Accessing Lua Tables from C

• Tables in Lua are accessed through the stack
  • Generally a Lua file is loaded and then the stack accessed

The rules for accessing a table are as follows:
  1\. The assumption is that the table is global in Lua and can be accessed using
     lua_getglobal(L, tablename);
  2\. lua_getglobal will cause the table to be put on the stack
  3\. test that the top element on the stack is a table
  4\. push a table key onto the stack, ends up on the stack top
  5\. do a lua_gettable which will take the key and access the table
  6\. do a lua_pop of the top element to remove the key
  7\. do the same thing for other keys
```

_An Example of accessing a table in Lua through the Stack_

```

#include 
#include 
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"

void load (lua_State *L, const char *fname, int *red, int *green, int *blue);
int getcolorfield(lua_State *L, const char *key);

#define MAX_COLOR 255
int main(void) {
    lua_State *L = luaL_newstate();   /* opens Lua */
    luaL_openlibs(L);             /* opens the standard libraries */
    char filename[] = "C:/Lua C and C++/workspace/configTable.lua";
    int red = 0;
    int green = 0;
    int blue = 0;
    load(L, filename, &red, &green, &blue);
    printf("red is %d\n", red);
    printf("green is %d\n", green);
    printf("blue is %d", blue);
    return 0;
}

void load (lua_State *L, const char *fname, int *red, int *green, int *blue) {
    if (luaL_loadfile(L, fname) || lua_pcall(L, 0, 0, 0)) {
        luaL_error(L, "cannot run config file: %s", lua_tostring(L, -1));
    }
    lua_getglobal(L, "background");
    if (!lua_istable(L, -1)) {
            luaL_error(L, "'background' is not a table\n");
    }
    int r = getcolorfield(L, "r");
    int g = getcolorfield(L, "g");
    int b = getcolorfield(L, "b");

    *red = r;
    *green = g;
    *blue = b;
}

int getcolorfield(lua_State *L, const char *key) {
    int result;
    lua_pushstring(L, key);
    lua_gettable(L, -2);
    if (!lua_isnumber(L, -1)) {
        luaL_error(L, "invalid component in background color");
    }
    result = (int)(lua_tonumber(L, -1) * MAX_COLOR);
    lua_pop(L, 1);
    return result;
}

```

_The Lua file with the table_

```

background = {r=0.30, g=0.10, b=0}
BLUE = {r=0, g=0, b=1.0}
background1 = BLUE
```