lua学习笔记

  1-- Two dashes start a one-line comment.
  2
  3--[[
  4     Adding two ['s and ]'s makes it a
  5     multi-line comment.
  6--]]
  7
  8----------------------------------------------------
  9-- 1. Variables and flow control.
 10----------------------------------------------------
 11
 12num = 42  -- All numbers are doubles.
 13-- Don't freak out, 64-bit doubles have 52 bits for
 14-- storing exact int values; machine precision is
 15-- not a problem for ints that need < 52 bits.
 16
 17s = 'walternate'  -- Immutable strings like Python.
 18t = "double-quotes are also fine"
 19u = [[ Double brackets
 20       start and end
 21       multi-line strings.]]
 22t = nil  -- Undefines t; Lua has garbage collection.
 23
 24-- Blocks are denoted with keywords like do/end:
 25while num < 50 do
 26  num = num + 1  -- No ++ or += type operators.
 27end
 28
 29-- If clauses:
 30if num > 40 then
 31  print('over 40')
 32elseif s ~= 'walternate' then  -- ~= is not equals.
 33  -- Equality check is == like Python; ok for strs.
 34  io.write('not over 40\n')  -- Defaults to stdout.
 35else
 36  -- Variables are global by default.
 37  thisIsGlobal = 5  -- Camel case is common.
 38
 39  -- How to make a variable local:
 40  local line = io.read()  -- Reads next stdin line.
 41
 42  -- String concatenation uses the .. operator:
 43  print('Winter is coming, ' .. line)
 44end
 45
 46-- Undefined variables return nil.
 47-- This is not an error:
 48foo = anUnknownVariable  -- Now foo = nil.
 49
 50aBoolValue = false
 51
 52-- Only nil and false are falsy; 0 and '' are true!
 53if not aBoolValue then print('twas false') end
 54
 55-- 'or' and 'and' are short-circuited.
 56-- This is similar to the a?b:c operator in C/js:
 57ans = aBoolValue and 'yes' or 'no'  --> 'no'
 58
 59karlSum = 0
 60for i = 1, 100 do  -- The range includes both ends.
 61  karlSum = karlSum + i
 62end
 63
 64-- Use "100, 1, -1" as the range to count down:
 65fredSum = 0
 66for j = 100, 1, -1 do fredSum = fredSum + j end
 67
 68-- In general, the range is begin, end[, step].
 69
 70-- Another loop construct:
 71repeat
 72  print('the way of the future')
 73  num = num - 1
 74until num == 0
 75
 76
 77----------------------------------------------------
 78-- 2. Functions.
 79----------------------------------------------------
 80
 81function fib(n)
 82  if n < 2 then return 1 end
 83  return fib(n - 2) + fib(n - 1)
 84end
 85
 86-- Closures and anonymous functions are ok:
 87function adder(x)
 88  -- The returned function is created when adder is
 89  -- called, and remembers the value of x:
 90  return function (y) return x + y end
 91end
 92a1 = adder(9)
 93a2 = adder(36)
 94print(a1(16))  --> 25
 95print(a2(64))  --> 100
 96
 97-- Returns, func calls, and assignments all work
 98-- with lists that may be mismatched in length.
 99-- Unmatched receivers are nil;
100-- unmatched senders are discarded.
101
102x, y, z = 1, 2, 3, 4
103-- Now x = 1, y = 2, z = 3, and 4 is thrown away.
104
105function bar(a, b, c)
106  print(a, b, c)
107  return 4, 8, 15, 16, 23, 42
108end
109
110x, y = bar('zaphod')  --> prints "zaphod  nil nil"
111-- Now x = 4, y = 8, values 15..42 are discarded.
112
113-- Functions are first-class, may be local/global.
114-- These are the same:
115function f(x) return x * x end
116f = function (x) return x * x end
117
118-- And so are these:
119local function g(x) return math.sin(x) end
120local g; g  = function (x) return math.sin(x) end
121-- the 'local g' decl makes g-self-references ok.
122
123-- Trig funcs work in radians, by the way.
124
125-- Calls with one string param don't need parens:
126print 'hello'  -- Works fine.
127
128
129----------------------------------------------------
130-- 3. Tables.
131----------------------------------------------------
132
133-- Tables = Lua's only compound data structure;
134--          they are associative arrays.
135-- Similar to php arrays or js objects, they are
136-- hash-lookup dicts that can also be used as lists.
137
138-- Using tables as dictionaries / maps:
139
140-- Dict literals have string keys by default:
141t = {key1 = 'value1', key2 = false}
142
143-- String keys can use js-like dot notation:
144print(t.key1)  -- Prints 'value1'.
145t.newKey = {}  -- Adds a new key/value pair.
146t.key2 = nil   -- Removes key2 from the table.
147
148-- Literal notation for any (non-nil) value as key:
149u = {['@!#'] = 'qbert', [{}] = 1729, [6.28] = 'tau'}
150print(u[6.28])  -- prints "tau"
151
152-- Key matching is basically by value for numbers
153-- and strings, but by identity for tables.
154a = u['@!#']  -- Now a = 'qbert'.
155b = u[{}]     -- We might expect 1729, but it's nil:
156-- b = nil since the lookup fails. It fails
157-- because the key we used is not the same object
158-- as the one used to store the original value. So
159-- strings & numbers are more portable keys.
160
161-- A one-table-param function call needs no parens:
162function h(x) print(x.key1) end
163h{key1 = 'Sonmi~451'}  -- Prints 'Sonmi~451'.
164
165for key, val in pairs(u) do  -- Table iteration.
166  print(key, val)
167end
168
169-- _G is a special table of all globals.
170print(_G['_G'] == _G)  -- Prints 'true'.
171
172-- Using tables as lists / arrays:
173
174-- List literals implicitly set up int keys:
175v = {'value1', 'value2', 1.21, 'gigawatts'}
176for i = 1, #v do  -- #v is the size of v for lists.
177  print(v[i])  -- Indices start at 1 !! SO CRAZY!
178end
179-- A 'list' is not a real type. v is just a table
180-- with consecutive integer keys, treated as a list.
181
182----------------------------------------------------
183-- 3.1 Metatables and metamethods.
184----------------------------------------------------
185
186-- A table can have a metatable that gives the table
187-- operator-overloadish behavior. Later we'll see
188-- how metatables support js-prototypey behavior.
189
190f1 = {a = 1, b = 2}  -- Represents the fraction a/b.
191f2 = {a = 2, b = 3}
192
193-- This would fail:
194-- s = f1 + f2
195
196metafraction = {}
197function metafraction.__add(f1, f2)
198  sum = {}
199  sum.b = f1.b * f2.b
200  sum.a = f1.a * f2.b + f2.a * f1.b
201  return sum
202end
203
204setmetatable(f1, metafraction)
205setmetatable(f2, metafraction)
206
207s = f1 + f2  -- call __add(f1, f2) on f1's metatable
208
209-- f1, f2 have no key for their metatable, unlike
210-- prototypes in js, so you must retrieve it as in
211-- getmetatable(f1). The metatable is a normal table
212-- with keys that Lua knows about, like __add.
213
214-- But the next line fails since s has no metatable:
215-- t = s + s
216-- Class-like patterns given below would fix this.
217
218-- An __index on a metatable overloads dot lookups:
219defaultFavs = {animal = 'gru', food = 'donuts'}
220myFavs = {food = 'pizza'}
221setmetatable(myFavs, {__index = defaultFavs})
222eatenBy = myFavs.animal  -- works! thanks, metatable
223
224-- Direct table lookups that fail will retry using
225-- the metatable's __index value, and this recurses.
226
227-- An __index value can also be a function(tbl, key)
228-- for more customized lookups.
229
230-- Values of __index,add, .. are called metamethods.
231-- Full list. Here a is a table with the metamethod.
232
233-- __add(a, b)                     for a + b
234-- __sub(a, b)                     for a - b
235-- __mul(a, b)                     for a * b
236-- __div(a, b)                     for a / b
237-- __mod(a, b)                     for a % b
238-- __pow(a, b)                     for a ^ b
239-- __unm(a)                        for -a
240-- __concat(a, b)                  for a .. b
241-- __len(a)                        for #a
242-- __eq(a, b)                      for a == b
243-- __lt(a, b)                      for a < b
244-- __le(a, b)                      for a <= b
245-- __index(a, b)  <fn or a table>  for a.b
246-- __newindex(a, b, c)             for a.b = c
247-- __call(a, ...)                  for a(...)
248
249----------------------------------------------------
250-- 3.2 Class-like tables and inheritance.
251----------------------------------------------------
252
253-- Classes aren't built in; there are different ways
254-- to make them using tables and metatables.
255
256-- Explanation for this example is below it.
257
258Dog = {}                                   -- 1.
259
260function Dog:new()                         -- 2.
261  newObj = {sound = 'woof'}                -- 3.
262  self.__index = self                      -- 4.
263  return setmetatable(newObj, self)        -- 5.
264end
265
266function Dog:makeSound()                   -- 6.
267  print('I say ' .. self.sound)
268end
269
270mrDog = Dog:new()                          -- 7.
271mrDog:makeSound()  -- 'I say woof'         -- 8.
272
273-- 1. Dog acts like a class; it's really a table.
274-- 2. function tablename:fn(...) is the same as
275--    function tablename.fn(self, ...)
276--    The : just adds a first arg called self.
277--    Read 7 & 8 below for how self gets its value.
278-- 3. newObj will be an instance of class Dog.
279-- 4. self = the class being instantiated. Often
280--    self = Dog, but inheritance can change it.
281--    newObj gets self's functions when we set both
282--    newObj's metatable and self's __index to self.
283-- 5. Reminder: setmetatable returns its first arg.
284-- 6. The : works as in 2, but this time we expect
285--    self to be an instance instead of a class.
286-- 7. Same as Dog.new(Dog), so self = Dog in new().
287-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.
288
289----------------------------------------------------
290
291-- Inheritance example:
292
293LoudDog = Dog:new()                           -- 1.
294
295function LoudDog:makeSound()
296  s = self.sound .. ' '                       -- 2.
297  print(s .. s .. s)
298end
299
300seymour = LoudDog:new()                       -- 3.
301seymour:makeSound()  -- 'woof woof woof'      -- 4.
302
303-- 1. LoudDog gets Dog's methods and variables.
304-- 2. self has a 'sound' key from new(), see 3.
305-- 3. Same as LoudDog.new(LoudDog), and converted to
306--    Dog.new(LoudDog) as LoudDog has no 'new' key,
307--    but does have __index = Dog on its metatable.
308--    Result: seymour's metatable is LoudDog, and
309--    LoudDog.__index = LoudDog. So seymour.key will
310--    = seymour.key, LoudDog.key, Dog.key, whichever
311--    table is the first with the given key.
312-- 4. The 'makeSound' key is found in LoudDog; this
313--    is the same as LoudDog.makeSound(seymour).
314
315-- If needed, a subclass's new() is like the base's:
316function LoudDog:new()
317  newObj = {}
318  -- set up newObj
319  self.__index = self
320  return setmetatable(newObj, self)
321end
322
323----------------------------------------------------
324-- 4. Modules.
325----------------------------------------------------
326
327
328--[[ I'm commenting out this section so the rest of
329--   this script remains runnable.
330-- Suppose the file mod.lua looks like this:
331local M = {}
332
333local function sayMyName()
334  print('Hrunkner')
335end
336
337function M.sayHello()
338  print('Why hello there')
339  sayMyName()
340end
341
342return M
343
344-- Another file can use mod.lua's functionality:
345local mod = require('mod')  -- Run the file mod.lua.
346
347-- require is the standard way to include modules.
348-- require acts like:     (if not cached; see below)
349local mod = (function ()
350  <contents of mod.lua>
351end)()
352-- It's like mod.lua is a function body, so that
353-- locals inside mod.lua are invisible outside it.
354
355-- This works because mod here = M in mod.lua:
356mod.sayHello()  -- Says hello to Hrunkner.
357
358-- This is wrong; sayMyName only exists in mod.lua:
359mod.sayMyName()  -- error
360
361-- require's return values are cached so a file is
362-- run at most once, even when require'd many times.
363
364-- Suppose mod2.lua contains "print('Hi!')".
365local a = require('mod2')  -- Prints Hi!
366local b = require('mod2')  -- Doesn't print; a=b.
367
368-- dofile is like require without caching:
369dofile('mod2.lua')  --> Hi!
370dofile('mod2.lua')  --> Hi! (runs it again)
371
372-- loadfile loads a lua file but doesn't run it yet.
373f = loadfile('mod2.lua')  -- Call f() to run it.
374
375-- loadstring is loadfile for strings.
376g = loadstring('print(343)')  -- Returns a function.
377g()  -- Prints out 343; nothing printed before now.
378
379--]]
380
381----------------------------------------------------
382-- 5. References.
383----------------------------------------------------
384
385--[[
386
387I was excited to learn Lua so I could make games
388with the Löve 2D game engine. That's the why.
389
390I started with BlackBulletIV's Lua for programmers.
391Next I read the official Programming in Lua book.
392That's the how.
393
394It might be helpful to check out the Lua short
395reference on lua-users.org.
396
397The main topics not covered are standard libraries:
398 * string library
399 * table library
400 * math library
401 * io library
402 * os library
403
404By the way, this entire file is valid Lua; save it
405as learn.lua and run it with "lua learn.lua" !
406
407This was first written for tylerneylon.com. It's
408also available as a github gist. Tutorials for other
409languages, in the same style as this one, are here:
410
411http://learnxinyminutes.com/
412
413Have fun with Lua!
414
415--]]