Re: Eval a Lua expression

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

On Apr 2, 2008, at 11:43 PM, Shmuel Zeigerman wrote:

> Eugen-Andrei Gavriloaie wrote:
> > Is there a method of evaluating an expression in the present lua  
> > context? Here is what I want:
> > lua_evaluate_expression(L,"applications[2].type"); // expect to  
> > find `mysql` string pushed into the stack after this call
> > or
> > lua_evaluate_expression(L,"#applications"); // expect to find value  
> > of 2 pushed into the stack after this call
> > This is supposed to push the result of the expression into the stack.
>
> The following is not very efficient, but seems to do what you want
> (not tested; error checks to be added):
>
> void lua_evaluate_expression(lua_State *L, const char *expr) {
>  lua_pushfstring(L, "return %s", expr);
>  luaL_loadstring(L, lua_tostring(L,-1));
>  lua_remove(L,-2);
>  lua_pcall(L,0,1,0);
> }
> --
> Shmuel


Thank you for your quick response but I have 3 questions:

1.

lua_pushfstring(L, "return %s", expr);
and
lua_remove(L,-2);

Are used only for appending `return ` in front of the expression, right?

2.

The lua_State *L will be reused (I need to read many things), so the  
end result script from memory will look like this after this sequence  
of calls from c++

--begin C++ code--
	lua_evaluate_expression(L,"#applications");
	...
	lua_evaluate_expression(L,"applications[1].database.name");
	...
--end C++ code--



--begin lua script represented from lua_State *L point of view --

	... (original code from the file that was loaded from the very  
beginning)

	return #applications --this was added by the first call of  
lua_evaluate_expression

	return  applications[1].database.name --this was added by the second  
call

--end lua script represented from lua_State *L point of view --

So, on the second lua_evaluate_expression call from C++ I will have a  
value of 2 instead of the string 'mysql' pushed on the stack because  
the entire script execution stops at the first return. Am I right?


3. I think the entire script will be executed entirely on each  
lua_evaluate_expression because of lua_pcall(L,0,1,0). This is not  
acceptable because I want to use this approach on other occasions.  
Here is ok, because we have to deal with a configuration file, but I  
will definitively have to deal with other types of Lua scripts that  
are not supposed to run over and over again. Am I right?