Learn Lua in 15 Minutes

-- 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

') -- 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.