Lua

How we are going to look at languages

  • Use chapters 5-10 of textbook as a guide
    • Variable Names, Binding, and Scope
    • Data types
    • Assignment, Epression, and Control Statements
    • Subprograms
  • What effect do these have on implementation and use of a language?

Brief Lua History

  • Designed by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes
    • All with Tecgraf at PUC-Rio at the time
  • Version 1.0 came out in 1993, version 5.3 is the current version
  • Very small - Lua interpreter is less than 150K
  • Notable Uses
    • Game Scripting
    • Adobe Lightroom Frontend
    • Deep Learning (torch library)

Lua Basics

Variables in Lua

  • Can be any combination of letters, numbers and _ (the underscore character)
  • Cannot start with a digit or be the same as a reserved word
  • Do not need to be declared
  • Are global by default

Scope in Lua

  • The scope of a variable is the portion of the code for which a variable is defined
  • In Lua, variables are global by default
  • To make the scope of a variable the current block, use the local keyword
In [3]:
x = 1
do 
    x = 2
    do 
        local q = 1
        print(q)
    end
    print(q)
end

do
    _y = 10
end

do
    local Z = -1
end

print(x)
print(_y)
print(Z)
Out[3]:
1	
nil	
2	
10	
nil
In [ ]:






















Data Types in Lua
    

  • Lua has a very simple type system, with just 8 types
      

    • number
      • 

    • string
      • 

    • boolean
      • 

    • nil
      • 

    • table
      • 

    • function
      • 

    • userdata
      • 

    • thread
      • 

    

    • 




















Number
    

  • There is only one type for all numbers in Lua
    • 

  • Stored internally as a float
    • 

  • Can be written in scientific notation
    • 

  • Operators for numbers are:  + , - , * , / , % ,  ^ 
    • 














In [5]:



    


print(2e0,2,02,2.00,2.00001,2e2,200000000000000000000)

















Out[5]:






2	2	2	2	2.00001	200	2e+20	




















In [10]:



    


type(0/0)

















Out[10]:






number	




















In [9]:



    


type(3/4)

















Out[9]:






number	




















In [7]:



    


3 / 0

















Out[7]:






inf	


























String
    

  • Can be denoted by using either single or double quotes
    • 

  • Multiline strings can be defined using [[ ]] 
    • 

  • Strings are immutable
    • 

  • Can hold any unicode character
    • 

  • .. is the concatination operator
    • 

  • # can be used to get the length of a string
    • 




















String Examples












In [11]:



    


'dog' == "dog"

















Out[11]:






true	




















In [12]:



    


[[these
are on
different lines
]]

















Out[12]:






these
are on
different lines
	




















In [15]:



    


'the'..' strings '..'are together'

















Out[15]:






the strings are together	




















In [21]:



    


string.lower('A')

















Out[21]:






a	




















In [22]:



    


'abc'[0]





















[string "'abc'[0]..."]:1: unexpected symbol near ''abc''


















In [14]:



    


#'how many characters are in this string?'

















Out[14]:






39	


























Booleans
    

  • false and nil evaluate to False
    • 

  • Everything else is true
    • 

  • and and or return their arguments
      

    • and returns the first argument when that argument evaluates to false, otherwise returns the second arguemnt
      • 

    • or returns the first argument when that argument evaluates to true, otherwise returns the second argument 
      • 

    

    • 














In [23]:



    


4 and 5

















Out[23]:






5	




















In [24]:



    


4 or 5

















Out[24]:






4	




















In [25]:



    


false or 5

















Out[25]:






5	




















In [26]:



    


x = 4
y = 100
print((x > y) and x or y)
--False or y

















Out[26]:






100	


























Nil
    

  • Represents absence of a useful value (Programming in Lua)
    • 














In [27]:



    


type(nil)

















Out[27]:






nil	




















In [28]:



    


print(empty)

















Out[28]:






nil	




















In [29]:



    


nil + nil





















[string "local f = function() return nil + nil end; lo..."]:1: attempt to perform arithmetic on a nil value
stack traceback:
	[string "local f = function() return nil + nil end; lo..."]:1: in function 'f'
	[string "local f = function() return nil + nil end; lo..."]:1: in main chunk
	[C]: in function 'xpcall'
	/home/bryan/torch/install/share/lua/5.1/itorch/main.lua:179: in function </home/bryan/torch/install/share/lua/5.1/itorch/main.lua:143>
	/home/bryan/torch/install/share/lua/5.1/lzmq/poller.lua:75: in function 'poll'
	/home/bryan/torch/install/share/lua/5.1/lzmq/impl/loop.lua:307: in function 'poll'
	/home/bryan/torch/install/share/lua/5.1/lzmq/impl/loop.lua:325: in function 'sleep_ex'
	/home/bryan/torch/install/share/lua/5.1/lzmq/impl/loop.lua:370: in function 'start'
	/home/bryan/torch/install/share/lua/5.1/itorch/main.lua:350: in main chunk
	[C]: in function 'require'
	(command line):1: in main chunk
	[C]: at 0x00406670
























Control Statements
    

  • All control statements must be terminated, usually with an end
    • 

  • if , elseif , else , then are the keywords
    • 














In [32]:



    


x = 10
y = 20
z = 0

if x > y then
    print(x)
elseif z > y then
    print(z)
    
else 
    print(y)
    print(z)
end

















Out[32]:






20	
0	


























Looping
    

  • Two types of conditional loops while loops and repeat loops
    • 

  • repeat is equivalent to do while loops in many other languages
    • 

  • for comes in two variations, numeric ( traditional ) and generic (foreach)
    • 














In [33]:



    


x = 10
while x < 20 do
    print(x)
    x = x + 1
end

















Out[33]:






10	
11	
12	
13	
14	
15	
16	
17	
18	
19	




















In [2]:



    


x = 10
repeat
    print(x)
    x = x + 1
until x > 20

















Out[2]:






10	
11	
12	
13	
14	
15	
16	
17	
18	
19	
20	




















In [3]:



    


for x = 10, 20, 1 do
    print(x)
end

















Out[3]:






10	
11	
12	
13	
14	
15	
16	
17	
18	
19	
20