Re: Nesting tables from C++ to Lua

[Thomas Tongue <ttongue@...>]

On Friday, January 23, 2004, at 11:21 AM, Virgil Smith wrote:

> Your code....
>
>     lua_pushstring(state,"a1");
>     lua_gettable(state,LUA_GLOBALSINDEX);
>     lua_pushstring(state,"a2");
>     lua_gettable(state,LUA_GLOBALSINDEX);
>     lua_settable(state,LUA_GLOBALSINDEX);
>
> Implies to me that you missed the fact that tables in Lua are assigned 
> by
> reference in Lua.  The above code results in a Global variable whose 
> "name"
> is the vector referenced by "a1".  It does NOT overwrite the vector a1 
> with
> the contents of the vector a2.  That statement might confuse the
> uninitiated.  Remember that global variables are merely entries in a 
> Lua
> table and that Lua tables may contain entries whose keys are "any" 
> value,
> not just strings and numbers.  Thus a settable operation given a table 
> for
> the key and a table for the value will NOT overwrite the key table it 
> will
> use the table AS A KEY.
>
> The following example is a bit simpler....
>
>     lua_pushstring(state,"a1");
>     lua_pushstring(state,"a2");
>     lua_gettable(state,LUA_GLOBALSINDEX);
>     lua_settable(state,LUA_GLOBALSINDEX);

<smacks forehead> Right. I read that, and then promptly forgot about 
it. This works fine.

However, I must still be missing something because building a nested 
table still does not work. Here is some sample code (assume that a1 and 
a2 are properly defined tables):

   lua_pushstring(state,"nested"); -- name for the nested table
   lua_newtable(state);
   lua_pushstring(state,"bz");
   lua_pushstring(state,"a1");
   lua_gettable(state,LUA_GLOBALSINDEX);
   lua_settable(state,-3);
   lua_pushstring(state,"by");
   lua_pushstring(state,"a2");
   lua_gettable(state,LUA_GLOBALSINDEX);
   lua_settable(state,-3);
   lua_pushliteral(state,"n");
   lua_pushnumber(state,2);
   lua_rawset(state,-3);
   lua_settable(state,LUA_GLOBALSINDEX); -- Assign the new table to 
"nested"

What I think this should do in Lua is:

   nested = {bz=a1, by=a2}

where a1 and a2 are predefined tables (globals). HOWEVER, the following 
*does* work:


   lua_pushstring(state,"nested"); -- name for the nested table
   lua_newtable(state);
   lua_settable(state,LUA_GLOBALSINDEX);
   lua_pushstring(state,"nested");
   lua_gettable(state,LUA_GLOBALSINDEX);
   lua_pushstring(state,"bz");
   lua_pushstring(state,"a1");
   lua_gettable(state,LUA_GLOBALSINDEX);
   lua_settable(state,-3);
   lua_pushstring(state,"by");
   lua_pushstring(state,"a2");
   lua_gettable(state,LUA_GLOBALSINDEX);
   lua_settable(state,-3);

Conceptually, I don't see how the two samples are different. I feel 
like I've found an acceptable work-around, but I'm missing something 
important in not understanding the difference in approaches. Thoughts?


=====================================================
Thomas Tongue <TTongue@imagiware.com>
Imagiware Inc. - http://www.imagiware.com/
Internet Presence and Consulting Services
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