GettingStarted
The syntax of NPL is 100% compatible with lua. If you are unfamiliar with lua, the only book you need is
Book: Programming in Lua
The following NPL code is modified from http://learnxinyminutes.com/
-- Two dashes start a one-line comment.
--[[
Adding two ['s and ]'s makes it a multi-line comment.
--]]
----------------------------------------------------
-- 1. Variables and flow control.
----------------------------------------------------
local num = 42; -- All numbers are doubles.
s = 'string is immutable'
-- comparing two strings is as fast as comparing two pointers
t = "double-quotes are also fine"; -- `;` is optional at line end
u = [[ Double brackets
start and end
multi-line strings.]]
-- 'nil' is like null_ptr. It undefines t. When data has no reference,
-- it will be garbage collected some time later.
t = nil
-- 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('less than 40')
elseif ("string" == 40 or s) then
print('lua has dynamic type,thus any two objects can be compared.');
elseif s ~= 'NPL' then
print('less than 40')
else
-- Variables are global by default.
thisIsGlobal = 5 -- variable case is sensitive.
-- How to make a variable local:
local line = "this is accessible until next `end` or end of containing script file"
-- String concatenation uses the .. operator:
print("First string " .. 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('false') end
-- 'or' and 'and' are short-circuited. They are like ||, && in C.
-- This is similar to the a?b:c operator in C:
ans = (aBoolValue and 'yes') or 'no' --> 'no'
local nSum = 0;
for i = 1, 100 do -- The range includes both ends. i is a local variable.
nSum = nSum + i
end
-- Use "100, 1, -1" as the range to count down:
-- In general, the range is begin, end[, step].
nSum = 0
for i = 100, 1, -1 do
nSum = nSum + i
end
-- Another loop construct:
repeat
nSum = nSum - 1
until nSum == 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(string.format("%s %s %s", a, b or "b", c))
return 4, 8, 15, 16, 23, 42
end
x, y = bar("NPL") --> prints "NPL b 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
-- Calls with one parameter don't need parenthesis:
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
-- Copyright: modified from http://learnxinyminutes.com/
-- Have fun with NPL/Lua!We have developed NPL Code Wiki to help you learn and practice NPL. To launch NPL Code Wiki, you need to
- install Paracraft
- create an empty world or load online world like this one (HourOfCode)
- press F11 to launch NPL Code Wiki.
You can then enter any NPL code in the console. See below:
Lua/NPL is easy to learn, but hard to master. Pay attention to following concepts.
- Lua string is immutable, so comparing two strings is as fast as comparing two pointers.
- Lua has lexical scoping, use it to accelerate your code by exposing frequently used objects as local variables in its upper lexical scope.
- Always use local variables.
- NPL recommends object oriented programming, one can implement all C++ class features with lua tables, like class inheritance and virtual functions, etc.