Here is the IO-Project cheat sheet to quickly learn the "Lua" programming language
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cheat-sheet-lua
Here is the IO-Project cheat sheet to quickly learn the "Lua" programming language. I suggest you to consult the documentation of lua for more information on the language, it is available by following this link.
Table of contents
Use this table of contents to travel more easily through this cheat sheet.
Basics
We'll now introduce the basics of lua, starting with comments, variables, loops and functions.
Code comments
-- Two dashes start a one-line comment.
--[[
by adding two "[" opening and two "]" closing,
you define a multi-line commentary. ;-)
--]]
Variables and loop
Introduction to variables, basic conditions, some loops (examples available here) and the equivalent of the ternary operator.
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 in 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 ==; 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 false; 0 and '' are true.
if not aBoolValue then print('that was 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
Functions
Introduction to function definition, recursion with lua and chain assignment followed by closure function.
-- The famous Fibonacci sequence.
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.
print(type(x)) --> number
-- 'type()' function allow to detemindthe type of a variable.
-- '...' is an elipse parameter, retrievable in context by '...'.
function e(...) print(...) end
e(2, 4, 8, 6) --> prints "2 4 8 6"
-- Functions are first-class, may be local/global.
-- (global) These are the same:
function f(x) return x * x end
f = function (x) return x * x end -- same of javascript definition
-- (local) 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.
Advanced
Let's move on to more advanced notions. With the notion of table, class, module, coroutine, meta-programming and module.
Tables, Array, dict..
Tables are the only compound data structure in Lua, 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.
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 in the section 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.
-- Main list. Here is a table with the metamethods.
-- __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(...)
Class-like tables and inheritance.
Classes aren't built in, there are different ways to emulate them with tables and metatables. The different ways to define a class in Lua are not easy to understand, so I suggest you to look at the following document implementing 3 types of class definition. The last one being the one I chose (my preferred method).
-- 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
Coroutine
Let's turn now to coroutines. Coroutines are functions that can be suspended and resumed at a later time. They are used to implement iterators, generators and event loops and represent a line of execution with its own stack. In other words, they can be compared to threads.
-- Create a coroutine that prints 'Hello' and then stops.
coHi = coroutine.create(function () print('Hello') end)
print(coHi) -- thread: 0x7f9c0c00a0c0
-- Coroutine status can be 'suspended', 'running' or 'dead'.
-- The coroutine is created in the 'suspended' state.
-- Resume the coroutine. It will print 'Hello' and stop.
coroutine.resume(coHi) -- return 'true'
print(coroutine.status(coHi)) -- 'dead'
-- We can also pass arguments to the coroutine.
-- The arguments of the first resume are passed to the
-- function of the coroutine. The following arguments
-- are passed to the yield function.
routine = coroutine.create(function (a, b, c)
print('first print: ', a, b, c)
print('yield1: ', coroutine.yield())
print('yield2: ', coroutine.yield('a variable'))
return(a+b+c)
end)
-- will run the coroutine until the first yield.
coroutine.resume(routine, 1, 2, 3)
-- run the coroutine until the second yield passing
-- the arguments 4, 5 and 6 to the 1er yield and
-- retrieve the return value of the second yield.
print('out routine: ' , coroutine.resume(routine, 4, 5, 6))
-- this will run out the second yield and made the
-- adition of 'a+b+c' and kill the coroutine.
print(coroutine.resume(routine, 7, 8, 9)) -- 1+2+3 = '6'
-- All these steps will print:
--[[
first print: 1 2 3
yield1: 4 5 6
out routine: true a variable
yield2: 7 8 9
true 6
--]]
Error handling
Lua allows low-level error handling with the error function and high-level error handling with the assert function. The error function raises an error and handles it with the pcall or xpcall function. The assert function checks a condition and raises an error if the condition is not met.
-- Throw an error if the first argument is false
-- the second argument is the error message.
assert(type(firstvariable) = 'string', 'not a string')
-- TODO to continue
Modules
Modules are a way to organize your code. They are a way to group functions and variables together in a single file. You can then use the module in other files by using the require function.
-- 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 -- Return the table 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.