Re: Table traversal

[Roberto Ierusalimschy <roberto@...>]

> In Lua 3.2, the lua.h file contains a lua_next function.  This looks like
> the Lua next function for traversing tables, but it's API isn't very
> clear.  (I can't find it in the docs.)

We put this function, but we are still not sure whether we will keep it;
that's why it has no docs.

Its typical use is something like

  lua_Object table = getyourtable();
  int i = 0;
  do {
    lua_beginblock();
    i = lua_next(table, i);
    if (i) {
      lua_Object index = lua_getresult(1);
      lua_Object value = lua_getresult(2);
      ...  /* use index-value for whatever you need */
    }
    lua_endblock();
  } while (i);

(warning: a common bug is to change the if to "if (!i) break;" and then
do not call the last "lua_endblock"). That is, "i" is an index with
only internal significance, that must start with 0, be passed from one
call to the next, and when it is 0 the traverse is over. For each call
to lua_next with return not 0, the results (index and value) are in lua2C.

With that code, you can traverse a table in an efficient way, without
the problem of how to keep the indices from one iteration to the next
accross different blocks.

-- Roberto
PS: the reason we are not very happy with this API is that it is not easy
to implement the builtin "next" function using "lua_next" :-(