Match expression deep dive in Rust - Step-by-Step Execution

Concept Flow - Match expression deep dive

Start with a value

↓

Match expression begins

↓

Check first pattern

No→Check next pattern

↓

Execute matched arm

↓

Return arm result

↓

End match expression

The match expression checks a value against patterns one by one, executes the first matching arm, then returns its result.

Execution Sample

Rust

let x = 3;
let result = match x {
    1 => "one",
    2 | 3 => "two or three",
    _ => "other",
};

This code matches the value of x to patterns and assigns a string describing it to result.

Execution Table

Step	Value of x	Pattern Checked	Match?	Action	Result
1	3	1	No	Check next pattern	None
2	3	2 | 3	Yes	Execute arm: return "two or three"	"two or three"
3	3	_ (wildcard)	Skipped	Match already found	"two or three"

💡 Match found at step 2 with pattern '2 | 3', so match expression ends returning "two or three".

Variable Tracker

Variable	Start	After match
x	3	3
result	uninitialized	"two or three"

Key Moments - 3 Insights

Why does the match stop checking after the second pattern even though there is a wildcard pattern after?

Because match expressions stop at the first pattern that matches. In the execution_table row 2, pattern '2 | 3' matches x=3, so the wildcard pattern is skipped (row 3).

What does the underscore (_) pattern mean in a match?

The underscore is a wildcard that matches any value not matched before. It acts like a default case, as shown in execution_table row 3.

Can multiple patterns be combined in one arm?

Yes, using the | operator. In the example, '2 | 3' matches if x is 2 or 3, as seen in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 2?

A "one"

B "two or three"

C "other"

D None

💡 Hint

Check the 'Result' column at step 2 in the execution_table.

At which step does the match expression stop checking patterns?

A Step 1

B Step 3

C Step 2

D It checks all patterns

💡 Hint

Look at the 'Match?' column and see where 'Yes' appears first in execution_table.

If x was 4 instead of 3, which pattern would match?

A _ (wildcard)

B 2 | 3

C 1

D No pattern matches

💡 Hint

Refer to the wildcard pattern '_' in the execution_table and key_moments about default matching.

Concept Snapshot

match value {
  pattern1 => expr1,
  pattern2 | pattern3 => expr2,
  _ => default_expr,
}
- Checks patterns top to bottom
- Executes first matching arm
- '_' matches anything else
- Arms can combine patterns with '|'
- Returns the arm's expression value

Full Transcript

This visual execution traces a Rust match expression. We start with a value x=3. The match checks patterns in order: first '1' (no match), then '2 | 3' (matches), so it executes that arm returning "two or three". The wildcard '_' pattern is skipped because a match was found. Variables x and result are tracked: x stays 3, result becomes "two or three". Key points: match stops at first match, '_' is a catch-all, and multiple patterns can be combined with '|'. The quiz tests understanding of these steps and outcomes.