Return values in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust function?

Consider the following Rust code. What will be printed when main runs?

Rust

fn add_one(x: i32) -> i32 {
    x + 1
}

fn main() {
    let result = add_one(5);
    println!("{}", result);
}

A 6

B 5

C Compilation error

D Runtime panic

💡 Hint

Look at what the function returns and what value is passed in.

The function add_one returns the input value plus one. Passing 5 returns 6, which is printed.

❓ Predict Output

intermediate

What does this Rust function return?

What is the value of result after calling compute()?

Rust

fn compute() -> i32 {
    let x = 10;
    let y = 20;
    x + y
}

fn main() {
    let result = compute();
    println!("{}", result);
}

A Compilation error

B 20

C 10

D 30

💡 Hint

The last expression without a semicolon is returned.

The function returns the sum of x and y, which is 30.

🔧 Debug

advanced

Why does this Rust function cause a compilation error?

Examine the code below. Why does it fail to compile?

Rust

fn faulty() -> i32 {
    let x = 5;
    return;
}

fn main() {
    let val = faulty();
    println!("{}", val);
}

A Missing return value in function with non-void return type

B Variable x is unused

C Syntax error in return statement

D Runtime panic due to missing return

💡 Hint

Check the function's declared return type and what it actually returns.

The function declares it returns i32 but returns nothing, causing a compilation error.

❓ Predict Output

advanced

What is printed by this Rust function with multiple return points?

What will be printed when main runs?

Rust

fn check_number(n: i32) -> &'static str {
    if n > 0 {
        return "Positive";
    } else if n < 0 {
        return "Negative";
    }
    "Zero"
}

fn main() {
    println!("{}", check_number(0));
}

A Compilation error

B Negative

C Zero

D Positive

💡 Hint

Look at the input value and which return path it triggers.

Input 0 triggers the last expression which returns "Zero".

🧠 Conceptual

expert

What is the type of the return value in this Rust function?

Consider this Rust function. What is the exact return type of mystery?

Rust

fn mystery(flag: bool) -> Option<i32> {
    match flag {
        true => Some(42),
        false => None,
    }
}

A i32

B Option<i32>

C bool

D Result<i32, bool>

💡 Hint

Look at the function signature and what is returned inside the match.

The function returns an Option wrapping an i32, either Some(42) or None.

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