Pragmatic Lua Basics in 30 Minutes

23 Mar 2013

I recently learnt Lua. By saying “recently” I actually mean two days ago. I did not learn it for any projects or work - I learnt it for fun. So I spent some time googling and playing around with different stuff. Lua is probably best known as the “WoW script language”, although Lua’s application extends far more than that.

However, Lua is not considered as a mainstream language and it does not have hype of some languages, say Ruby, so there are not a lot of tutorials, especially the quick introduction of the language for programmers of other language. So I decide to write one. I have to say I am by no means a Lua expert (You probably figure that out already), so please correct me for if I make any mistake. Also, this is an pragmatic introduction to the language for (experienced) programmers. So I am not gonna spend time on programming basics.

Data Types

Lua has 8 basic data types: nil, boolean, number, string, function, table, userdata, thread. The first five types are common across lots of languages. You can use type(variable) to get the type of a variable (returned in string). There is no object in Lua.

• Lua booleans are true or false, lower case. Only false and nil are regarded as false, other values (like 0) are regarded as true.
• Lua numbers are floating points (double). There is no integer. So what looks like integer is actually represented by floating points, which is similar to Javascript
• String can be quoted in either 'single quote' or "double quote". You can create literal strings by using [[]], which kind of like """ in Python. String is immutable in Lua, you can concat string using .. but not +. When you try to do 1 + "10", Lua will try to cast string to number.

Operators and Assignment

Most of them are pretty “standard” but there are several interesting features.

• Logical operators: and or not. You cannot use && || !. Both and and or are use short-cut evaluation.
• Mathematical operators: + - * /
• Relational operators: < > <= >= == ~=. The difference is ~=, which is != in most other languages. Tables, userdata and functions are compared by reference.
• Assignment is done via =. An interesting feature in Lua is multiple assignment, which is like a, b = 0, 1. In case you write a, b, c = 0, 1, c will be nil. There is no syntax sugar like a++ or a += 1

Variable, Functions and Control Structure

Variables declared like this a = 1 are global. To scope a variable, use local a = 1, which is similar to var a = 1 behavior in JavaScript. In Lua, semicolons are optional, as in JavaScript. However, the convention is to omit the semicolon.

If-else syntax is like this. There is no switch statement.

if cond1 then
	...
elseif cond2 then
	...
elseif cond3 then
	...
else
	...
end

Define function like this. Anonymous function is supported and since function is a data type, you can assign it to a variable, very much like JavaScript. A function can return multiple value can have variable number of arguments

function func_name(a, b, c)
	return a, b
end

-- Anonymous function. And yes, I am a comment
function (a, b, c)
	...
end

There are three types of loop

while cond do
	...
end

-- Repeat until the condition is true.
repeat
	...
until cond

-- Numeric for, step can be ignored, which will default to 1. There is another generic for, which will be introduced later
for var=from,to,step do
	...
end

One more thing about variable scope: local variables are local to a block, which can be a function body, a control statement body or a file. There are return and break. They can only be the last statement of a block (Lua 5.2 allows break in the middle)

Tables

Table is the only built-in data structure. If you have done PHP programming (unfortunately I have), it is very similar to PHP’s array - it’s a mixture of numerical array (list, sequence) and associative array (dictionary, map, hash). To construct a new table, you can

local t1 = {"apple", "orange"}
local t2 = {1="apple", 2="orange"}
local t3 = {[1]="apple", [2]="orange"}

The above three are equivalent. Note that table index start at 1 by default, which is different from most other languages. If you construct a table with string as key, it will become an associative array

local t2 = {one="apple", two="orange"}
local t3 = {["one"]="apple", ["two"]="orange"}

These two are equivalent, but the second form allows keys to be any string while the first form only allows keys which are valid identifier.

To loop through a table with integer keys, you can do

for i = 1, #t do
	print(t[i])
end

# is a very handy way to get the length of a table or string.

To loop through a table with string keys, you have to do

for key,value in pairs(t) do
	print(key .. value)
end

ipairs is a built in iterators that can help you iterate through a table.

That’s it. All the important features and facts are here. The language is very lightweight and easy to learn. In my next post (I promise), I will cover some of the “advanced” topics like functions as closures, use table for object oriented programming, error handling and some built in standard libraries - these are pretty common tasks in writing “real” programs. After that, hopefully you should be able to make full use of Lua.

As always, please discuss on HN and correct me for any mistake.