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Page History: The LUA 5.1 Programming Language


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Page Revision: 2007/09/18 14:38


Intro

WoW add-ons are developed using the LUA 5.1 programming language:

LUA 5.1 Manual
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

-- This is a single line comment

--[[ This is a block
     of comment text ]]

Arithmetic Operators

+  addition        1 + 2 => 3
-  substraction    2 - 1 => 1
*  multiplication  3 * 4 => 12
/  division        9 / 3 => 3
%  modulo          8 % 6 => 2
^  exponent        2 ^ 5 => 32

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"
cNumber = 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

for i = 1, 5, 1   -- start, limit, increment
do
   print(i.."\n")
end

results:

1
2
3
4
5

WHILE and REPEAT Statements

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

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:

a = {101, 202, 303, 404, 505}

print(a[2])                   result => 202

Instead of accepting the default key, though, it can be specified. Any unspecified key names will assume the next available one-based integer index:

a = {[3] = 101, [4] = 202, 303, 404, fifth = 505}

print(a[2])                   result => 404
print(a[3])                   result => 101
print(a.fifth)                result => 505

The square bracket notation for key assignment substitutes the value inside the brackets (either a literal or a variable's value):

name = "Fred"
a = {[name] = "Flintstone", name = "Barney"}

print(a[name])                result => Flintstone
print(a.name)                 result => Barney
print(a.Fred)                 result => Flintstone   

Note that in this sense, table "properties" are indeed case sensitive, since they are just syntactic sugar for a string-based key:

a = { }
a.Name = "Jason"

is equivalent to:

a = { ["Name"] = "Jason" }
(work in progress)

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