---
title: Lua
layout: 2017/sheet
---

## Basic examples

### References

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

### 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")
    elseif 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

## More concepts

### 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

    -- 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

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

### Tables

    -- 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

### API: Global Functions

    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

    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()