Match overview in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the match expression in Rust?

The match expression in Rust allows you to compare a value against a series of patterns and execute code based on which pattern matches. It's similar to a switch statement but more powerful and exhaustive.

beginner

How does Rust ensure all cases are handled in a match expression?

Rust requires that match expressions be exhaustive, meaning all possible cases must be covered. If not, the compiler will give an error. You can use a catch-all pattern _ to handle any remaining cases.

beginner

What is the purpose of the underscore _ pattern in a match?

The underscore _ pattern acts as a wildcard that matches any value not matched by previous arms. It is used as a catch-all to ensure exhaustiveness without specifying every possible value.

intermediate

Can match arms bind variables? Give an example.

Yes, match arms can bind variables to parts of the matched value. For example:

match some_option {
    Some(x) => println!("Value is {}", x),
    None => println!("No value"),
}

beginner

What happens if no match arm matches and there is no catch-all?

The Rust compiler will produce an error because match expressions must be exhaustive. You must handle all possible cases or include a catch-all _ arm.

What keyword is used in Rust to compare a value against multiple patterns?

A match

B switch

C case

D select

Rust uses the match keyword to compare a value against patterns.

What does the underscore _ pattern represent in a match?

A A variable binding

B A wildcard matching any value

C A specific value

D An error case

The underscore _ is a wildcard pattern that matches any value not matched before.

What will happen if a match expression is not exhaustive?

A It will ignore unmatched cases silently

B It will run but skip unmatched cases

C It will panic at runtime

D It will cause a compiler error

Rust requires match expressions to be exhaustive; otherwise, the compiler throws an error.

Can you bind variables inside match arms?

A Yes, to parts of the matched value

B Only in the catch-all arm

C Only constants can be used

D No, variables cannot be bound

You can bind variables to parts of the matched value inside match arms.

Which of these is a valid match arm pattern?

A case x

B default

C Some(x)

D switch(x)

Some(x) is a valid pattern matching an enum variant with a variable binding.

Explain how the match expression works in Rust and why it is useful.

Think about how you decide what to do based on different cases.

Describe the role of the underscore _ pattern in a match expression.

It’s like saying 'anything else' in a list of options.

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