lua学习笔记
<p>lua是一门轻量级的脚本语言…好像比较适合写游戏？在 <a href="https://github.com/Mogara/QSanguosha">太阳神三国杀</a> 中见过很多lua脚本。 由于<a href="https://splash.readthedocs.io/en/stable/scripting-tutorial.html">splash</a> 的渲染脚本需要用lua来写，因此来学习一波。</p>
直接上语法…看到了python和pascal的影子orz
-- 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 40n') – 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) <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.lua') –> Hi!
dofile('mod2.lua') –> Hi! (runs it again)
– loadfile loads a lua file but doesn't run it yet.
f = loadfile('mod2.lua') – Call f() to run it.
– 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. It's
also available as a github gist. Tutorials for other
languages, in the same style as this one, are here:
Have fun with Lua!
–]]
<p class="tip"><i></i><span>真诚赞赏，手留余香</span></p>
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