The LUA 5.1 Programming Language

===Intro===
WoW add-ons are developed using the LUA 5.1 programming language: {BR}{BR}[http://www.lua.org/manual/5.1/|LUA 5.1 Manual] {BR}
[http://lua-users.org/files/wiki_insecure/users/thomasl/luarefv51.pdf|A Condensed Summary]
Of course, the LUA language itself does not implement the WoW API, which uses an event-driven model, so simply reading the manual will not help you to write that killer add-on.  But, you've got to crawl before you can run, so let's dive into some of the syntax features of LUA itself.
===Reserved Keywords===
A lot can be learned about a language just by knowing what words are reserved keywords (and thus, cannot be used as variable names).
{{{{and       break     do        else      elseif
end       false     for       function  if
in        local     nil       not       or
repeat    return    then      true      until   while}}}}
===Comment===
{{{{<nowiki>-- This is a single line comment
--[[ This is a block
     of comment text ]]</nowiki>}}}}
===Arithmetic Operators===
{{{{<nowiki>+  addition        1 + 2 => 3
-  substraction    2 - 1 => 1
*  multiplication  3 * 4 => 12
/  division        9 / 3 => 3
%  modulo          8 % 6 => 2
^  exponent        2 ^ 5 => 32</nowiki>}}}}
===Relational Operators===
{{{{== equality  
~= inequality
<  less-than
<= less-than or equal
>  greater-than
>= greater-than or equal}}}}
===Logical Operators===
'''false''' and '''nil''' are both considered '''false''', everything else is '''true'''.  Assigning the results of a logical operation to a variable might give some unexpected results (stated below).
{{{{and  (returns first argument if false/nil, otherwise returns second argument)
or   (returns the first argument if not false/nil, otherwise returns the second argument)
not  (returns true or false, representing the opposite of the supplied argument)
10  or  20  --> 10
10  and 20  --> 20
nil or  10  --> 10
nil and 10  --> nil}}}}
===Variable Assignment===
{{{{a = 1
a, b = 1, 2        Multiple assignment at the same time
a, b = b, a        Variable Swap
_, _, a = 1, 2, 3  Common usage of "throw away" variables}}}}
===Literals===
{{{{aString = 'single quotes work'
bString = "double quotes work"
cString = <nowiki>[[hell, even double square brackets work]]</nowiki>
dNumber = 12345.6789}}}}
===Concatenation===
The double-dot (..) represents the concatenation operator.
{{{{name = "Jason"
print("Hello, "..name..", it is very good to meet you!")
results:
Hello, Jason, it is very good to meet you!}}}}
===Length Operator===
The pound-sign (#) preceeding a variable returns the length of that variable.  If the variable is a string, then the returned value is the number of characters.  If the variable is a table, then the returned value is the highest index number such that the table element at the next higher index is nil.
{{{{name = "Jason"
print(#name)
results:
5}}}}
===IF Statements===
{{{{if i == 1 then
   print("One")
elseif i==2 then
   print("Two")
else
   print(i)
end}}}}
===FOR Statements===
{{{{<nowiki>for i = 1, 5, 1   -- start, limit, increment
do
   print(i.."\n")
end
results:
1
2
3
4
5</nowiki>}}}}
===WHILE and REPEAT Statements===
{{{{<nowiki>i = 3
while i > 1
do
   print(i.."\n")
   i = i - 1
end
repeat
   print(i.."\n")
   i = i + 1
until i = 3
results:
3
2  <-- last output of the WHILE
1  <-- first output of the REPEAT
2</nowiki>}}}}
===Functions===
{{{{function add (input1, input2)
  return input1 + input2
end
(equivalent to)
add = function(input1, input2)
   return input1 + input2
end
function MultiValue ()
   return 1, 2, 3, 4
end
i = add(1, 2)
print(i)                      result => 3
_, _, a, b = MultiValue()
print(a..","..b)              result => 3,4
}}}}
===Tables===
A table is very much like a Dictionary in other languages.  By default, tables use a one-based index as the key:
{{{{<nowiki>a = {101, 202, 303, 404, 505}
print(a[2])                   result => 202</nowiki>}}}}
Instead of accepting the default key, though, it can be specified.  Any unspecified key names will assume the next available one-based integer index:
{{{{<nowiki>a = {[3] = 101, [4] = 202, 303, 404, fifth = 505}
print(a[2])                   result => 404
print(a[3])                   result => 101
print(a.fifth)                result => 505</nowiki>}}}}
The square bracket notation for key assignment substitutes the value inside the brackets (either a literal or a variable's value):
{{{{<nowiki>name = "Fred"
a = {[name] = "Flintstone", name = "Barney"}
print(a[name])                result => Flintstone
print(a.name)                 result => Barney
print(a.Fred)                 result => Flintstone   
</nowiki>}}}}
Note that in this sense, table "properties" are indeed case sensitive, since they are just syntactic sugar for a string-based key:
{{{{<nowiki>a = { }
a.Name = "Jason"
is equivalent to:
a = { ["Name"] = "Jason" }
but not:
a = { ["name"] = "Jason" }
</nowiki>}}}}
''(work in progress)''
Jason Follas.com

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The LUA 5.1 Programming Language
