Learn.lua

```
-- 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!

--]]

```