Re: C API question - modifing table

[Michael Roth <mroth@...>]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Dmitry Samersoff wrote:

| I can add new value to table before iterate it,
| but code below doesn't work where is my mistake ?

There are three possible problems:

|  /* table is in the stack at index `t' */

a.) Maybe, your stack index 't' is a relative index. This won't work
with your loop. Make sure, 't' is absolute.

|     lua_pushnil(L);  /* first key */
|     while (lua_next(L, t) != 0)
|     {
|          /* `key' is at index -2 and `value' at index -1 */
|         const char *key = lua_tostring(L, -2);

b.) The Lua manual states: "While traversing a table, do not call
lua_tostring directly on a key, unless you know that the key is actually
a string. Recall that lua_tostring changes the value at the given index;
this confuses the next call to lua_next."

|         const char *val = lua_tostring(L, -1);
|         cerr << "Key: " << key << " Val:" << val << endl;
|         lua_pushstring(L,key);

c.) You fill up the stack because you leave every key value pair on the
stack. The Lua manual states: "When you interact with Lua API, you are
responsible for controlling stack overflow."
Either, pop the old value and don't push a new key, or make sure the
stack will grow using lua_checkstack() before you push a new key.


An example of your loop that should work. It will keep the key and
values on the stack.

~      /* somewhere in your code, push your table and
~         get the absolute index of it after you pushed it */
~      int t = lua_gettop(L);

~      /* first key */
~      lua_pushnil(L);

~      /* iterate, leave k,v pairs on stack */
~      while (lua_next(L, t))
~      {
~        /* make sure the stack is large enought for the
~           next iteration and push a copy of the unmodified
~           key for usage in the next iteration */
~        lua_checkstack(L, 2);
~        lua_pushvalue(L, -2);

~        /* convert the key and value to a string. this is
~           save because we copied the key earlier to the top
~           of the stack */
~        const char *key = lua_tostring(L, -3); /* Note: index changed */
~        const char *val = lua_tostring(L, -2);
~        cerr << "Key: " << key << " Val:" << val << endl;
~      }

~      /* If the loop terminates, lua_next() popped the copy of
~         the last key. Only key value pairs are left on the stack. */


The above code is untested. Maybe it contains errors and bugs. ;-)




-----BEGIN PGP SIGNATURE-----
Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org

iD8DBQFBGfvWSIrOxc3jOmoRApdbAJ0fD0gaLqn4nNBIMnK7Cns06PQ3UgCgvnEt
q3XB1UgM5znCmqUGUjHKL4w=
=V1kV
-----END PGP SIGNATURE-----