Option enum usage in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an Option with a value.

Rust

let some_number = Some([1]);

Drag options to blanks, or click blank then click option'

A5

B"five"

CNone

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using None instead of Some(value).

Putting a string when a number is expected.

The Some variant holds a value. Here, 5 is the value wrapped in Some.

2fill in blank

medium

Complete the code to extract the value from an Option using match.

Rust

match maybe_value {
    Some(val) => println!("Value is: {}", [1]),
    None => println!("No value"),
}

Drag options to blanks, or click blank then click option'

Aval

Bmaybe_value

C"val"

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using the Option variable name instead of the bound value.

Using None inside Some arm.

Inside the Some(val) arm, val holds the extracted value.

3fill in blank

hard

Fix the error in unwrapping an Option safely.

Rust

if let [1] = optional_value {
    println!("Got a value");
} else {
    println!("No value");
}

Drag options to blanks, or click blank then click option'

Aoptional_value

BSome(value)

Cvalue

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using None instead of Some(value).

Using the Option variable name directly.

The if let syntax matches Some(value) to unwrap the Option safely.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters Some values and unwraps them.

Rust

let numbers = vec![Some(1), None, Some(3)];
let unwrapped: Vec<_> = numbers.iter().filter_map(|&[1]| [2]).collect();

Drag options to blanks, or click blank then click option'

Aopt

Copt.unwrap()

Attempts:

3 left

💡 Hint

Common Mistakes

Using None as variable name.

Not unwrapping the Option inside filter_map.

Use opt as the variable name and call unwrap() inside filter_map to get the values.

5fill in blank

hard

Fill all three blanks to create a function that returns the square of an Option<i32> if it has a value.

Rust

fn square_option(num: Option<i32>) -> Option<i32> {
    match num {
        [1] => Some([2] * [3]),
        None => None,
    }
}

Drag options to blanks, or click blank then click option'

ASome(x)

Bx

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using None in place of Some(x).

Using different variable names inconsistently.

The function matches Some(x) and returns Some(x * x). For None, it returns None.

Practice

1. What does the Option enum represent in Rust?

easy

A. A way to handle errors explicitly.

B. A value that can be either something (Some) or nothing (None).

C. A type of loop in Rust.

D. A macro for printing output.

Solution

1. Step 1: Understand the purpose of Option enum

   The Option enum is used to represent a value that might be present or absent.

2. Step 2: Match the description to the correct option

   A value that can be either something (Some) or nothing (None). correctly describes Option as either Some(value) or None, representing presence or absence.

3. Final Answer:

   A value that can be either something (Some) or nothing (None). -> Option B

4. Quick Check:

   Option enum = Some or None [OK]

Hint: Option means value or no value, like a yes/no box [OK]

Common Mistakes:

• Confusing Option with error handling Result type
• Thinking Option is a loop or macro
• Assuming Option always contains a value

2. Which of the following is the correct way to create an Option containing the integer 5?

easy

A. let x = Some(5);

B. let x = Option(5);

C. let x = Some[5];

D. let x = Option::new(5);

Solution

1. Step 1: Recall the syntax for creating an Option with a value

   In Rust, Some(value) wraps a value inside an Option.

2. Step 2: Identify the correct syntax among options

   let x = Some(5); uses Some(5), which is the correct syntax. Others are invalid or do not exist.

3. Final Answer:

   let x = Some(5); -> Option A

4. Quick Check:

   Use Some(value) to create Option with value [OK]

Hint: Use Some(value) to wrap a value in Option [OK]

Common Mistakes:

• Using Option(5) instead of Some(5)
• Using square brackets instead of parentheses
• Trying to call Option::new which doesn't exist

3. What will be the output of this Rust code?

let x: Option<i32> = Some(10);
match x {
    Some(val) if val > 5 => println!("Greater than 5: {}", val),
    Some(val) => println!("Value: {}", val),
    None => println!("No value"),
}

medium

A. Value: 10

B. No value

C. Greater than 5: 10

D. Compilation error

Solution

1. Step 1: Analyze the Option value and match arms

   The variable x is Some(10). The match checks if val > 5 first.

2. Step 2: Determine which match arm runs

   Since 10 > 5, the first arm runs, printing "Greater than 5: 10".

3. Final Answer:

   Greater than 5: 10 -> Option C

4. Quick Check:

   Some(10) with val > 5 prints "Greater than 5: 10" [OK]

Hint: Match with if guards runs first matching condition [OK]

Common Mistakes:

• Ignoring the if guard and picking second arm
• Thinking None arm runs for Some value
• Assuming compilation error due to if guard

4. Identify the error in this Rust code snippet using Option:

let x: Option<i32> = None;
let y = x.unwrap();
println!("Value: {}", y);

medium

A. No error, prints "Value: None"

B. unwrap returns Option, not value

C. Compilation error due to type mismatch

D. Runtime panic because unwrap on None

Solution

1. Step 1: Understand unwrap behavior on None

   Calling unwrap() on None causes a runtime panic because there is no value.

2. Step 2: Check the code behavior

   Since x is None, unwrap() panics and program crashes at runtime.

3. Final Answer:

   Runtime panic because unwrap on None -> Option D

4. Quick Check:

   unwrap on None panics at runtime [OK]

Hint: unwrap on None causes panic, avoid without check [OK]

Common Mistakes:

• Thinking unwrap returns None safely
• Expecting compilation error instead of runtime panic
• Assuming unwrap returns Option instead of value

5. Given a vector of integers, how can you use Option to safely find the first even number and print it, or print "No even number" if none exist? Choose the correct Rust code snippet.

hard

A. let nums = vec![1, 3, 5];
if let Some(even) = nums.iter().find(|&&x| x % 2 == 0) {
println!("First even: {}", even);
} else {
println!("No even number");
}

B. let nums = vec![1, 3, 5];
let even = nums.iter().find(|x| x % 2 == 0).unwrap();
println!("First even: {}", even);

C. let nums = vec![1, 3, 5];
let even = nums.iter().filter(|x| x % 2 == 0).next().unwrap();
println!("First even: {}", even);

D. let nums = vec![1, 3, 5];
let even = nums.iter().find(|&&x| x % 2 == 0).unwrap_or(0);
println!("First even: {}", even);

Solution

1. Step 1: Understand safe Option handling with find()

   nums.iter().find returns Option<&i32>. Using if let Some(even) safely handles Some or None.

2. Step 2: Check each option for correctness and safety

   let nums = vec![1, 3, 5]; if let Some(even) = nums.iter().find(|&&x| x % 2 == 0) { println!("First even: {}", even); } else { println!("No even number"); } uses if let to print the even number or fallback message safely. Others use unwrap() which panics if no even number.

3. Final Answer:

   Option A code safely finds and prints first even or fallback message. -> Option A

4. Quick Check:

   Use if let Some(...) to safely handle Option from find() [OK]

Hint: Use if let Some(...) to safely unwrap Option without panic [OK]

Common Mistakes:

• Using unwrap() without checking for None causes panic
• Incorrect closure syntax in find/filter
• Using unwrap_or with incompatible types