What are Lua's standard libraries? Choose one standard library and explain its purpose and functionality in Lua programming.

Lua provides a set of standard libraries that offer a wide range of functionality for various tasks, including string manipulation, mathematical operations, file I/O, and more. These libraries are built into the Lua language and are available for use without any additional installation. One of the standard libraries in Lua is the "io" library, which handles input/output operations.

The "io" library in Lua provides functions for performing input and output operations, such as reading from and writing to files, manipulating file handles, and managing the standard input/output streams. This library is essential for file handling and is commonly used in Lua programs to interact with external files and perform I/O operations.

The "io" library offers several key functions and concepts:

1. Opening and Closing Files:

* `io.open(filename, mode)`: Opens a file with the specified filename and mode. It returns a file handle that can be used for subsequent operations on the file.
* `file:close()`: Closes the file associated with the file handle, releasing any system resources used by the file.
2. Reading from Files:

* `file:read(...)`: Reads data from the file. It can read a specified number of characters, a line, or the entire file, depending on the arguments passed.
* `file:lines()`: Returns an iterator function that can be used in a loop to read the file line by line.
3. Writing to Files:

* `file:write(...)`: Writes the provided arguments to the file. Multiple arguments can be passed, and they are concatenated and written to the file.
4. File Positioning:

* `file:seek([whence [, offset]])`: Sets or retrieves the current position in the file. It allows moving the file pointer to a specific location within the file.

These functions, along with other utility functions available in the "io" library, enable Lua programmers to interact with files, read and write data, and manage file operations efficiently. The "io" library plays a vital role in scenarios where data needs to be stored, retrieved, or manipulated from external files.

Here's an example that demonstrates the usage of the "io" library to read from and write to a file:

```
lua`-- Open a file in read mode
local file = io.open("data.txt", "r")

-- Read the entire content of the file
local content = file:read("a")

-- Close the file
file:close()

-- Print the content
print(content)

-- Open a file in write mode
file = io.open("output.txt", "w")

-- Write data to the file
file:write("Hello, Lua!")

-- Close the file
file:close()`
```
In this example, we first open a file named "data.txt" in read mode using `io.open()`. We then use the `read()` function with the `*a` argument to read the entire content of the file into the `content` variable. After reading, we close the file using the `close()` method.

Next, we open a file named "output.txt" in write mode and write the string "Hello, Lua!" to the file using the `write()` method. Finally, we close the file.

The "io" library simplifies file I/O operations in Lua, allowing programmers to read data from files, write data to files, and manipulate files in a straightforward and efficient manner.