Lua programming/Introduction

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Lua (not "LUA", which is incorrect although common) is a powerful, fast, lightweight and embeddable programming language. It is used by many frameworks, games and other applications. While it can be used by itself, it has been designed to be easy to embed in another application. It is implemented in ANSI C, a subset of the C programming language that is very portable, which means it can run on many systems and many devices where most other scripting languages would not be able to run. The purpose of this book is to teach Lua programming to anyone regardless of previous programming experience. The book can be used as an introduction to programming, for someone who has never programmed before, or as an introduction to Lua, for people who have programmed before but not in Lua. Since there are many development platforms and games that use Lua, this book can also be used to learn to use Lua and then to use it in that development platform.

This book aims to teach usage of the latest version of Lua. This means it will be attempted to regularly update it as new versions of Lua come out (Lua releases are infrequent enough that this should not be too difficult). Currently, the book is up-to-date for Lua 5.2, which is the latest version. If you are using Lua in an embedded environment that uses an older version of Lua in the 5.x branch (Lua 5.0 and Lua 5.1), the material is probably sufficiently relevant for you.

Lua was designed and is being maintained at the Pontifical Catholic University of Rio de Janeiro, which is located in Brazil. Its creators are Roberto Ierusalimschy, Waldemar Celes and Luiz Henrique de Figueiredo.

"Lua" (pronounced LOO-ah) means "Moon" in Portuguese. As such, it is neither an acronym nor an abbreviation, but a noun. More specifically, "Lua" is a name, the name of the Earth's moon and the name of the language. Like most names, it should be written in lower case with an initial capital, that is, "Lua". Please do not write it as "LUA", which is both ugly and confusing, because then it becomes an acronym with different meanings for different people. So, please, write "Lua" right!
—Lua authors, About Lua

Lua comes from two languages that were designed by TeCGraf (a laboratory at the Pontifical Catholic University of Rio de Janeiro): DEL and Sol. DEL means "data entry language", while Sol means "simple object language" and also means sun in Portuguese, which is why the name Lua was chosen, since it means "moon" in Portuguese. It was created for Petrobras, a Brazilian oil company, but was also used in many other projects in TeCGraf, and is now used in a multitude of projects world-wide. Lua is one of the leading languages in the field of embedded game development.

One of the main advantages of Lua is its simplicity. Some companies use it exclusively because of that advantage: they think their employees would be able to work better if they could use a programming language to perform certain tasks, but they cannot afford to give to their employees a full course on a complicated programming language. Some very simple languages like Bash or Batch here would not be powerful enough to perform these tasks, but Lua is both powerful and simple. Another of the important advantages of Lua is its capacity to be embedded, which was one of the most important characteristics of it throughout all of its development. Games like or World of Warcraft or ROBLOX need to be able to embed Lua in their application so users of the application can use it.

Programming, which is also sometimes called scripting in the case of programs that run inside embedded applications, is the process of writing computer programs. A programming language is a language used to give instructions to a computer through computer code that is contained in a computer program. A programming language consists of two things: a syntax, which is like grammar in English, and libraries, basic functions provided with the language. These libraries could be compared with vocabulary in English.

Hello, world![edit]

Lua can either be used embedded in an application or by itself. This book will not describe the process to install Lua on your computer, but you can execute code using codepad or the Lua demo. The first example of Lua code in this book will be the basic and traditional hello world program.

A "Hello world" program is a computer program that outputs "Hello, world" on a display device. Because it is typically one of the simplest programs possible in most programming languages, it is by tradition often used to illustrate to beginners the most basic syntax of a programming language, or to verify that a language or system is operating correctly.
—Wikipedia, Hello world program
print("Hello, world!")

The code above prints the text "Hello, world!" to the output, printing referring to displaying text in the output, not to printing something on paper. It does so by calling the print function with the string "Hello, world!" as an argument. This will be explained in the chapter about functions.

Note that Lua is most of the time embedded in a lower level application, which means that the print function will not always display text in an area that is visible to the user. The documentation of the programming interface of these applications will generally explain how text may be displayed to users.

Comments[edit]

A comment is a code annotation that is ignored by the programming language. Comments can be used to describe one or many lines of code, to document a program, to temporarily disable code, or for any other reason. They need to be prefixed by two hyphens to be recognized by Lua and they can be put either on their own line or at the end of another line:

print("This is normal code.")
-- This is a comment
print("This is still normal code.") -- This is a comment at the end of a line of code.

These comments are called short comments. It is also possible to create long comments, which start with a long bracket and can continue on many lines:

print("This is normal code")
--[[Line 1
Line 2
]]

Long brackets consist of two brackets in the middle of which any number of equality signs may be put. That number is called the level of the long bracket. Long brackets will continue until the next bracket of the same level, if there is one. A long bracket with no equal sign is called a long bracket of level 0. This approach makes it possible to use closing double brackets inside of long comments by adding equal signs in the middle of the two brackets. It is often useful to do this when using comments to disable blocks of code.

--[==[
This is a comment that contains a closing long bracket of level 0 which is here: ]]
However, the closing double bracket doesn't make the comment end, because the comment was opened with an opening long bracket of level 2, and only a closing long bracket of level 2 can close it.
]==]

In the example above, the closing long bracket of level 0 (]]) does not close the comment, but the closing long bracket of level 2 (]==]) does.

Syntax[edit]

The syntax of a programming language defines how statements and expressions must be written in that programming language, just like grammar defines how sentences and words must be written. Statements and expressions can be respectively compared to sentences and words. Expressions are pieces of code that have a value and that can be evaluated, while statements are pieces of code that can be executed and that contain an instruction and one or many expressions to use that instruction with. For example, 3 + 5 is an expression and variable = 3 + 5 is a statement that sets the value of variable to that expression.

The entire syntax of Lua can be found in extended Backus–Naur form on the Lua website, but you wouldn't understand anything if you read it. Extended Backus–Naur Form is a metalanguage, a language used to describe another language, just like a metawebsite is a website about a website, and just like metatables, in Lua, are tables that define the behavior of other tables (you'll learn about metatables and tables later in this book). But you're not going to have to learn extended Backus–Naur form in this book, because, while a language like Lua can be described using a metalanguage, it can also be described using words and sentences, in English, and this is exactly what this book is going to do.

Since English can be used to describe another language, then it must itself be a metalanguage (because it corresponds to the definition of a metalanguage). This is indeed the case. And since the purpose of a programming language is to describe instructions, and you can do that with English, English must also be a programming language. This, in a way, is also the case. In fact, English is a language that can be used for many things. But extended Backus–Naur form is a specialized language, and programming languages are also specialized languages. Specialization is the characteristic of being very good at doing something in particular, but not being capable of doing other things. Extended Backus–Naur form is very good at describing other languages, but it cannot be used to write instructions or to communicate a message. Programming languages are very good at giving instructions, but they cannot be used to describe languages or to communicate messages.

English is capable of doing everything: describing languages, giving instructions and communicating messages. But it is not very good at doing some of these. In fact, it is so bad at giving instructions that, if it is used to give instructions to a computer, the computer won't understand anything. That's because computers need the instructions to be very precise and unambiguous.

Obtaining Lua[edit]

Lua can be obtained on the official Lua website, on the download page. Instructions are also available there: the download button is for the source code, which is probably not what you want. You are probably looking for binaries, so you should look around the page to find information about those (what exactly you are looking for depends on the platform you are using). The purpose of this book is only to teach the Lua language, not to teach usage of the Lua tools. It is generally assumed that the reader will be using Lua in an embedded environment, but this does not need to be the case for the book to be useful, only does it mean that the book will not describe the usage of Lua as a standalone language.

Quiz[edit]

There are some questions you can answer to verify that you have understood the material in this chapter. Note that finding the answer to some of those questions could require having knowledge that is not presented in this chapter. This is normal: the quizzes are part of the learning experience, and they can introduce information that is not available elsewhere in the book.

1. What does "Lua" mean in Portuguese?


2. Which of these is a long comment of level 0?

--Comment
This is a short comment. Short comments do not have a level.
[[Comment]]
This is a long string, not a comment, because the two opening hyphens are missing.
--[[Comment]]
The level is equivalent to the number of equality symbols between the two brackets in both the opening and the closing parts. Since there are no such symbols in this case, the level is 0.
--[=[Comment]=]
This is a long comment of level 1 because there is one equality symbol between the two brackets in the opening and closing parts.
[=[Comment]=]
This is a long string, not a comment, for the same reason as the second choice.

3. What is extended Backus–Naur form?

A language
A programming language
While extended Backus–Naur form is used to describe programming languages, it is not itself a programming language.
A natural (or ordinary) language
A notation
Extended Backus–Naur form is indeed a notation. The language described using a metalanguage is described by a metasyntax, which uses the notation of the metalanguage to describe the syntax of the language described.
A metalanguage
Extended Backus–Naur form is a language that is used to describe languages that generally are programming languages. Since it is a language used to describe other languages, it is a metalanguage.
A markup language
Extended Backus–Naur form can be used to describe markup languages, although it is rarely used to do so, but it is not by itself a markup language.

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