Reaching a table element - . and [] differs?

[Attila <ejjeliorjarat@...>]

Hello!

I am working on a little project, where the C code passes to Lua some  
parameters, then I build the basic architecture from it. The type of the  
passed value is lightuserdata as I see, and when I used  
print(tostring(id)) it wrote: userdata: <address>. I use this as a table  
field to store the built table. It works well, but I experience some  
strange thing. When I want to select a table, I use this:

function get(id) -- id is that lightuserdata that C core passes
	for a in pairs(t) do -- I store my tables in t
		if a == id then -- Checked, they are the same
			return t.id
		end
	end
end

No matter if I use return t.id or t.a, the returning value is nil. After  
that I used return t[id], and strange, that works perfectly. Can somebody  
say me, what is the difference between . and []? Because the reference  
manual says:
"The language supports this representation by providing a.name as  
syntactic sugar for a["name"]. There are several convenient ways to create  
tables in Lua (see §2.5.7)."

Thanks: Attila