Return values in Rust - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the sum of two numbers.

Rust

fn add(a: i32, b: i32) -> i32 {
    [1]
}

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

Aa + b

Bprintln!("{}", a + b)

Creturn;

Da - b

Attempts:

3 left

💡 Hint

Common Mistakes

Using println! instead of returning a value.

Adding a semicolon after the expression, which prevents returning the value.

The function should return the sum of a and b. Using a + b returns the correct value.

2fill in blank

medium

Complete the code to return the length of the string slice.

Rust

fn length(s: &str) -> usize {
    [1]
}

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

Areturn s.length();

Bs.len()

Cs.len();

Ds.size()

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a semicolon after s.len(), which makes the function return ().

Using incorrect method names like length() or size().

In Rust, len() returns the length of a string slice. The last expression without a semicolon is returned.

3fill in blank

hard

Fix the error in the function to correctly return the boolean result.

Rust

fn is_even(num: i32) -> bool {
    if num % 2 == 0 {
        [1]
    } else {
        false
    }
}

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

Atrue;

Breturn true;

Ctrue

Dreturn true

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a semicolon after true, which causes the function to return ().

Using return true; inside the if block but missing return in else.

In Rust, the last expression without a semicolon is returned. Using true without a semicolon returns the value correctly.

4fill in blank

hard

Fill both blanks to return a tuple with the sum and product of two numbers.

Rust

fn sum_and_product(a: i32, b: i32) -> (i32, i32) {
    let sum = a [1] b;
    let product = a [2] b;
    (sum, product)
}

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or division instead of addition or multiplication.

Mixing up the operators for sum and product.

The sum is calculated with + and the product with *.

5fill in blank

hard

Fill all three blanks to return a HashMap with words as keys and their lengths as values, only for words longer than 3 characters.

Rust

use std::collections::HashMap;

fn word_lengths(words: Vec<&str>) -> HashMap<String, usize> {
    let mut lengths = HashMap::new();
    for word in words {
        if word.[1]() > 3 {
            lengths.insert(word.[2](), word.[3]());
        }
    }
    lengths
}

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

Alen

Bto_string

Dchars

Attempts:

3 left

💡 Hint

Common Mistakes

Using chars() instead of len() for length.

Forgetting to convert &str to String for the HashMap key.

We check the length with len(), convert the word to a String with to_string(), and get the length again with len() for the value.

Practice

1. What does the return value of a Rust function represent?

easy

A. The type of variables inside the function

B. The name of the function

C. The number of lines in the function

D. The result the function sends back to the caller

Solution

1. Step 1: Understand function purpose

   A function processes data and often produces a result.

2. Step 2: Identify return value role

   The return value is the output sent back to where the function was called.

3. Final Answer:

   The result the function sends back to the caller -> Option D

4. Quick Check:

   Return value = function output [OK]

Hint: Return value is what function gives back [OK]

Common Mistakes:

• Confusing return value with function name
• Thinking return value is the number of lines
• Mixing return value with variable types

2. Which of the following is the correct way to declare a function in Rust that returns an i32 value?

easy

A. fn add() returns i32 { 5 + 3 }

B. fn add() -> i32 { 5 + 3 }

C. fn add() : i32 { 5 + 3 }

D. fn add() -> int { 5 + 3 }

Solution

1. Step 1: Check Rust return syntax

   Rust uses -> Type to specify return type after function parentheses.

2. Step 2: Validate options

   fn add() -> i32 { 5 + 3 } uses -> i32 correctly; others use invalid syntax or wrong type name.

3. Final Answer:

   fn add() -> i32 { 5 + 3 } -> Option B

4. Quick Check:

   Return type syntax = -> Type [OK]

Hint: Use -> Type after () to declare return type [OK]

Common Mistakes:

• Using colon : instead of -> for return type
• Writing 'returns' keyword incorrectly
• Using 'int' instead of 'i32' in Rust

3. What is the output of this Rust code?

fn multiply(x: i32, y: i32) -> i32 {
    x * y
}

fn main() {
    println!("{}", multiply(4, 5));
}

medium

A. 20

B. 45

C. 9

D. Compilation error

Solution

1. Step 1: Understand function multiply

   It returns the product of x and y without explicit return keyword.

2. Step 2: Calculate multiply(4, 5)

   4 * 5 = 20, so the function returns 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   4 * 5 = 20 [OK]

Hint: Last expression is returned automatically [OK]

Common Mistakes:

• Expecting explicit return keyword
• Confusing multiplication with addition
• Thinking code causes error due to missing return

4. Identify the error in this Rust function that is supposed to return an i32:

fn get_number() -> i32 {
    let x = 10;
    return;
}

medium

A. Variable x is unused

B. Function should not have a return type

C. Missing return value after return keyword

D. Function should use println! instead of return

Solution

1. Step 1: Check return statement

   The function declares return type i32 but uses return; without value.

2. Step 2: Understand Rust return rules

   Functions with return type must return a value of that type; missing value causes error.

3. Final Answer:

   Missing return value after return keyword -> Option C

4. Quick Check:

   Return type requires value [OK]

Hint: Return type needs a value after return [OK]

Common Mistakes:

• Omitting return value when return type is specified
• Ignoring unused variable warnings
• Confusing return with print statements

5. Given this Rust function, what will be the output?

fn check_value(x: i32) -> bool {
    if x > 0 {
        true
    } else {
        false
    }
}

fn main() {
    println!("{}", check_value(-3));
}

hard

A. false

B. Compilation error

C. 0

D. true

Solution

1. Step 1: Analyze function check_value

   Returns true if x > 0, else false.

2. Step 2: Evaluate check_value(-3)

   -3 is not greater than 0, so function returns false.

3. Final Answer:

   false -> Option A

4. Quick Check:

   x > 0 ? true : false = false [OK]

Hint: If condition returns last expression automatically [OK]

Common Mistakes:

• Assuming negative numbers return true
• Expecting numeric output instead of bool
• Thinking missing return causes error