Accessing Lua global tables with C++ - c++

Question

How can I access a global table that already exists in Lua using C++ ?
Below are code which I tried. I tried creating a global variable and try modifying a that local to the local in Lua but things dont seem to work
lua_State *lua_state = luaL_newstate();
luaL_openlibs(lua_state);
// lua_createtable(lua_state, 0, 81);
// for (int i = 1; i <= 81; i++)
// {
// lua_pushnumber(lua_state, i);
// lua_pushnumber(lua_state, grid_[i - 1]);
// lua_settable(lua_state, -3);
// }
//
// lua_setglobal(lua_state, "arg");
// lua_createtable(lua_state, 81, 1);
//
// for (int i = 1; i <= 81; i++)
// {
// lua_pushnumber(lua_state, i);
// lua_pushnumber(lua_state, grid_[i - 1]);
// lua_settable(lua_state, -3);
// }
// lua_setglobal(lua_state, "arg");
luaL_loadfile(lua_state, "main.lua");
lua_call(lua_state, 0, 0);
int t = 2;
/* table is in the stack at index 't' */
lua_pushnil(lua_state); /* first key */
while (lua_next(lua_state, t) != 0) {
/* uses 'key' (at index -2) and 'value' (at index -1) */
printf("%s - %s\n",
lua_typename(lua_state, lua_type(lua_state, -2)),
lua_typename(lua_state, lua_type(lua_state, -1)));
/* removes 'value'; keeps 'key' for next iteration */
lua_pop(lua_state, 1);
}
Lua
problem =
{
{9, 0, 0, 1, 0, 0, 0, 0, 5},
{0, 0, 5, 0, 9, 0, 2, 0, 1},
{8, 0, 0, 0, 4, 0, 0, 0, 0},
{0, 0, 0, 0, 8, 0, 0, 0, 0},
{0, 0, 0, 7, 0, 0, 0, 0, 0},
{0, 0, 0, 0, 2, 6, 0, 0, 9},
{2, 0, 0, 3, 0, 0, 0, 0, 6},
{0, 0, 0, 2, 0, 0, 9, 0, 0},
{0, 0, 1, 9, 0, 4, 5, 7, 0},
}
Update 1
int main()
{
lua_State *lua_state = luaL_newstate();
luaL_openlibs(lua_state);
luaL_loadfile(lua_state, "main.lua");
lua_getglobal(lua_state, "problem");
//lua_pushglobaltable(lua_state); // Get global table
lua_pushnil(lua_state); // put a nil key on stack
while (lua_next(lua_state, -2) != 0) { // key(-1) is replaced by the next key(-1) in table(-2)
std::string name = lua_tostring(lua_state, -2); // Get key(-2) name
std::cout << name << std::endl;
lua_pop(lua_state, 1); // remove value(-1), now key on top at(-1)
}
lua_pop(lua_state, 1); // remove global table(-1)
lua_call(lua_state, 0, 0);
return 0;
}
Lua
problem =
{
{9, 0, 0, 1, 0, 0, 0, 0, 5},
{0, 0, 5, 0, 9, 0, 2, 0, 1},
{8, 0, 0, 0, 4, 0, 0, 0, 0},
{0, 0, 0, 0, 8, 0, 0, 0, 0},
{0, 0, 0, 7, 0, 0, 0, 0, 0},
{0, 0, 0, 0, 2, 6, 0, 0, 9},
{2, 0, 0, 3, 0, 0, 0, 0, 6},
{0, 0, 0, 2, 0, 0, 9, 0, 0},
{0, 0, 1, 9, 0, 4, 5, 7, 0},
}
print("Lua Works")
user_input = io.read();

Answer 1

You don't have any values to iterate on Lua stack.
That int t=2; doesn't reflect anything, and your script doesn't return values to be left on stack.
See PIL book: 25.1 – Table Manipulation for examples on accessing global table.