Result enum in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Result that holds a success value of 10.

Rust

let result: Result<i32, &str> = [1](10);

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

AErr

BSome

COk

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using Err instead of Ok for success

Using Option variants like Some or None

The Ok variant of Result represents success and holds the value.

2fill in blank

medium

Complete the code to handle a Result and print the success value.

Rust

match result {
    [1](val) => println!("Success: {}", val),
    Err(e) => println!("Error: {}", e),
}

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

AOk

BNone

CSome

DErr

Attempts:

3 left

💡 Hint

Common Mistakes

Matching Err instead of Ok for success

Using Option variants in match

When matching a Result, the success case is Ok.

3fill in blank

hard

Fix the error in the code to unwrap the Result safely.

Rust

let value = result.[1](0);

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

Aunwrap

Bunwrap_or

Cexpect

Dunwrap_err

Attempts:

3 left

💡 Hint

Common Mistakes

Using unwrap which panics on error

Using unwrap_err which unwraps the error variant

unwrap_or returns the success value or a default if it's an error.

4fill in blank

hard

Fill both blanks to create a Result and handle the error case.

Rust

let result: Result<i32, &str> = [1]("Failed");
match result {
    Ok(val) => println!("Value: {}", val),
    [2](e) => println!("Error: {}", e),
}

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

AErr

BOk

DSome

Attempts:

3 left

💡 Hint

Common Mistakes

Using Ok to create an error

Matching Ok instead of Err for errors

Err creates an error Result and matches the error case.

5fill in blank

hard

Fill all three blanks to map a Result's success value and handle errors.

Rust

let result: Result<i32, &str> = Ok(5);
let new_result = result.[1](|x| x [2] 2);
match new_result {
    Ok(val) => println!("Doubled: {}", val),
    [3](e) => println!("Error: {}", e),
}

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

Amap

B*

CErr

DOk

Attempts:

3 left

💡 Hint

Common Mistakes

Using map_err instead of map

Using + instead of * for multiplication

Matching Ok instead of Err for errors

map applies a function to the success value, * multiplies, and Err matches errors.

Practice

1. What does the Result enum in Rust represent?

easy

A. A type for storing multiple values in a list

B. A way to loop over numbers

C. A value that can be either success (Ok) or failure (Err)

D. A function that always returns Ok

Solution

1. Step 1: Understand the purpose of Result

   Result is used to show if an operation succeeded or failed.

2. Step 2: Identify the two variants

   Ok means success, Err means failure or error.

3. Final Answer:

   A value that can be either success (Ok) or failure (Err) -> Option C

4. Quick Check:

   Result = success or failure [OK]

Hint: Remember: Result = Ok (success) or Err (failure) [OK]

Common Mistakes:

• Thinking Result is a list or collection
• Confusing Result with loops or arrays
• Assuming Result always means success

2. Which of the following is the correct way to create a Result with a success value of 10?

easy

A. let x: Result = Result::new(10);

B. let x: Result = Err(10);

C. let x: Result = Ok("10");

D. let x: Result = Ok(10);

Solution

1. Step 1: Check the correct variant for success

   Success uses Ok, so Ok(10) is correct.

2. Step 2: Verify the type matches

   The type is Result, so Ok(10) matches i32 for success.

3. Final Answer:

   let x: Result<i32, &str> = Ok(10); -> Option D

4. Quick Check:

   Success = Ok(value) [OK]

Hint: Use Ok(value) for success, Err(value) for error [OK]

Common Mistakes:

• Using Err for success value
• Mismatching types inside Ok or Err
• Trying to use Result::Ok instead of Ok directly

3. What will be printed by this Rust code?

fn divide(a: i32, b: i32) -> Result {
    if b == 0 {
        Err(String::from("Divide by zero"))
    } else {
        Ok(a / b)
    }
}

fn main() {
    match divide(10, 2) {
        Ok(result) => println!("Result: {}", result),
        Err(e) => println!("Error: {}", e),
    }
}

medium

A. Result: 0

B. Result: 5

C. Error: Divide by zero

D. Compile error

Solution

1. Step 1: Check the divide function call

   Calling divide(10, 2) means b is 2, not zero, so it returns Ok(10 / 2) which is Ok(5).

2. Step 2: Match on the Result

   The match sees Ok(5), so it prints Result: 5.

3. Final Answer:

   Result: 5 -> Option B

4. Quick Check:

   Ok(5) prints Result: 5 [OK]

Hint: Match Ok prints result, Err prints error message [OK]

Common Mistakes:

• Assuming divide by 2 returns error
• Confusing Ok and Err branches
• Expecting compile error due to String

4. Identify the error in this Rust code using Result:

fn get_number(flag: bool) -> Result {
    if flag {
        Ok(42)
    } else {
        Err(42)
    }
}

medium

A. The Err variant has wrong type; should be a String

B. The Ok variant should return a String

C. Missing semicolon after Ok(42)

D. Function must return Option not Result

Solution

1. Step 1: Check the function return type

   The function returns Result<i32, String>, so Ok must hold i32 and Err must hold String.

2. Step 2: Identify the error in Err(42)

   42 is an integer, but Err expects a String. This causes a type mismatch.

3. Final Answer:

   The Err variant has wrong type; should be a String -> Option A

4. Quick Check:

   Err type must match declared error type [OK]

Hint: Err must match error type declared in Result [OK]

Common Mistakes:

• Using wrong type inside Err
• Thinking Ok needs String
• Ignoring type mismatch errors

5. Given a list of strings representing numbers, which Rust code correctly converts them to integers and collects only successful parses using Result handling?

let strings = vec!["10", "abc", "20", "xyz"];
// Fill in the blank:
let numbers: Vec<i32> = strings.iter()
    .map(|s| s.parse::<i32>())
    .__________;
println!("{:?}", numbers);

hard

A. filter_map(Result::ok).collect()

B. filter(|r| r.is_ok()).collect()

C. map(Result::unwrap).collect()

D. collect::()

Solution

1. Step 1: Understand parse returns Result

   Each s.parse::<i32>() returns Result<i32, _>, so we get an iterator of Results.

2. Step 2: Use filter_map(Result::ok) to keep only successes

   filter_map(Result::ok) extracts the Ok values and skips errors, then collect() gathers them into a vector.

3. Final Answer:

   filter_map(Result::ok).collect() -> Option A

4. Quick Check:

   filter_map(Result::ok) keeps only successful parses [OK]

Hint: Use filter_map(Result::ok) to keep only Ok values [OK]

Common Mistakes:

• Using filter without extracting Ok values
• Calling unwrap on all results causing panic
• Trying to collect without filtering errors