初识lua

转自：http://www.oschina.net/question/12_115993
-- 两个横线是单行注释（译者注：这跟 SQL 一样）

--[[
     增加两个 [ 和 ] 变成多行注释
     我是多行注释：）
--]]

----------------------------------------------------
-- 1. 变量和程序流程控制 Variables and flow control.
----------------------------------------------------

num = 42  -- 所有的数值都是双精度的
-- 别吓一跳，64位的双精度需要52位
-- 存储，特别是 int 值，机器精度对小于 52 位的 ints 不是一个问题

s = 'walternate'  -- 字符串常量，跟 Python 差不多
t = "也可以用双引号"
u = [[ 在开始和结束位置两个中括号
       表示多行字符串]]
t = nil  -- t 赋值为空，Lua 会进行垃圾收集

-- 块使用关键字如 do/end 来表示：
while num < 50 do
  num = num + 1  -- Lua 没有 ++ 和 += 这样的操作符
end

-- If 语句:
if num > 40 then
  print('over 40')
elseif s ~= 'walternate' then  -- ~= 是不等于的意思
  -- == 是相等检查，类似 Python 和 Java; ok for strs.
  io.write('not over 40\n')  -- 默认写到标准输出
else
  -- 变量默认是全局的
  thisIsGlobal = 5  -- 常见的驼峰式大小写

  -- 如何定义局部变量
  local line = io.read()  -- 读入标准输入下一行

  -- 使用 .. 操作符进行字符串连接
  print('Winter is coming, ' .. line)
end

-- 未定义的变量返回 nil
-- 这不是一个错误
foo = anUnknownVariable  -- 现在 foo 的值为 nil

aBoolValue = false

-- 只有 nil 和 false 为假，'' 为真
if not aBoolValue then print('twas false') end

-- 'or' 和 'and' are short-circuited.
-- 这个类似 C/Java/JavaScript 里的 a?b:c 操作符
ans = aBoolValue and 'yes' or 'no'  --> 'no'

karlSum = 0
for i = 1, 100 do  -- 范围包括两端的值
  karlSum = karlSum + i
end

-- 使用 "100, 1, -1" 进行数值递减
fredSum = 0
for j = 100, 1, -1 do fredSum = fredSum + j end

-- 一般情况下，范围是 begin, end[, step].

-- 另外一种循环的方式
repeat
  print('the way of the future')
  num = num - 1
until num == 0

----------------------------------------------------
-- 2. 函数.
----------------------------------------------------

function fib(n)
  if n < 2 then return 1 end
  return fib(n - 2) + fib(n - 1)
end

-- 闭包和匿名函数
function adder(x)
  -- adder 被调用时才会创建返回函数，记住 x 的值
  return function (y) return x + y end
end
a1 = adder(9)
a2 = adder(36)
print(a1(16))  --> 25
print(a2(64))  --> 100

-- 返回, 函数调用和赋值都可以工作
-- 列表可能不匹配长度
-- 无法匹配的接收者就是 nil
-- 无法匹配的发送者被丢弃 x, y, z = 1, 2, 3, 4
-- 现在 x = 1, y = 2, z = 3, 和 4 被丢弃

function bar(a, b, c)
  print(a, b, c)
  return 4, 8, 15, 16, 23, 42
end

x, y = bar('zaphod')  --> 打印 "zaphod  nil nil"
-- 现在 x = 4, y = 8, 而 15..42 被丢弃

-- 函数可以是全局的也可以是局部的
-- 下面是相同的
function f(x) return x * x end
f = function (x) return x * x end

-- 局部函数
local function g(x) return math.sin(x) end
local g = function (x) return math.sin(x) end

-- Trig funcs work in radians, by the way.

-- 调用一个字符串参数无需括号
print 'hello'  -- Works fine.

----------------------------------------------------
-- 3. Tables.
----------------------------------------------------

-- Tables = Lua's only compound data structure;
--          they are associative arrays.
-- Similar to php arrays or js objects, they are
-- hash-lookup dicts that can also be used as lists.

-- Using tables as dictionaries / maps:

-- Dict literals have string keys by default:
t = {key1 = 'value1', key2 = false}

-- String keys can use js-like dot notation:
print(t.key1)  -- Prints 'value1'.
t.newKey = {}  -- Adds a new key/value pair.
t.key2 = nil   -- Removes key2 from the table.

-- Literal notation for any (non-nil) value as key:
u = {['@!#'] = 'qbert', [{}] = 1729, [6.28] = 'tau'}
print(u[6.28])  -- prints "tau"

-- Key matching is basically by value for numbers
-- and strings, but by identity for tables.
a = u['@!#']  -- Now a = 'qbert'.
b = u[{}]     -- We might expect 1729, but it's nil:
-- b = nil since the lookup fails. It fails
-- because the key we used is not the same object
-- as the one used to store the original value. So
-- strings & numbers are more portable keys.

-- A one-table-param function call needs no parens:
function h(x) print(x.key1) end
h{key1 = 'Sonmi~451'}  -- Prints 'Sonmi~451'.

for key, val in pairs(u) do  -- Table iteration.
  print(key, val)
end

-- _G is a special table of all globals.
print(_G['_G'] == _G)  -- Prints 'true'.

-- Using tables as lists / arrays:

-- List literals implicitly set up int keys:
v = {'value1', 'value2', 1.21, 'gigawatts'}
for i = 1, #v do  -- #v is the size of v for lists.
  print(v[i])  -- Indices start at 1 !! SO CRAZY!
end
-- A 'list' is not a real type. v is just a table
-- with consecutive integer keys, treated as a list.

----------------------------------------------------
-- 3.1 Metatables and metamethods.
----------------------------------------------------

-- A table can have a metatable that gives the table
-- operator-overloadish behavior. Later we'll see
-- how metatables support js-prototypey behavior.

f1 = {a = 1, b = 2}  -- Represents the fraction a/b.
f2 = {a = 2, b = 3}

-- This would fail:
-- s = f1 + f2

metafraction = {}
function metafraction.__add(f1, f2)
  sum = {}
  sum.b = f1.b * f2.b
  sum.a = f1.a * f2.b + f2.a * f1.b
  return sum
end

setmetatable(f1, metafraction)
setmetatable(f2, metafraction)

s = f1 + f2  -- call __add(f1, f2) on f1's metatable

-- f1, f2 have no key for their metatable, unlike
-- prototypes in js, so you must retrieve it as in
-- getmetatable(f1). The metatable is a normal table
-- with keys that Lua knows about, like __add.

-- But the next line fails since s has no metatable:
-- t = s + s
-- Class-like patterns given below would fix this.

-- An __index on a metatable overloads dot lookups:
defaultFavs = {animal = 'gru', food = 'donuts'}
myFavs = {food = 'pizza'}
setmetatable(myFavs, {__index = defaultFavs})
eatenBy = myFavs.animal  -- works! thanks, metatable

-- Direct table lookups that fail will retry using
-- the metatable's __index value, and this recurses.

-- An __index value can also be a function(tbl, key)
-- for more customized lookups.

-- Values of __index,add, .. are called metamethods.
-- Full list. Here a is a table with the metamethod.

-- __add(a, b)                     for a + b
-- __sub(a, b)                     for a - b
-- __mul(a, b)                     for a * b
-- __div(a, b)                     for a / b
-- __mod(a, b)                     for a % b
-- __pow(a, b)                     for a ^ b
-- __unm(a)                        for -a
-- __concat(a, b)                  for a .. b
-- __len(a)                        for #a
-- __eq(a, b)                      for a == b
-- __lt(a, b)                      for a < b
-- __le(a, b)                      for a <= b
-- __index(a, b)  <fn or a table>  for a.b
-- __newindex(a, b, c)             for a.b = c
-- __call(a, ...)                  for a(...)

----------------------------------------------------
-- 3.2 Class-like tables and inheritance.
----------------------------------------------------

-- Classes aren't built in; there are different ways
-- to make them using tables and metatables.

-- Explanation for this example is below it.

Dog = {}                                   -- 1.

function Dog:new()                         -- 2.
  newObj = {sound = 'woof'}                -- 3.
  self.__index = self                      -- 4.
  return setmetatable(newObj, self)        -- 5.
end

function Dog:makeSound()                   -- 6.
  print('I say ' .. self.sound)
end

mrDog = Dog:new()                          -- 7.
mrDog:makeSound()  -- 'I say woof'         -- 8.

-- 1. Dog acts like a class; it's really a table.
-- 2. function tablename:fn(...) is the same as
--    function tablename.fn(self, ...)
--    The : just adds a first arg called self.
--    Read 7 & 8 below for how self gets its value.
-- 3. newObj will be an instance of class Dog.
-- 4. self = the class being instantiated. Often
--    self = Dog, but inheritance can change it.
--    newObj gets self's functions when we set both
--    newObj's metatable and self's __index to self.
-- 5. Reminder: setmetatable returns its first arg.
-- 6. The : works as in 2, but this time we expect
--    self to be an instance instead of a class.
-- 7. Same as Dog.new(Dog), so self = Dog in new().
-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.

----------------------------------------------------

-- Inheritance example:

LoudDog = Dog:new()                           -- 1.

function LoudDog:makeSound()
  s = self.sound .. ' '                       -- 2.
  print(s .. s .. s)
end

seymour = LoudDog:new()                       -- 3.
seymour:makeSound()  -- 'woof woof woof'      -- 4.

-- 1. LoudDog gets Dog's methods and variables.
-- 2. self has a 'sound' key from new(), see 3.
-- 3. Same as LoudDog.new(LoudDog), and converted to
--    Dog.new(LoudDog) as LoudDog has no 'new' key,
--    but does have __index = Dog on its metatable.
--    Result: seymour's metatable is LoudDog, and
--    LoudDog.__index = LoudDog. So seymour.key will
--    = seymour.key, LoudDog.key, Dog.key, whichever
--    table is the first with the given key.
-- 4. The 'makeSound' key is found in LoudDog; this
--    is the same as LoudDog.makeSound(seymour).

-- If needed, a subclass's new() is like the base's:
function LoudDog:new()
  newObj = {}
  -- set up newObj
  self.__index = self
  return setmetatable(newObj, self)
end

----------------------------------------------------
-- 4. Modules.
----------------------------------------------------

--[[ I'm commenting out this section so the rest of
--   this script remains runnable.

-- Suppose the file mod.lua looks like this:
local M = {}

local function sayMyName()
  print('Hrunkner')
end

function M.sayHello()
  print('Why hello there')
  sayMyName()
end

return M

-- Another file can use mod.lua's functionality:
local mod = require('mod')  -- Run the file mod.lua.

-- require is the standard way to include modules.
-- require acts like:     (if not cached; see below)
local mod = (function ()
  <contents of mod.lua>
end)()
-- It's like mod.lua is a function body, so that
-- locals inside mod.lua are invisible outside it.

-- This works because mod here = M in mod.lua:
mod.sayHello()  -- Says hello to Hrunkner.

-- This is wrong; sayMyName only exists in mod.lua:
mod.sayMyName()  -- error

-- require's return values are cached so a file is
-- run at most once, even when require'd many times.

-- Suppose mod2.lua contains "print('Hi!')".
local a = require('mod2')  -- Prints Hi!
local b = require('mod2')  -- Doesn't print; a=b.

-- dofile is like require without caching:
dofile('mod2')  --> Hi!
dofile('mod2')  --> Hi! (runs again, unlike require)

-- loadfile loads a lua file but doesn't run it yet.
f = loadfile('mod2')  -- Calling f() runs mod2.lua.

-- loadstring is loadfile for strings.
g = loadstring('print(343)')  -- Returns a function.
g()  -- Prints out 343; nothing printed before now.

--]]

----------------------------------------------------
-- 5. References.
----------------------------------------------------

--[[

I was excited to learn Lua so I could make games
with the Löve 2D game engine. That's the why.

I started with BlackBulletIV's Lua for programmers.
Next I read the official Programming in Lua book.
That's the how.

It might be helpful to check out the Lua short
reference on lua-users.org.

The main topics not covered are standard libraries:
 * string library
 * table library
 * math library
 * io library
 * os library

By the way, this entire file is valid Lua; save it
as learn.lua and run it with "lua learn.lua" !

This was first written for tylerneylon.com, and is
also available as a github gist. Have fun with Lua!

--]]