lua入门指南：Programming in Lua 4th Edition

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《Programming in Lua, Fourth Edition》是一本由Lua语言的创始人Roberto Ierusalimschy编著的权威指南，专为Lua编程初学者设计，适合想要深入理解并掌握这种轻量级脚本语言的人士。本书于2016年进行了更新，最后一次修订是在2017年3月18日，对应版本build 21。该书的版权属于Roberto Ierusalimschy，并由Feisty Duck Digital进行Book Distribution，可以在其官方网站www.feistyduck.com上获取授权。这本书分为多个章节，旨在逐步引导读者从基础概念到高级特性的学习。首先，"Getting Started"章节介绍了如何开始使用Lua，包括基本的代码块（chunks）和一些语法规则，如全局变量的使用。章节中还涵盖了Lua的基本数据类型，如nil、布尔值，以及如何使用Lua的内置解释器来运行代码。在"Interlude: The Eight-Queen Puzzle"这一插曲中，作者通过解决经典的八皇后问题，展示了一种实际应用编程逻辑的方式，帮助读者更好地理解程序设计的逻辑思维。接着，"Numbers"章节详细讨论了数值处理，包括数值表示法、算术和关系运算符，以及Lua的数学库，如随机数生成和数值转换功能。 "Strings"章节探讨字符串处理，包括字符串的两种表示形式（字面字符串和长字符串）、类型转换和字符串库的使用，以及对Unicode的支持。对于处理复杂文本数据的开发者来说，这部分内容至关重要。 "Tables"是Lua的核心特性之一，它既是数据结构又是函数调用的机制。章节介绍了表的索引、构造方法、数组、列表和序列的使用，以及如何遍历和安全访问表中的元素。此外，还有关于Lua表库的详细介绍。 "Functions"章节深入讨论了函数的定义、多返回值以及如何组织复杂的逻辑。这对于理解面向对象编程和函数式编程在Lua中的实现至关重要。《Programming in Lua, Fourth Edition》是一本实用且全面的教程，涵盖了Lua语言的基础和进阶知识，对于希望精通此语言的程序员来说，无论是初学者还是有一定经验的开发者，都是不可或缺的学习资料。通过阅读和实践书中的例子，读者能够掌握Lua的核心概念，提高编程技能。

Table of Contents

1. Getting Started ................................................................................................................ 4

Chunks ...................................................................................................................... 4

Some Lexical Conventions ........................................................................................... 6

Global Variables ......................................................................................................... 7

Types and Values ........................................................................................................ 7

Nil .................................................................................................................... 8

Booleans ............................................................................................................ 8

The Stand-Alone Interpreter .......................................................................................... 9

2. Interlude: The Eight-Queen Puzzle .................................................................................... 12

3. Numbers ....................................................................................................................... 15

Numerals .................................................................................................................. 15

Arithmetic Operators .................................................................................................. 16

Relational Operators .................................................................................................. 17

The Mathematical Library ........................................................................................... 18

Random-number generator .................................................................................. 18

Rounding functions ............................................................................................ 18

Representation Limits ................................................................................................. 19

Conversions .............................................................................................................. 21

Precedence ............................................................................................................... 22

Lua Before Integers ................................................................................................... 22

4. Strings ......................................................................................................................... 24

Literal strings ............................................................................................................ 24

Long strings ............................................................................................................. 25

Coercions ................................................................................................................. 26

The String Library ..................................................................................................... 27

Unicode ................................................................................................................... 29

5. Tables .......................................................................................................................... 33

Table Indices ............................................................................................................ 33

Table Constructors ..................................................................................................... 35

Arrays, Lists, and Sequences ....................................................................................... 36

Table Traversal ......................................................................................................... 38

Safe Navigation ......................................................................................................... 38

The Table Library ..................................................................................................... 39

6. Functions ...................................................................................................................... 42

Multiple Results ........................................................................................................ 43

Variadic Functions ..................................................................................................... 45

The function table.unpack .................................................................................... 47

Proper Tail Calls ....................................................................................................... 48

7. The External World ........................................................................................................ 50

The Simple I/O Model ............................................................................................... 50

The Complete I/O Model ............................................................................................ 53

Other Operations on Files ........................................................................................... 54

Other System Calls .................................................................................................... 55

Running system commands ................................................................................. 55

8. Filling some Gaps .......................................................................................................... 57

Local Variables and Blocks ......................................................................................... 57

Control Structures ...................................................................................................... 58

if then else ....................................................................................................... 58

while ............................................................................................................... 59

repeat ............................................................................................................. 59

Numerical for ................................................................................................... 60

Chapter 1. Getting Started

To keep with tradition, our first program in Lua just prints "Hello World":

print("Hello World")

If you are using the stand-alone Lua interpreter, all you have to do to run your first program is to call the

interpreter —usually named lua or lua5.3— with the name of the text file that contains your program.

If you save the above program in a file hello.lua, the following command should run it:

% lua hello.lua

As a more complex example, the next program defines a function to compute the factorial of a given

number, asks the user for a number, and prints its factorial:

-- defines a factorial function

function fact (n)

if n == 0 then

return 1

else

return n * fact(n - 1)

end

print("enter a number:")

a = io.read("*n") -- reads a number

print(fact(a))

Chunks

We call each piece of code that Lua executes, such as a file or a single line in interactive mode, a chunk.

A chunk is simply a sequence of commands (or statements).

A chunk can be as simple as a single statement, such as in the “Hello World” example, or it can be composed

of a mix of statements and function definitions (which are actually assignments, as we will see later), such

as the factorial example. A chunk can be as large as we wish. Because Lua is used also as a data-description

language, chunks with several megabytes are not uncommon. The Lua interpreter has no problems at all

with large chunks.

Instead of writing your program to a file, you can run the stand-alone interpreter in interactive mode. If

you call lua without any arguments, you will get its prompt:

% lua

Thereafter, each command that you type (such as print "Hello World") executes immediately after

you enter it. To exit the interactive mode and the interpreter, just type the end-of-file control character

(ctrl-D in POSIX, ctrl-Z in Windows), or call the function os.exit, from the Operating System

library —you have to type os.exit().

Starting in version 5.3, we can enter expressions directly in the interactive mode, and Lua will print their

values:

Getting Started

% lua

> math.pi / 4 --> 0.78539816339745

> a = 15

> a^2 --> 225

> a + 2 --> 17

In older versions, we need to precede these expressions with an equals sign:

% lua5.2

> a = 15

> = a^2 --> 225

For compatibility, Lua 5.3 still accepts these equals signs.

To run that code as a chunk (not in interactive mode), we must enclose the expressions inside calls to

print:

print(math.pi / 4)

a = 15

print(a^2)

print(a + 2)

Lua usually interprets each line that we type in interactive mode as a complete chunk or expression. How-

ever, if it detects that the line is not complete, it waits for more input, until it has a complete chunk. This

way, we can enter a multi-line definition, such as the factorial function, directly in interactive mode. How-

ever, it is usually more convenient to put such definitions in a file and then call Lua to run the file.

We can use the -i option to instruct Lua to start an interactive session after running a given chunk:

% lua -i prog

A command line like this one will run the chunk in the file prog and then prompt for interaction. This is

especially useful for debugging and manual testing. At the end of this chapter, we will see other options

for the stand-alone interpreter.

Another way to run chunks is with the function dofile, which immediately executes a file. For instance,

suppose we have a file lib1.lua with the following code:

function norm (x, y)

return math.sqrt(x^2 + y^2)

end

function twice (x)

return 2.0 * x

end

Then, in interactive mode, we can type this code:

> dofile("lib1.lua") -- load our library

> n = norm(3.4, 1.0)

> twice(n) --> 7.0880180586677

The function dofile is useful also when we are testing a piece of code. We can work with two windows:

one is a text editor with our program (in a file prog.lua, say) and the other is a console running Lua

Getting Started

in interactive mode. After saving a modification in our program, we execute dofile("prog.lua")

in the Lua console to load the new code; then we can exercise the new code, calling its functions and

printing the results.

Some Lexical Conventions

Identifiers (or names) in Lua can be any string of letters, digits, and underscores, not beginning with a

digit; for instance

i j i10 _ij

aSomewhatLongName _INPUT

You should avoid identifiers starting with an underscore followed by one or more upper-case letters (e.g.,

_VERSION); they are reserved for special uses in Lua. Usually, I reserve the identifier _ (a single under-

score) for dummy variables.

The following words are reserved; we cannot use them as identifiers:

and break do else elseif

end false for function goto

if in local nil not

or repeat return then true

until while

Lua is case-sensitive: and is a reserved word, but And and AND are two different identifiers.

A comment starts anywhere with two consecutive hyphens (--) and runs until the end of the line. Lua

also offers long comments, which start with two hyphens followed by two opening square brackets and

run until the first occurrence of two consecutive closing square brackets, like here:

--[[A multi-line

long comment

]]

A common trick that we use to comment out a piece of code is to enclose the code between --[[ and

--]], like here:

--[[

print(10) -- no action (commented out)

--]]

To reactivate the code, we add a single hyphen to the first line:

---[[

print(10) --> 10

--]]

In the first example, the --[[ in the first line starts a long comment, and the two hyphens in the last line

are still inside that comment. In the second example, the sequence ---[[ starts an ordinary, single-line

comment, so that the first and the last lines become independent comments. In this case, the print is

outside comments.

Lua needs no separator between consecutive statements, but we can use a semicolon if we wish. Line

breaks play no role in Lua's syntax; for instance, the following four chunks are all valid and equivalent:

Long comments can be more complex than that, as we will see in the section called “Long strings”.

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