Lua

From AQA Computer Science
Jump to: navigation, search

Comments

Lua implements comments by:

-- Two dashes start a one-line comment.

--[[
     Adding two ['s and ]'s makes it a
     multi-line comment.
--]]

Write to Console

Lua provides 2 ways to write to the console:

print("helloworld")
io.write('helloworld')

Escape Characters

These can also include escape characters in the string, eg '\n':

io.write('helloworld\n')

Read from Console

Lua can read from the keyboard (ie Console.ReadLine() in C# or input() in Python), you can also convert this input to a number:

input = io.read()
-- Convert input to a number
num = tonumber(io.read())

Variables

Lua uses variables similar to python, you don't need to specify a data type:

Numbers

num = 42  -- All numbers are doubles.

Strings

s = 'walternate'  -- Immutable strings like Python.
t = "double-quotes are also fine"
u = [[ Double brackets
       start and end
       multi-line strings.]]
-- String concatenation uses the .. operator:
message = 'Winter is coming, ' .. line

Empty / Null

t = nil  -- Undefines t; Lua has garbage collection.

Global Variables

The default in Lua is that all variables are global (in python all variables are local):

  -- Variables are global by default.
  thisIsGlobal = 5  -- Camel case is common.

Local Variables

To specify a variable is only local, you need to include 'local' before the variable name:

  -- How to make a variable local:
  local line = io.read()  -- Reads next stdin line.

If Statement

Lua implements IF statements, but doesn't implement a Switch Case statement. An example if is shown below:

if num < 40 then
  print('below 40')
elseif name ~= 'wayne' then  
  -- ~= is not equals.
  -- Equality check is == like Python; ok for strs.
  io.write('over 40 and Name is wayne\n')  -- Defaults to stdout.
else
  print('above 40')
end

Operators

Within if statements and also within loops, the following relational and conditional operates exist:

Relational

Symbols Explanation
== Equal
~= Not equal
<= Less than or equal
>= Greater than or equal
< Less than
> Greater than

Conditional

Operator Description Example
and Called Logical AND operator. If both the operands are non zero then condition becomes true. (A and B) is false
or Called Logical OR Operator. If any of the two operands is non zero then condition becomes true. (A or B) is true.
not Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false. not(A and B) is true.

Loops

Just like other languages Lua has the standard 3 types of loop. While will run 0 or more times (may never run), Repeat will run at least once, For will run an exact number of times:

While

-- Blocks are denoted with keywords like do/end:
while num < 50 do
  num = num + 1  -- No ++ or += type operators.
end

For

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].

Repeat / Do While

-- Another loop construct:
repeat
  print('the way of the future')
  num = num - 1
until num == 0

Functions

Declaring a function

You can declare a function using the command 'function', the parameters will be within the round brackets '()'. The use of 'local' is not really required:

local function add(first_number, second_number)
	print(first_number + second_number)
end

add(2, 3) -- calling a method

Returning a value

You can return a value from a function by using 'return'

local function add(first_number, second_number)
	return first_number + second_number
end

print(add(5, 6))

Classes - Table Based

Just like other programming languages, Lua is can be used in an OOP way. Lua calls '{}' tables or meta tables, but they are the same as a class really:

Define a class

-- Meta class, class defined with {}, variables can be set like 'area'
Shape = {area = 0} 

-- Base class is named, followed by ':' to create a method for the class
function Shape:new (o,side)
   o = o or {} -- if an object is passed, use it or create an empty one
   setmetatable(o, self)
   self.__index = self
   side = side or 0 -- if side is passed, use it or set side to 0
   self.area = side*side;
   return o
end

-- Base class method printArea
function Shape:printArea ()
   print("The area is ",self.area)
end

Creating an object

-- Creating an object
myshape = Shape:new(nil,10)

--to access the methods of the object you need to use ':'
myshape:printArea()

Inheritance

Square = Shape:new()

-- Derived class method new

function Square:new (o,side)
   o = o or Shape:new(o,side) -- if an object is passed, use it or create a new shape
   setmetatable(o, self)
   self.__index = self
   return o
end

You can override an inherited method by redeclaring it:

-- Derived class method printArea

function Square:printArea ()
   print("The area of square is ",self.area)
end

Classes - Closure Based

Lua can implement classes in a second way:

Define a Class

function MyClass(init)
  -- the new instance
  local self = {
    -- public fields go in the instance table
    public_field = 0
  }

  -- private fields are implemented using locals
  -- they are faster than table access, and are truly private, so the code that uses your class can't get them
  local private_field = init

  function self.foo()
    return self.public_field + private_field
  end

  function self.bar()
    private_field = private_field + 1
  end

  -- return the instance
  return self
end

Creating an Object

Creating and using an instance can be done with:

local i = MyClass(5)
print(i.foo()) --> 5
i.public_field = 3
i.bar()

Inheritance

This is the base class, and below a subclass derived from it:

function BaseClass(init)
  local self = {}

  local private_field = init

  function self.foo()
    return private_field
  end

  function self.bar()
    private_field = private_field + 1
  end

  -- return the instance
  return self
end

function DerivedClass(init, init2)
  local self = BaseClass(init)

  -- Public fields can be added to the derived version
  self.public_field = init2

  -- this is independent from the base class's private field that has the same name
  -- the derived value would be self.private_field
  local private_field = init2

  -- save the base version of foo for use in the derived version
  local base_foo = self.foo

  -- this function overrides the derived version
  function self.foo() 
    return private_field + self.public_field + base_foo()
  end

  -- return the instance
  return self
end