Propagating errors with ? in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to propagate the error using the ? operator.

Rust

fn read_file() -> Result<String, std::io::Error> {
    let content = std::fs::read_to_string("file.txt")[1];
    Ok(content)
}

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

A?

B;

C!

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using ; instead of ? causes the error to be ignored and code to not compile.

Using ! is for macros, not error propagation.

The ? operator propagates the error if read_to_string fails, otherwise returns the content.

2fill in blank

medium

Complete the code to propagate the error from parsing an integer using the ? operator.

Rust

fn parse_number(s: &str) -> Result<i32, std::num::ParseIntError> {
    let num = s.parse::<i32>()[1];
    Ok(num)
}

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

A!

B.

C?

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using ; ignores the error and causes a type mismatch.

Using ! is incorrect here.

The ? operator propagates the parse error if parsing fails.

3fill in blank

hard

Fix the error in the code by correctly propagating the error with the ? operator.

Rust

fn open_and_read() -> Result<String, std::io::Error> {
    let mut file = std::fs::File::open("data.txt")[1];
    let mut buf = String::new();
    std::io::Read::read_to_string(&mut file, &mut buf)?;
    Ok(buf)
}

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

A?

B;

C!

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using ; causes a type mismatch error.

Using ! is invalid here.

The ? operator propagates the error from File::open if it fails.

4fill in blank

hard

Fill both blanks to propagate errors correctly when opening and reading a file.

Rust

fn read_file_contents() -> Result<String, std::io::Error> {
    let mut file = std::fs::File::open("input.txt")[1];
    let mut contents = String::new();
    file.read_to_string(&mut contents)[2];
    Ok(contents)
}

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

A?

B;

C!

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using ; ignores errors and causes compile errors.

Using ! is incorrect for error propagation.

Both File::open and read_to_string can fail, so both need ? to propagate errors.

5fill in blank

hard

Fill all three blanks to propagate errors correctly when opening, reading, and parsing a file.

Rust

fn read_and_parse() -> Result<i32, Box<dyn std::error::Error>> {
    let mut file = std::fs::File::open("number.txt")[1];
    let mut contents = String::new();
    file.read_to_string(&mut contents)[2];
    let number = contents.trim().parse::<i32>()[3];
    Ok(number)
}

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

A?

B;

C!

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using ; causes errors to be ignored and compilation to fail.

Using ! is not for error propagation.

All three operations can fail, so each needs ? to propagate errors properly.