2.1 — Introduction to functions

In this lesson, you'll learn the fundamentals of functions in C++, including what they are, why they're useful, and how to create and use them.

What is a function?

A function is a collection of statements that performs a specific task. Functions are one of the most important features of C++ programming because they allow you to break your code into smaller, manageable pieces.

Think of functions like recipes in a cookbook. Each recipe (function) has:

• A name (like "Chocolate Chip Cookies")
• Ingredients (inputs, called parameters)
• Instructions (the code inside the function)
• A result (output, called the return value)

Why use functions?

Functions provide several important benefits:

1. Organization: Break large problems into smaller, manageable pieces
2. Reusability: Write code once and use it many times
3. Readability: Make code easier to understand and maintain
4. Testing: Test individual pieces of functionality separately
5. Collaboration: Different people can work on different functions

Anatomy of a function

Here's the basic structure of a C++ function:

return_type function_name(parameter_list)
{
    // function body
    // statements to execute
    return value; // if return_type is not void
}

Let's break this down:

• return_type: The type of value the function returns (e.g., int, double, void)
• function_name: The name you give to identify the function
• parameter_list: Input values the function can receive (can be empty)
• function body: The code that runs when the function is called
• return statement: Sends a value back to the caller (optional for void functions)

Your first function

Here's a simple function that calculates the area of a rectangle:

#include <iostream>

double calculateRectangleArea(double length, double width)
{
    double area = length * width;
    return area;
}

int main()
{
    double length = 5.0;
    double width = 3.0;
    
    double area = calculateRectangleArea(length, width);
    
    std::cout << "The area is: " << area << std::endl;
    
    return 0;
}

Output:

The area is: 15

Let's analyze this function:

1. Function declaration: double calculateRectangleArea(double length, double width)

   • Returns a double value
   • Named calculateRectangleArea
   • Takes two double parameters: length and width

2. Function body: Calculates area = length * width

3. Return statement: Returns the calculated area

4. Function call: calculateRectangleArea(length, width) in the main function

Function calls

To use a function, you call it by writing its name followed by parentheses:

function_name(arguments);

When you call a function:

1. Program execution jumps to the function
2. The function's code executes
3. If there's a return value, it's sent back
4. Program execution continues from where the function was called

Example: Multiple function calls

#include <iostream>

int addNumbers(int a, int b)
{
    return a + b;
}

int main()
{
    int result1 = addNumbers(5, 3);
    int result2 = addNumbers(10, 20);
    int result3 = addNumbers(result1, result2);
    
    std::cout << "5 + 3 = " << result1 << std::endl;
    std::cout << "10 + 20 = " << result2 << std::endl;
    std::cout << result1 << " + " << result2 << " = " << result3 << std::endl;
    
    return 0;
}

Output:

5 + 3 = 8
10 + 20 = 30
8 + 30 = 38

Key concepts to remember

1. Functions must be defined before they are used (we'll learn about forward declarations in a later lesson)

2. Every C++ program must have a main() function - this is where program execution begins

3. Functions can call other functions, including themselves (recursion)

4. Variables created inside functions are local to that function

Best practices

• Use descriptive names: calculateArea() is better than calc() or a()
• Keep functions focused: Each function should do one specific task
• Use consistent naming: Choose a naming style and stick to it
• Add comments: Explain what complex functions do

Summary

Functions are essential building blocks of C++ programs. They help organize code, promote reusability, and make programs easier to understand and maintain. In the next lesson, you'll learn about functions that return values and how to use them effectively.

Quiz

1. What are the four main parts of a function definition?
2. What happens when you call a function?
3. Why are functions useful in programming?
4. What would happen if you tried to call a function before defining it?

Practice exercises

Try creating these simple functions:

1. A function that calculates the area of a circle given its radius
2. A function that converts Celsius to Fahrenheit
3. A function that finds the maximum of two numbers