output' columns in execution_table.

If x was 2, which arm would execute?

A Arm 3 (_)

B Arm 1 (n if n < 3)

C Arm 2 (n if n < 7)

D No arm executes

💡 Hint

Check the guard conditions and their results for x=2 in the variable_tracker and execution_table logic.

Concept Snapshot

Match guards add extra conditions to match arms.
Syntax: pattern if condition => expression
The arm runs only if pattern matches AND condition is true.
If guard is false, matching continues to next arm.
Useful for fine control in pattern matching.

Full Transcript

This example shows how Rust's match guards work. The value x is matched against patterns with extra conditions called guards. Each arm has a pattern and a guard condition. The program checks each arm in order. If the pattern matches and the guard condition is true, it executes that arm's code and stops. Otherwise, it moves to the next arm. In the example, x=5 does not satisfy the first guard (5 < 3 is false), so it skips that arm. The second arm's guard (5 < 7) is true, so it runs that arm and returns "medium". The default arm is not reached. Variables x and n keep their values throughout. This helps control matching with extra checks beyond just the pattern.

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