Function parameters in Rust - Mini Project: Build & Apply

Function parameters

📖 Scenario: You are creating a simple calculator program that adds two numbers. You will learn how to use function parameters to pass values into a function.

🎯 Goal: Build a Rust program with a function called add that takes two numbers as parameters and returns their sum. Then call this function and print the result.

📋 What You'll Learn

Create a function named add with two parameters named a and b of type i32.

The function add should return the sum of a and b as i32.

Call the function add with the arguments 5 and 7.

Print the result of the function call.

💡 Why This Matters

🌍 Real World

Functions with parameters are used everywhere in programming to reuse code and perform tasks with different inputs, like calculators, games, and web apps.

💼 Career

Understanding how to define and use function parameters is essential for writing clean, reusable code in any software development job.

1

Create the add function with parameters

Write a function named add that takes two parameters: a and b, both of type i32. The function should return an i32. Do not write the function body yet.

Rust

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

Hint

Define the function header with parameters and return type. Use a + b as the return expression.

2

Call the add function with arguments

Create a variable named result and assign it the value returned by calling the function add with arguments 5 and 7.

Rust

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

// Your code here

Hint

Use let result = add(5, 7); to call the function and store the result.

3

Print the result

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

Rust

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

let result = add(5, 7);

// Your code here

Hint

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

4

Complete program with main function

Wrap all the code inside a fn main() function so the program runs correctly.

Rust

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

fn main() {
    // Your code here
}

Hint

Put the function call and print statement inside fn main() { ... }.

Practice

1. What is the purpose of function parameters in Rust?

easy

A. To send data into functions so they can use it

B. To define the return type of a function

C. To declare variables inside a function

D. To create loops inside functions

Solution

1. Step 1: Understand what parameters do

   Parameters allow functions to receive input data when called.

2. Step 2: Identify the correct purpose

   Sending data into functions makes them flexible and reusable.

3. Final Answer:

   To send data into functions so they can use it -> Option A

4. Quick Check:

   Function parameters = input data [OK]

Hint: Parameters pass data into functions for flexibility [OK]

Common Mistakes:

• Confusing parameters with return types
• Thinking parameters declare variables inside functions
• Mixing parameters with control flow like loops

2. Which of the following is the correct way to declare a function with one parameter named num of type i32 in Rust?

easy

A. fn example(num) -> i32 {}

B. fn example(i32 num) {}

C. fn example(num: i32) {}

D. fn example(num: int) {}

Solution

1. Step 1: Recall Rust function parameter syntax

   Parameters are declared as name: type, e.g., num: i32.

2. Step 2: Check each option

   fn example(num: i32) {} matches correct syntax. fn example(i32 num) {} reverses order. fn example(num) -> i32 {} misses type. fn example(num: int) {} uses wrong type name.

3. Final Answer:

   fn example(num: i32) {} -> Option C

4. Quick Check:

   Parameter syntax = name: type [OK]

Hint: Use name: type for parameters in Rust functions [OK]

Common Mistakes:

• Swapping type and name order
• Omitting the parameter type
• Using incorrect type names like 'int'

3. What will be the output of this Rust code?

fn greet(name: &str) {
    println!("Hello, {}!", name);
}

fn main() {
    greet("Alice");
}

medium

A. Hello, Alice!

B. Hello, name!

C. Hello, &str!

D. Compilation error

Solution

1. Step 1: Understand the function call and parameter

   The function greet takes a string slice &str and prints a greeting with the given name.

2. Step 2: Trace the function call in main

   Calling greet("Alice") passes "Alice" as the parameter, so it prints "Hello, Alice!".

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   Function prints greeting with passed name [OK]

Hint: Parameters replace placeholders in print statements [OK]

Common Mistakes:

• Printing the parameter name instead of its value
• Confusing &str with variable name
• Expecting a compilation error due to &str

4. Identify the error in this Rust function declaration:

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

medium

A. Parameters should be declared as 'i32 a' and 'i32 b'

B. Missing return type declaration

C. Function body missing braces

D. Missing colon between parameter name and type for 'a'

Solution

1. Step 1: Check parameter syntax

   Each parameter must have a colon between name and type, e.g., a: i32.

2. Step 2: Identify the error

   Parameter a i32 is missing the colon, causing a syntax error.

3. Final Answer:

   Missing colon between parameter name and type for 'a' -> Option D

4. Quick Check:

   Parameter syntax requires colon [OK]

Hint: Always put colon between parameter name and type [OK]

Common Mistakes:

• Omitting colon between name and type
• Confusing parameter order syntax
• Forgetting return type after ->

5. Given this function:

fn calculate_area(width: f64, height: f64) -> f64 {
    width * height
}

Which call correctly uses the function parameters to calculate the area of a 5.0 by 3.0 rectangle?

hard

A. calculate_area(5, 3)

B. calculate_area(5.0, 3.0)

C. calculate_area("5", "3")

D. calculate_area(width=5.0, height=3.0)

Solution

1. Step 1: Check parameter types

   The function expects two f64 (floating-point) parameters.

2. Step 2: Evaluate each call

   calculate_area(5, 3) uses integer literals, which Rust automatically coerces to f64 (implicit conversion). calculate_area(5.0, 3.0) uses floating-point literals matching the f64 parameter type exactly. calculate_area("5", "3") uses strings, type mismatch. calculate_area(width=5.0, height=3.0) uses named arguments, which Rust does not support.

3. Final Answer:

   calculate_area(5.0, 3.0) -> Option B

4. Quick Check:

   Use correct types and order for parameters [OK]

Hint: Match argument types exactly to parameter types [OK]

Common Mistakes:

• Using integers instead of floats without suffix
• Passing strings instead of numbers
• Trying named arguments (not supported in Rust)