learn.lua
| -- 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) <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: | |
| http://learnxinyminutes.com/ | |
| Have fun with Lua! | |
| --]] | |