If expression in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an if expression in Rust?

An if expression in Rust lets you run code only when a condition is true. It can also return a value, making it more than just a decision maker.

beginner

How do you write a simple if expression in Rust?

Use if condition { /* code */ }. For example:
if x > 5 { println!("x is big"); }

intermediate

Can if expressions return values in Rust?

Yes! You can assign the result of an <code>if</code> expression to a variable. For example: <br><code>let number = if condition { 5 } else { 10 };</code>

intermediate

What happens if you omit the else branch in an if expression used for assignment?

Rust will give an error because the if expression must return a value in all cases when assigned to a variable.

beginner

How do you write multiple conditions with if in Rust?

Use else if for additional conditions. For example:
if x > 5 { ... } else if x == 5 { ... } else { ... }

What does an if expression in Rust return when used in an assignment?

A The value from the executed branch

B Always a boolean

C Nothing, it cannot return a value

D The last executed statement in the program

An if expression returns the value from the branch that runs, which can be assigned to a variable.

Which of these is a correct way to write an if expression in Rust?

A if x > 5: { println!("big"); }

B if (x > 5) then { println!("big"); }

C if x > 5 print!("big");

D if x > 5 { println!("big"); }

Rust uses if condition { } syntax without 'then' or colons.

What error occurs if you assign an if expression without an else branch?

A Missing else branch error

B Mismatched types error

C No error, it's allowed

D Syntax error due to missing semicolon

Rust expects all branches to return the same type; missing else means no value for some cases, causing a type mismatch.

How do you check multiple conditions in Rust?

A Use multiple <code>if</code> statements without <code>else</code>

B Use <code>switch</code> statement

C Use <code>else if</code> after <code>if</code>

D Use <code>elif</code> keyword

Rust uses else if to chain multiple conditions.

What type must the values returned by all branches of an if expression have?

A They must all be the same type

B They can be any types

C They must be booleans

D They must be integers

All branches must return the same type so Rust knows what type the if expression returns.

Explain how an if expression works in Rust and how it can be used to assign values.

Think about how you decide between two options and store the result.

Describe what happens if you use an if expression without an else branch when assigning to a variable.

Consider what Rust expects when it needs a value from all paths.

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