If expression in Rust - Step-by-Step Execution

Concept Flow - If expression

Evaluate condition

↓

Condition true?

No→Execute else block

↓

Result from else

↓

Execute if block

↓

Result from if

↓

Use result

The if expression evaluates a condition. If true, it runs the if block and returns its value; otherwise, it runs the else block and returns that value.

Execution Sample

Rust

let x = 5;
let y = if x > 3 { 10 } else { 20 };
println!("y = {}", y);

This code sets y to 10 if x is greater than 3, otherwise 20, then prints y.

Execution Table

Step	Condition (x > 3)	Branch Taken	Value from Branch	Variable y
1	5 > 3	True (if block)	10	10
2	End of if expression	-	-	10
3	Print y	-	-	Outputs: y = 10

💡 Condition is true, so if block runs and y is set to 10.

Variable Tracker

Variable	Start	After if expression	Final
x	5	5	5
y	uninitialized	10	10

Key Moments - 2 Insights

Why does the if expression return a value?

In Rust, if is an expression, not just a statement. It returns the value from the executed branch, as shown in the execution_table step 1 where y gets the value 10.

What happens if the else block is missing?

If else is missing and the condition is false, Rust will give a compile error because the if expression must return a value in all cases. This is why the else branch is needed to cover the false case.

Visual Quiz - 3 Questions

Look at the execution table, what value does y have after the if expression?

A 5

B 10

C 20

D uninitialized

💡 Hint

Check the 'Variable y' column in the first row of the execution_table.

At which step does the program decide which branch to take?

A Step 3

B Step 2

C Step 1

D Before Step 1

💡 Hint

Look at the 'Condition (x > 3)' and 'Branch Taken' columns in the execution_table.

If x was 2 instead of 5, what would y be after the if expression?

A 20

B 10

C 2

D uninitialized

💡 Hint

If condition is false, else branch value is used, see execution_table logic.

Concept Snapshot

if expression syntax:
if condition { expr1 } else { expr2 }

Evaluates condition:
- if true, returns expr1
- else, returns expr2

Must cover all cases (else required)
Returns a value usable in assignments.

Full Transcript

This visual execution shows how Rust's if expression works. First, the condition x > 3 is checked. Since x is 5, the condition is true, so the if block runs and returns 10. This value is assigned to y. The else block is skipped. Finally, y is printed as 10. The key point is that if is an expression returning a value, not just a statement. The else block is necessary to cover the false case and ensure the expression always returns a value.