Match guards in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a match guard in Rust?

A match guard is an additional condition specified after a pattern in a match arm using if. It lets you add extra checks to decide if that arm should be chosen.

beginner

How do you write a match guard in Rust?

You write a match guard by adding if condition after the pattern in a match arm. For example: Some(x) if x > 5 => ...

intermediate

Why use match guards instead of just matching patterns?

Match guards let you check extra conditions that patterns alone can't express, like comparing values or calling functions, making your matching more precise.

intermediate

What happens if multiple match arms match but only one has a true guard?

Rust chooses the first arm where the pattern matches and the guard condition is true. If the guard is false, Rust continues checking the next arms.

intermediate

Can match guards use variables bound in the pattern?

Yes! Variables bound in the pattern can be used in the guard condition to make decisions based on their values.

What keyword introduces a match guard in Rust?

A if

B else

C match

D guard

Match guards use the if keyword after a pattern to add extra conditions.

Which arm will be chosen in this code?<br>

match x {
  Some(n) if n > 10 => "big",
  Some(_) => "small",
  None => "none",
}

A The first arm if n &gt; 10

B The third arm if x is None

C Both A and C depending on x

D The second arm always

If x is Some(n) and n > 10, the first arm matches. If x is Some(n) but n ≤ 10, the second arm matches. If x is None, the third arm matches.

Can a match guard call functions?

A Yes, any function can be called in a guard

B No, only simple comparisons are allowed

C Only functions without parameters

D Only functions returning booleans

Match guards can call any function as long as it returns a boolean value.

What happens if a guard condition is false?

A Rust restarts the match

B Rust panics

C Rust chooses that arm anyway

D Rust skips that arm and checks the next

If the guard condition is false, Rust ignores that arm and continues checking the next arms.

Can match guards be used with all pattern types?

A No, only with enum variants

B Yes, with any pattern

C Only with tuple patterns

D Only with literals

Match guards can be used with any pattern to add extra conditions.

Explain what a match guard is and how it changes pattern matching behavior in Rust.

Think about how you can add an extra check after matching a pattern.

Describe a situation where using a match guard is better than just matching a pattern.

Consider when you want to match a pattern only if a value meets a certain condition.