Lua编程语言第四版精华解读

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"Programming in Lua, 4th Edition" 本书是关于编程语言Lua的第四版，由Roberto Ierusalimschy编写，版权归属2016年及2003年的作者。Feisty Duck Digital负责数字图书的发行，该书授权给特定用户Eric Taylor使用。书中的内容涵盖了Lua语言的基础知识，包括以下几个部分： 1. 开始编程：介绍了如何开始使用Lua，涉及块（Chunks）的概念，以及一些词法约定，如全局变量。还讨论了Lua中的不同类型和值，如nil和布尔类型。此外，书中提到了独立的解释器，它是学习和使用Lua的重要工具。 2. 插曲：八皇后问题：通过解决这个经典的编程问题，展示了Lua的逻辑和控制结构，帮助读者在实践中理解语言的运用。 3. 数字：详述了Lua中的数字表示，包括数值、算术运算符和关系运算符。还介绍了数学库，如随机数生成器和舍入函数，以及数字表示的限制和转换规则。特别指出，Lua在早期版本中没有整数类型。 4. 字符串：讲解了字符串的字面量表示、长字符串以及字符串与其它类型之间的转换。还介绍了字符串库，用于处理和操作字符串。此外，还简要讨论了Unicode支持。 5. 表：这是Lua的核心特性，表可以作为数组、列表或序列来使用。书中介绍了表的索引、构造器和遍历方式，以及安全导航的方法。同时，列出了表库的一些功能。 6. 函数：详细讨论了Lua中的函数，包括多返回值、局部变量和闭包。函数也可以作为第一类值，这意味着它们可以被赋值给变量、作为参数传递和作为返回值。 7. 更深入的话题：接下来的章节可能会涵盖元表、模块、错误处理、垃圾收集、协程以及C语言接口等高级主题，这些内容有助于开发者更全面地理解和利用Lua的强大功能。这本书对于初学者和有经验的程序员来说都是一个宝贵的资源，它深入浅出地讲解了Lua语言，并提供了大量的实例来帮助读者理解和掌握这个灵活且高效的脚本语言。

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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