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.

Practice

1. What does an if expression do in Rust?

easy

A. It chooses between code blocks based on a condition.

B. It repeats code multiple times.

C. It defines a new function.

D. It declares a variable.

Solution

1. Step 1: Understand the purpose of if

   The if expression checks a condition to decide which code block to run.

2. Step 2: Compare with other options

   Repeating code is done by loops, functions define behavior, and variables store data, so these are incorrect.

3. Final Answer:

   It chooses between code blocks based on a condition. -> Option A

4. Quick Check:

   If expression = choose code [OK]

Hint: If checks true/false to pick code [OK]

Common Mistakes:

• Confusing if with loops
• Thinking if declares variables
• Mixing if with function definitions

2. Which of the following is the correct syntax for an if expression in Rust?

easy

A. if condition: { /* code */ } else: { /* code */ }

B. if condition { /* code */ } else { /* code */ }

C. if (condition) then { /* code */ } else { /* code */ }

D. if condition -> { /* code */ } else -> { /* code */ }

Solution

1. Step 1: Recall Rust if syntax

   Rust uses if condition { ... } else { ... } without parentheses or arrows.

2. Step 2: Check each option

   if condition { /* code */ } else { /* code */ } matches Rust syntax; others use invalid keywords or symbols.

3. Final Answer:

   if condition { /* code */ } else { /* code */ } -> Option B

4. Quick Check:

   Rust if syntax = braces, no parentheses [OK]

Hint: Rust if uses braces, no parentheses [OK]

Common Mistakes:

• Adding parentheses like other languages
• Using 'then' keyword
• Using colons or arrows incorrectly

3. What is the output of this Rust code?

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

medium

A. Compilation error

B. small

C. big

D. 10

Solution

1. Step 1: Evaluate the condition x > 5

   Since x is 10, which is greater than 5, the condition is true.

2. Step 2: Determine which branch runs

   The if expression returns "big" because the condition is true, so y is assigned "big".

3. Final Answer:

   big -> Option C

4. Quick Check:

   10 > 5 is true, so output = big [OK]

Hint: Check condition true or false to pick output [OK]

Common Mistakes:

• Confusing output with variable x
• Thinking else branch runs
• Expecting numeric output

4. Identify the error in this Rust code snippet:

let number = 7;
let result = if number % 2 == 0 {
    "even"
} else {
    "odd"
};
println!("{}", result);

medium

A. No error, code runs correctly

B. Mismatched types in if and else branches

C. Missing semicolon after if expression

D. Using % operator incorrectly

Solution

1. Step 1: Check syntax and types

   The if expression returns string slices "even" or "odd", both same type, so no type mismatch.

2. Step 2: Verify semicolons and operators

   The semicolon after the if expression is present, and the modulo operator is used correctly.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Correct syntax and types = no error [OK]

Hint: Check types and semicolons carefully [OK]

Common Mistakes:

• Thinking semicolon is missing
• Confusing types in branches
• Misusing modulo operator

5. How can you use an if expression to assign a value to a variable based on multiple conditions in Rust? Choose the correct code snippet.

hard

A. let grade = if score >= 90 -> 'A' else if score >= 80 -> 'B' else -> 'C';

B. let grade = if (score >= 90) then 'A' else if (score >= 80) then 'B' else 'C';

C. let grade = if score >= 90: 'A' elif score >= 80: 'B' else: 'C';

D. let grade = if score >= 90 { 'A' } else if score >= 80 { 'B' } else { 'C' };

Solution

1. Step 1: Understand Rust's if-else if syntax

   Rust uses if condition { value } and else if condition { value } blocks with braces, no keywords like then or arrows.

2. Step 2: Check each option for syntax correctness

   let grade = if score >= 90 { 'A' } else if score >= 80 { 'B' } else { 'C' }; uses correct Rust syntax; others use invalid keywords or punctuation.

3. Final Answer:

   let grade = if score >= 90 { 'A' } else if score >= 80 { 'B' } else { 'C' }; -> Option D

4. Quick Check:

   Rust if-else if uses braces, no then [OK]

Hint: Use braces and else if without extra keywords [OK]

Common Mistakes:

• Adding 'then' keyword
• Using colons or arrows
• Incorrect else if syntax