Creating a simple table with Lua tables C API

Question

I'm running a MySQL query which always returns 4 rows:

row->name, row->date, row->ip, row->custom

What I want to achieve is create a simple table basing on the above results so it would look like:

{
     "name" = result of row->name,
     "date" = result of row->date,
     "ip"   = result of row->ip,
     "custom" = result of row->custom
}

I have tried multiple possibilities, but the examples posted are really varied and I got a problems making it working.

My last unsuccessful try:

lua_createtable(L, 0, 4);
top = lua_gettop(L);
lua_pushstring(L, "name");
lua_pushstring(L, row->name);
lua_pushstring(L, "date");
lua_pushnumber(L, row->date);
lua_pushstring(L, "ip");
lua_pushstring(L, row->ip);
lua_pushstring(L, "custom");
lua_pushstring(L, row->custom);
lua_settable(L, top);

lua_settable() sets only 1 value and pops only 1 pair of key, value from stack. Repeat it 4. — Kamiccolo
– Kamiccolo, Commented Nov 22, 2013 at 14:58

Kamiccolo · Accepted Answer · 2025-09-29 10:57:25Z

22

As I mentioned in comment, lua_settable() takes care only of one key, value pair. Must repeat that if You need more.

I'd prefer saving the Lua stack space like this:

lua_createtable(L, 0, 4);

lua_pushstring(L, "name");
lua_pushstring(L, row->name);
lua_settable(L, -3);  /* 3rd element from the stack top */

lua_pushstring(L, "date");
lua_pushstring(L, row->date);
lua_settable(L, -3);

lua_pushstring(L, "ip");
lua_pushstring(L, row->ip);
lua_settable(L, -3);

lua_pushstring(L, "custom");
lua_pushstring(L, row->custom);
lua_settable(L, -3);

/* We still have table left on top of the Lua stack. */

Also, You could write some kind of C struct iterator or something.

NOTE: if this is for some kind of Lua wrapper - You should do it the standard way. In the following example applied @lhf comment about shortening it a bit:

int 
l_row_push(lua_State *L)
{
    lua_createtable(L, 0, 4); /* creates and pushes new table on top of Lua stack */

    lua_pushstring(L, row->name); /* Pushes table value on top of Lua stack */
    lua_setfield(L, -2, "name");  /* table["name"] = row->name. Pops key value */

    lua_pushstring(L, row->date);
    lua_setfield(L, -2, "date");

    lua_pushstring(L, row->ip);
    lua_setfield(L, -2, "ip");

    lua_pushstring(L, row->custom);
    lua_setfield(L, -2, "custom");

    /* Returning one table which is already on top of Lua stack. */
    return 1;
}

EDIT: Fixes usage of lua_setfield() by @lhf note. Thanks!

edited Sep 29 at 10:57

answered Nov 22, 2013 at 15:08

Kamiccolo

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7 Comments

Kamiccolo Over a year ago

@Lucas yeah, it might look tricky at the beginning. Overall - it's not that bad at all.

Lucas Over a year ago

Can I use the lua_newtable instead of the lua_createtable ?

Kamiccolo Over a year ago

@Lucas definitely. lua_createtable() saves some time of increasing memory allocated for the table each time inserting stuff. And recommended to be used. If size of table is not known, You're free to use lua_newtable.

Lucas Over a year ago

The code is compiling itself, but in Lua when I do print(variable["name"]); nothing is being printed.

Lucas Over a year ago

Nevermind, return 1 worked for it, just seen that its not going to return 4 tables, lol. Thank you very much!

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