c - Iterate through Lua Table

Question

I am trying to iterate through a lua table but I keep getting this error:

invalid key to 'next'

I know that index starts off as -8 and I know that there is a table there because it gets the first (and only) value in it. However, it tries to loop round again even though I know there is only one string in the table.

if (lua_istable(L, index))
{
    lua_pushnil(L);

    // This is needed for it to even get the first value
    index--;

    while (lua_next(L, index) != 0)
    {
        const char *item = luaL_checkstring(L, -1);
        lua_pop(L, 1);

        printf("%s
", item);
    }
}
else
{
    luaL_typerror(L, index, "string table");
}

Any help would be appreciated.

This works fine when I use a positive index (as long as I don't remove 1 from it)

Edit: I've noticed that I don't get this error if I leave the value of item alone. Only when I start reading the value of item do I get this error. When I've got the value from the table, I call another Lua function, could this be disrupting lua_next?

Answer 1

There are 2 things you need to watch:

• ensure the original key is left on the stack before the next call to lua_next. luaL_checkstring will convert non-string keys to strings (as the resulting string is not in the table, it becomes an invalid key.) This is most easily done by passing luaL_checkstring a copy of the key instead of the original.
• ensure you preserve the stack structure (i.e. pop as many values as you push) on each pass through the loop

Your function will only work for negative values of index. You are correct that index--; will ensure that index still points to the table after pushing the key, but only if index was negative (i.e. relative to the top of the stack.) If index is an absolute or pseudo index then it this will cause it to point to the wrong item. The easiest workaround is to push another reference to the table onto the top of the stack.

Here's a minimal C program to demonstrate:

#include <lauxlib.h>
#include <lua.h>

static void iterate_and_print(lua_State *L, int index);

int main(int ac, char **av)
{
   lua_State *L = luaL_newstate();
   luaL_openlibs(L);

   // Create a table and put it on the top of the stack
   luaL_loadstring(L, "return {one=1,[2]='two',three=3}");
   lua_call(L, 0, 1);

   iterate_and_print(L, -1);
   return 0;
}

static void iterate_and_print(lua_State *L, int index)
{
    // Push another reference to the table on top of the stack (so we know
    // where it is, and this function can work for negative, positive and
    // pseudo indices
    lua_pushvalue(L, index);
    // stack now contains: -1 => table
    lua_pushnil(L);
    // stack now contains: -1 => nil; -2 => table
    while (lua_next(L, -2))
    {
        // stack now contains: -1 => value; -2 => key; -3 => table
        // copy the key so that lua_tostring does not modify the original
        lua_pushvalue(L, -2);
        // stack now contains: -1 => key; -2 => value; -3 => key; -4 => table
        const char *key = lua_tostring(L, -1);
        const char *value = lua_tostring(L, -2);
        printf("%s => %s
", key, value);
        // pop value + copy of key, leaving original key
        lua_pop(L, 2);
        // stack now contains: -1 => key; -2 => table
    }
    // stack now contains: -1 => table (when lua_next returns 0 it pops the key
    // but does not push anything.)
    // Pop table
    lua_pop(L, 1);
    // Stack is now the same as it was on entry to this function
}