Return values in Rust - Mini Project: Build & Apply

Return values

📖 Scenario: You are creating a simple calculator function in Rust that adds two numbers and returns the result.

🎯 Goal: Build a Rust function that takes two numbers, adds them, and returns the sum. Then print the returned value.

📋 What You'll Learn

Create a function named add that takes two i32 parameters

The function add must return an i32 value

Call the add function with the numbers 5 and 7

Print the returned result from the add function

💡 Why This Matters

🌍 Real World

Functions that return values are used everywhere in programming to get results from calculations or data processing.

💼 Career

Understanding how to write and use functions with return values is essential for software development, enabling modular and reusable code.

1

Create the add function signature

Write a function named add that takes two parameters: a and b, both of type i32. The function should return an i32 value.

Rust

fn add(a: i32, b: i32) -> i32 {
    // Your code here
}

Hint

Start by writing the function header with the correct parameter types and return type.

2

Add the two parameters inside the add function

Inside the add function, return the sum of a and b without using the return keyword.

Rust

fn add(a: i32, b: i32) -> i32 {
    // Your code here
}

Hint

In Rust, the last expression without a semicolon is returned automatically.

3

Call the add function with 5 and 7

Create a variable named result and assign it the value returned by calling add(5, 7).

Rust

fn add(a: i32, b: i32) -> i32 {
    a + b
}

// Your code here

Hint

Use let to create the variable and call the function with the exact numbers.

4

Print the result variable

Write a println! macro to print the value of the variable result.

Rust

fn add(a: i32, b: i32) -> i32 {
    a + b
}

fn main() {
    let result = add(5, 7);
    // Your code here
}

Hint

Use println!("{}", result); to display the sum.

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