Result enum in Rust

Introduction

The Result enum helps your program handle success or failure clearly. It tells you if something worked or if there was an error.

When reading a file that might not exist.

When dividing numbers and you want to avoid dividing by zero.

When making a network request that could fail.

When parsing user input that might be wrong.

Syntax

Rust

enum Result<T, E> {
    Ok(T),
    Err(E),
}

T is the type for a successful value.

E is the type for an error value.

Examples

This shows a Result with an integer on success and a string on error.

Rust

let success: Result<i32, &str> = Ok(10);
let failure: Result<i32, &str> = Err("Oops");

This function returns Ok with the division result or Err with an error message.

Rust

fn divide(a: i32, b: i32) -> Result<i32, String> {
    if b == 0 {
        Err(String::from("Cannot divide by zero"))
    } else {
        Ok(a / b)
    }
}

Sample Program

This program tries to divide numbers safely. It prints the result if successful or an error message if not.

Rust

fn divide(a: i32, b: i32) -> Result<i32, String> {
    if b == 0 {
        Err(String::from("Cannot divide by zero"))
    } else {
        Ok(a / b)
    }
}

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

    let fail = divide(5, 0);
    match fail {
        Ok(value) => println!("Success: {}", value),
        Err(e) => println!("Error: {}", e),
    }
}

Important Notes

You can use match to check if a Result is Ok or Err.

Using Result helps avoid crashes by handling errors properly.

Summary

Result shows success or failure clearly.

Use Ok for success and Err for errors.

Handle Result with match to respond to each case.

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