Modular Functions in Programming

# Introduction

In the previous programs, we only used one `main` function. However, as the program becomes more complex, we can split it into several modular functions. By abstracting the operations with the same functionality into a function, we can simply call the function when needed. It also improves collaboration among multiple people and enhances code readability.

Now, let's step into the world of functions together.

**Knowledge Points:**

- What is a function
- Function declaration
- Pass by value
- Pass by reference
- Multiple return values
- Variadic parameters