Re: Another table question...

[Eugen-Andrei Gavriloaie <shiretu@...>]

On Apr 9, 2008, at 9:42 PM, Alex Davies wrote:

> ----- Original Message ----- From: "Eugen-Andrei Gavriloaie"
>
>
> > The table might have keys 1,2,3,55,34,22,"sdfds",etc. This table is  
> > returned by calling a lua function (is on the C++ stack). After  
> > doing some c++ processing I'd like to add more "anonymous" values  
> > (with no keys) to the table and pass it as a parameter to anoter  
> > lua function.  So, I have no guarantee that the key is not in use. :(
>
> Um, just so we're clear, the table: {1, 2, 3, 55, 34, 22}
> does not have any "anonymous" values. It's merely a short hand way  
> of writing:
> { [1] = 1, [2] = 2, [3] = 3, [4] = 55, [5] = 34, [6] = 22 }
> As in Lua there is no such thing as a value in a table without a key.
Yes you are right. But here is an example of an unwanted value  
overwrite:
1. Lua calls a c++ with the following argument:  
{"first_anon_value","second_anon_value",false,22}. I know that  
internally this table is represented like this:
	1=> first_anon_value
	2=> second_anon_value
	3=> false
	4=> 22
1,2,3 and 4 key values are computed by lua internally (I really don't  
know the algorithm behind the scene)


2. the c++ function is making some computing and decide that row  
number 2 must be deleted. So, internal representation is now like this:
	1=> first_anon_value
	3=> false
	4=> 22


3. We now try to add a new value using your method. Let this value be  
the string "this is a string". lua_objlen(L, -2) + 1 evaluates to 4 in  
this case.
I think (not sure) that after lua_rawset the table will look like this:
	1=> first_anon_value
	3=> false
	4=> this is a string
As you can see 22 was overwritten with "this is a string".

Extract from the docs:

--begin cut--
lua_rawseti

[-1, +0, m]

void lua_rawseti (lua_State *L, int index, int n);

Does the equivalent of t[n] = v, where t is the value at the given  
valid index and v is the value at the top of the stack.

This function pops the value from the stack. The assignment is raw;  
that is, it does not invoke metamethods.
--end cut--

So the question is: n denotes a position or a key?

> How would you retrieve such a value?
>
>
> (Your wording was a little vague).
>
> So to add more values, simply use the code provided earlier  
> (lua_rawseti(L, -2, lua_objlen(L, -2) + 1)).
> See, t[#t+1] is always nil, by definition. So you're always safe to  
> add more values there.
>
> (If you mean that want you want a way of attaching additional info  
> to a table without modifying the table in anyway, that's a separate  
> problem that can be solved by weak tables).
>
> - Alex