How to create nested Lua tables using the C API

c++, lua, lua-api, lua-table

Solution

Here's a full and minimal program demonstrating how to nest tables. Basically what you are missing is the `lua_setfield` function.

#include <stdio.h>
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"

int main()
{
    int res;
    lua_State *L = lua_open();
    luaL_openlibs(L);

    lua_newtable(L); /* bottom table */

    lua_newtable(L); /* upper table */

    lua_pushinteger(L, 4);
    lua_setfield(L, -2, "four"); /* T[four] = 4 */
    lua_setfield(L, -2, "T");  /* name upper table field T of bottom table */
    lua_setglobal(L, "t"); /* set bottom table as global variable t */

    res = luaL_dostring(L, "print(t.T.four == 4)");
    if(res)
    {
        printf("Error: %s\n", lua_tostring(L, -1));
    }

    return 0;
}

The program will simply print `true`.

If you need numeric indices, then you continue using `lua_settable`:

#include <stdio.h>
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"

int main()
{
    int res;
    lua_State *L = lua_open();
    luaL_openlibs(L);

    lua_newtable(L); /* bottom table */

    lua_newtable(L); /* upper table */

    lua_pushinteger(L, 0);
    lua_pushinteger(L, 4);
    lua_settable(L, -3);  /* uppertable[0] = 4; pops 0 and 4 */
    lua_pushinteger(L, 0);
    lua_insert(L, -2); /* swap uppertable and 0 */
    lua_settable(L, -3); /* bottomtable[0] = uppertable */
    lua_setglobal(L, "t"); /* set bottom table as global variable t */

    res = luaL_dostring(L, "print(t[0][0] == 4)");
    if(res)
    {
        printf("Error: %s\n", lua_tostring(L, -1));
    }

    return 0;
}

Rather than using absolute indices of 0 like I did, you might want to use `lua_objlen` to generate the index.

Problem

I want to create a table like ``` myTable = { [0] = { ["a"] = 4, ["b"] = 2 }, [1] = { ["a"] = 13, ["b"] = 37 } } ``` using the C API? My current approach is ``` lua_createtable(L, 0, 2); int c = lua_gettop(L); lua_pushstring(L, "a"); lua_pushnumber(L, 4); lua_settable(L, c); lua_pushstring(L, "b"); lua_pushnumber(L, 2); lua_settable(L, c); ``` to create the inner tables in a loop. Before, this loop, I use ``` lua_createtable(L, 2, 0); int outertable = lua_gettop(L); ``` to create the outer table for 2 numeric slots. But how can I save the inner tables to the outer table?

Original source