| language | contributors | filename | |||
|---|---|---|---|---|---|
lua |
|
learnlua.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 n 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') --> Hi!
dofile('mod2') --> Hi! (runs again, unlike require)
-- loadfile loads a lua file but doesn't run it yet.
f = loadfile('mod2') -- Calling f() runs mod2.lua.
-- loadstring is loadfile for strings.
g = loadstring('print(343)') -- Returns a function.
g() -- Prints out 343; nothing printed before now.
--]]
I was excited to learn Lua so I could make games with the Love 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:
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!