15分钟入门lua

原文地址：http://tylerneylon.com/a/learn-lua/

先粘下来，英文不难，回头再看，怕以后找不到或者想不起来了，囧

Learn Lua in 15 Minutes
more or less
– Two dashes start a one-line comment.

–[[

 Adding two ['s and ]'s makes it a
 multi-line comment.

–]]

– 1. Variables and flow control.

num = 42 – All numbers are doubles.
– Don't freak out, 64-bit doubles have 52 bits for
– storing exact int values; machine precision is
– not a problem for ints that need < 52 bits.

s = 'walternate' – Immutable strings like Python.
t = “double-quotes are also fine”
u = [[ Double brackets

   start and end
   multi-line strings.]]

t = nil – Undefines t; Lua has garbage collection.

– Blocks are denoted with keywords like do/end:
while num < 50 do
num = num + 1 – No ++ or += type operators.
end

– If clauses:
if num > 40 then
print('over 40')
elseif s ~= 'walternate' then – ~= is not equals.
– Equality check is == like Python; ok for strs.
io.write('not over 40\n') – Defaults to stdout.
else
– Variables are global by default.
thisIsGlobal = 5 – Camel case is common.

– How to make a variable local:
local line = io.read() – Reads next stdin line.

– String concatenation uses the .. operator:
print('Winter is coming, ' .. line)
end

– Undefined variables return nil.
– This is not an error:
foo = anUnknownVariable – Now foo = nil.

aBoolValue = false

– Only nil and false are falsy; 0 and '' are true!
if not aBoolValue then print('twas false') end

– 'or' and 'and' are short-circuited.
– This is similar to the a?b:c operator in C/js:
ans = aBoolValue and 'yes' or 'no' –> 'no'

karlSum = 0
for i = 1, 100 do – The range includes both ends.
karlSum = karlSum + i
end

– Use “100, 1, -1” as the range to count down:
fredSum = 0
for j = 100, 1, -1 do fredSum = fredSum + j end

– In general, the range is begin, end[, step].

– Another loop construct:
repeat
print('the way of the future')
num = num - 1
until num == 0

– 2. Functions.

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

– Closures and anonymous functions are ok:
function adder(x)
– The returned function is created when adder is
– called, and remembers the value of x:
return function (y) return x + y end
end
a1 = adder(9)
a2 = adder(36)
print(a1(16)) –> 25
print(a2(64)) –> 100

– Returns, func calls, and assignments all work
– with lists that may be mismatched in length.
– Unmatched receivers are nil;
– unmatched senders are discarded.

x, y, z = 1, 2, 3, 4
– Now x = 1, y = 2, z = 3, and 4 is thrown away.

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

x, y = bar('zaphod') –> prints “zaphod nil nil”
– Now x = 4, y = 8, values 15..42 are discarded.

– Functions are first-class, may be local/global.
– These are the same:
function f(x) return x * x end
f = function (x) return x * x end

– And so are these:
local function g(x) return math.sin(x) end
local g; g = function (x) return math.sin(x) end
– the 'local g' decl makes g-self-references ok.

– Trig funcs work in radians, by the way.

– Calls with one string param don't need parens:
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) 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 ()

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!

–]]