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Learn Lua in 15 minutes

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Posted by Nick Gammon   Australia  (21,322 posts)  [Biography] bio   Forum Administrator
Date Sat 28 Jan 2017 03:59 AM (UTC)
Message

Learn Lua in 15 minutes

by Tyler Neylon

Learn Lua quickly with this short yet comprehensive and friendly script. It's written as both an introduction and a quick reference. It's also a valid Lua script so you can verify that the code does what it says, and learn more by modifying and running this script in your Lua interpreter.

-- 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\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 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.

-- 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()  -- 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.

--]]

----------------------------------------------------
-- 5. References.
----------------------------------------------------

--[[

I was excited to learn Lua so I could make games
with the Löve 2D 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, this entire file is valid Lua; save it
as learn.lua and run it with "lua learn.lua" !

This was first written for tylerneylon.com. It's
also available as a github gist. Tutorials for other
languages, in the same style as this one, are here:

http://learnxinyminutes.com/

Have fun with Lua!

--]]

Credit


- Nick Gammon

www.gammon.com.au, www.mushclient.com
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Posted by Fiendish   USA  (1,641 posts)  [Biography] bio   Global Moderator
Date Reply #1 on Sat 28 Jan 2017 01:21 PM (UTC)

Amended on Sat 28 Jan 2017 11:52 PM (UTC) by Nick Gammon

Message
Yes! That is an excellent resource for people who know some programming but are new to Lua.

And if a person has never programmed before at all, I definitely recommend the three part series:

1) Learn Lua the Hard Way

2) Learn Lua the Hard Way – Tables

3) Problem Solving with Lua

https://github.com/fiendish/aardwolfclientpackage
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To show them in your local time you can join the forum, and then set the 'time correction' field in your profile to the number of hours difference between your location and UTC time.


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