Re: traversing nested tables in C

["Grisha" <grisha@...>]

Hi, Tassilo

When you traverse nested table, you call walk(L, -1), so your walk function
does this:

     void walk (L, -1) {
         lua_pushnil(L);    // now nil is at -1 position !!!!

         printf("idx=%i\n", -1);
         while (lua_next(L,  -1) != 0) { // ERROR: iterating nil value
             print_pair(L);
             lua_pop(L, 1);
         }
     }

Best Regards,
Grisha


----- Original Message ----- 
From: "Tassilo von Parseval" <tassilo.von.parseval@rwth-aachen.de>
To: "Lua list" <lua@bazar2.conectiva.com.br>
Sent: Wednesday, June 16, 2004 11:44 AM
Subject: traversing nested tables in C


> Hi there,
>
> I am currently experiencing unpleasant segfaults when recursively
> traversing Lua tables. This is the Lua function whose return value I'd
> like to traverse:
>
>     function foo(a)
>         local table = { x="a", y="a", z="a"}
>         local table2 = { 1, 2 }
>         table["4"] = table2
>         return table
>     end
>
> The corresponding C-code:
>
>     #define KEY -2
>     #define VAL -1
>
>     void walk (lua_State *, int);
>
>     void
>     print_pair (lua_State *L) {
>
>         switch (lua_type(L, KEY)) {
>             case LUA_TNUMBER:
>                 Warn("key: %i\n", lua_tonumber(L, KEY));
>                 break;
>             case LUA_TSTRING:
>                 Warn("key: %s\n", lua_tostring(L, KEY));
>         }
>
>         switch (lua_type(L, VAL)) {
>             case LUA_TNUMBER:
>                 printf("val: %i (number)\n", lua_tonumber(L, VAL));
>                 break;
>             case LUA_TSTRING:
>                 printf("val: %s (string)\n", lua_tostring(L, VAL));
>                 break;
>             case LUA_TTABLE:
>                 walk(L, VAL);
>                 break;
>             default:
>                 Warn("val: other\n");
>         }
>     }
>
>     void walk (lua_State *L, int idx) {
>         lua_pushnil(L);
>
>         printf("idx=%i\n", idx);
>         while (lua_next(L, idx) != 0) {
>             print_pair(L);
>             lua_pop(L, 1);
>         }
>     }
>
> This yields the output:
>
>     idx=1
>     key: y
>     val: a (string)
>     key: x
>     val: a (string)
>     key: z
>     val: a (string)
>     key: 4
>     idx=-1
>     key: 0
>     val: 0 (number)
>     Segmentation fault (core dumped)
>
> Are there some special steps I have to take when descending into a
> nested table thusly? If so, I suppose it would have to happen in the
> LUA_TTABLE case of print_pair(), but I don't know which.
>
> Thanks for any help,
> Tassilo
> -- 
>
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>