15分钟掌握Lua

-- 15分钟掌握Lua------------------------------------------------------ 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 = 0for i = 1, 100 do  -- The range includes both ends.  karlSum = karlSum + iend-- Use "100, 1, -1" as the range to count down:fredSum = 0for 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 - 1until 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 endenda1 = adder(9)a2 = adder(36)print(a1(16))  --> 25print(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, 42endx, 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 endf = function (x) return x * x end-- And so are these:local function g(x) return math.sin(x) endlocal 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) endh{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 + f2metafraction = {}function metafraction.__add(f1, f2)  sum = {}  sum.b = f1.b * f2.b  sum.a = f1.a * f2.b + f2.a * f1.b  return sumendsetmetatable(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.endfunction Dog:makeSound()                   -- 6.  print('I say ' .. self.sound)endmrDog = 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)endseymour = 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')endfunction M.sayHello()  print('Why hello there')  sayMyName()endreturn 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.--]]