learnxinyminutes-docs/lua.html.markdown

language	contributors	filename
Lua	Tyler Neylon http://tylerneylon.com/	learnlua.lua

-- Two dashes start a one-line comment.

--[[
     Adding two ['s and ]'s makes it a
     multi-line comment.
--]]

print("Hello, Lua")

----------------------------------------------------
---- Variables, Data Types, and some operations ----
----------------------------------------------------

-- integer and float values --
num1 = 42
num2 = 10.5

-- Mathematical Operations :
print(num1 + num2)  -- Addition
print(num1 - num2)  -- Subtraction
print(num1 * num2)  -- Multiplication
print(num1 / num2)  -- Division
print(num1 // num2) -- floor Division (round down to the closest integer)
print(num1 % num2)  -- Modulus (gives the remainder of num1/num2)
print(num1 ^ num2)  -- Exponentiation


-- string values --
str1 = 'single quotes string'
str2 = "double quotes string"
str3 = [[ multi
          line
          string]]

-- string concatenation is done using two dots :
print(str1 .. str2)


-- nil values (the absence of a value) -- 
-- an undefined variable returns nil
print(undefined_variable)
-- a previously defined variable can be undefined with nil :
t = 4
t = nil  -- Undefines t; Lua has garbage collection.


-- Boolean values --
a = true
b = false

-- note : other than writing false, you can write nil to represent a false value, you can't however write 0 or '' like in other languages.

-- boolean operations :
print(not a)
print(a and b)
print(a or b)


-- Variables are global by default
-- to make them local you write :
local variable_name = variable_value


----------------------------------------------------
------------ Standard Input and Output -------------
----------------------------------------------------

-- Input :
io.read() -- allow the user to input data through the console

-- example :
print("write your name :")
name = io.read() -- takes the user input and stores it in a variable named "name"
print("Your name is " .. name)

-- Output :
--[[
The io.write() function is used to print text or data to the standard output (usually the console) without automatically adding a newline at the end. Unlike print()
]]

io.write("Hello, Lua")

-- you can add a new line with \n if you want
io.write("Hello, Lua\n")
-- or a tab with -t, or a backslash (\) with \\


----------------------------------------------------
------------------ If Statements -------------------
----------------------------------------------------
-- Blocks of code are enclosed between key words like
-- then/end or do/end

-- syntax :
-- if (condition is true) then (do this) end

-- if (condition is true) then (do this)
-- elseif (this condition is true) then (do this instead)
-- else (if no condition is true, do this) end

--example :
if true then
  print('something')
end

if false then
    print('nothing')
end

-- comparison operations :
a == b -- Equality
a ~= b -- Inequality
a > b -- greater than
a >= -- greater or equal to
a < b -- less than
a >= -- less or equal to
-- alongside the (not, and, or) operations we learned earlier

----------------------------------------------------
------------------- while loops --------------------
----------------------------------------------------
num = 0

while num < 50 do
    print(num)
    num = num + 1
end

-- note : this language doesn't have ++ or -- or += or -= operations like in some other languages.

----------------------------------------------------
-------------------- for loops ---------------------
----------------------------------------------------
-- syntax :
-- for (variable_name) = (start_value , end_value , step) do (something) end
-- note : both the start_value and end_value are included in the range.
-- note : the step value is optional, and it's default value is +1

-- examples :
for i = 1, 10 do
  print(i)
end

for j = 10, 1, -1 do
    print(j)
end

for j = 10, 1, -2 do
    print(j)
end

----------------------------------------------------
---------------- repeat until loops ----------------
----------------------------------------------------

--[[The repeat, until loop in Lua is similar to the do, while loop in other languages. It repeatedly executes a block of code until a specified condition becomes true. Unlike the while loop, the repeat ... until loop will always execute at least once, since the condition is checked after each iteration.]]

-- syntax :
repeat
    -- code to execute
until condition

-- example
k = 10

repeat
  print(k)
  k = k - 1
until k == 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-prototype 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() -- Prints: Why hello there 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.

-- load is loadfile for strings.
-- (loadstring is deprecated, use load instead)
g = load('print(343)')  -- Returns a function.
g()  -- Prints out 343; nothing printed before now.

--]]

Community

If you need support join the official Lua mailing list, IRC channel, or forum.

References

I was excited to learn Lua so I could make games with the LÖVE 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, the entire file is valid Lua; save it as learn.lua and run it with "lua learn.lua" !

This was first written for tylerneylon.com, and is also available as a GitHub gist. Have fun with Lua!