cheatsheets/lua.md

---
title: Lua
---

## Comments

    -- comment
    --[[ Multiline
         comment ]]

## Invoking functions

    print()
    print("Hi")

    -- You can omit parentheses if the argument is one string or table literal
    print "Hello World"     <-->     print("Hello World")
    dofile 'a.lua'          <-->     dofile ('a.lua')
    print [[a multi-line    <-->     print([[a multi-line
     message]]                        message]])
    f{x=10, y=20}           <-->     f({x=10, y=20})
    type{}                  <-->     type({})

## Tables / arrays

    t = {}
    t = { a = 1, b = 2 }
    t.a = function() ... end

    t = { ["hello"] = 200 }
    t.hello

    -- Remember, arrays are also tables
    array = { "a", "b", "c", "d" }
    print(array[2])       -- "b" (one-indexed)
    print(#array)         -- 4 (length)

## Loops

    while condition do
    end

    for i = 1,5 do
    end

    for i = start,finish,delta do
    end

    for k,v in pairs(tab) do
    end

    repeat
    until condition

    -- Breaking out:
    while x do
      if condition then break end
    end

## Conditionals

    if condition then
      print("yes")
    elsif condition then
      print("maybe")
    else
      print("no")
    end

## Variables

    local x = 2
    two, four = 2, 4

## Functions

    function myFunction()
      return 1
    end

    function myFunctionWithArgs(a, b)
      -- ...
    end

    myFunction()

    anonymousFunctions(function()
      -- ...
    end)

    -- Not exported in the module
    local function myPrivateFunction()
    end

    -- Splats
    function doAction(action, ...)
      print("Doing '"..action.."' to", ...)
      --> print("Doing 'write' to", "Shirley", "Abed")
    end

    doAction('write', "Shirley", "Abed")

## Lookups

    mytable = { x = 2, y = function() .. end }

    -- The same:
    mytable.x
    mytable['x']

    -- Syntactic sugar, these are equivalent:
    mytable.y(mytable)
    mytable:y()

    mytable.y(mytable, a, b)
    mytable:y(a, b)

    function X:y(z) .. end
    function X.y(self, z) .. end

## Metatables


    mt = {}

    -- A metatable is simply a table with functions in it.
    mt.__tostring = function() return "lol" end
    mt.__add      = function(b) ... end       -- a + b
    mt.__mul      = function(b) ... end       -- a * b
    mt.__index    = function(k) ... end       -- Lookups (a[k] or a.k)
    mt.__newindex = function(k, v) ... end    -- Setters (a[k] = v)

    -- Metatables allow you to override behavior of another table.
    mytable = {}
    setmetatable(mytable, mt)

    print(myobject)

## Classes

    Account = {}

    function Account:new(balance)
      local t = setmetatable({}, { __index = Account })

      -- Your constructor stuff
      t.balance = (balance or 0)
      return t
    end

    function Account:withdraw(amount)
      print("Withdrawing "..amount.."...")
      self.balance = self.balance - amount
      self:report()
    end

    function Account:report()
      print("Your current balance is: "..self.balance)
    end

    a = Account:new(9000)
    a:withdraw(200)    -- method call

## Constants

    nil
    false
    true

## Operators (and their metatable names)

    -- Relational (binary)
    -- __eq  __lt  __gt  __le  __ge
       ==    <     >     <=    >=
    ~=   -- Not equal, just like !=

    -- Arithmetic (binary)
    -- __add  __sub  __muv  __div  __mod  __pow
       +      -      *      /      %      ^

    -- Arithmetic (unary)
    -- __unm (unary minus)
       -

    -- Logic (and/or)
    nil and 10     --> 10
    false and nil  --> false
    10 and 20      --> 20


    -- Length
    -- __len(array)
    #array


    -- Indexing
    -- __index(table, key)
    t[key]
    t.key

    -- __newindex(table, key, value)
    t[key]=value

    -- String concat
    -- __concat(left, right)
    "hello, "..name

    -- Call
    -- __call(func, ...)


## API: Global functions  [(ref)](http://lua.gts-stolberg.de/en/Basis.php)

    dofile("hello.lua")
    loadfile("hello.lua")

    assert(x)    -- x or (raise an error)
    assert(x, "failed")

    type(var)   -- "nil" | "number" | "string" | "boolean" | "table" | "function" | "thread" | "userdata"

    -- Does /not/ invoke meta methods (__index and __newindex)
    rawset(t, index, value)    -- Like t[index] = value
    rawget(t, index)           -- Like t[index]

    _G  -- Global context
    setfenv(1, {})  -- 1: current function, 2: caller, and so on -- {}: the new _G

    pairs(t)     -- iterable list of {key, value}
    ipairs(t)    -- iterable list of {index, value}

    tonumber("34")
    tonumber("8f", 16)

## API: Strings

    'string'..'concatenation'

    s = "Hello"
    s:upper()
    s:lower()
    s:len()    -- Just like #s

    s:find()
    s:gfind()

    s:match()
    s:gmatch()

    s:sub()
    s:gsub()

    s:rep()
    s:char()
    s:dump()
    s:reverse()
    s:byte()
    s:format()

## API: Tables

    table.foreach(t, function(row) ... end)
    table.setn
    table.insert(t, 21)          -- append (--> t[#t+1] = 21)
    table.insert(t, 4, 99)
    table.getn
    table.concat
    table.sort
    table.remove(t, 4)

## API: Math [(ref)](http://lua-users.org/wiki/MathLibraryTutorial)

    math.abs     math.acos    math.asin       math.atan    math.atan2
    math.ceil    math.cos     math.cosh       math.deg     math.exp
    math.floor   math.fmod    math.frexp      math.ldexp   math.log
    math.log10   math.max     math.min        math.modf    math.pow
    math.rad     math.random  math.randomseed math.sin     math.sinh
    math.sqrt    math.tan     math.tanh

    math.sqrt(144)
    math

## API: Misc

    io.output(io.open("file.txt", "w"))
    io.write(x)
    io.close()

    for line in io.lines("file.txt")

    file = assert(io.open("file.txt", "r"))
    file:read()
    file:lines()
    file:close()

## Reference

  http://www.lua.org/pil/13.html
  http://lua-users.org/wiki/ObjectOrientedProgramming