Writing Iterator in C

[Ralph Hempel <rhempel@...>]

I have a C function that I would like to rewrite as an
iterator, but I'm having great difficulty in reproducing
the results that I get with my pure Lua test program.

I've read the appropriate chapters in PiL and the reference manual..

To test my understanding (or lack thereof) I
wrote a little test harness in pure Lua, like so:

This function will return the string x and a true or false flag
depending on wether the length of the string in x is even or odd.
n is always nil and the function returns nil when the length of
the string is 10 or greater.

function fooIter(n,x)
  local i = string.len(x)

  if i < 10 then
    if 0 == i%2 then
      return(x,true)
    else
      return(x,false)
    end
  end

  return(nil)
end

Then the constructor:

function foo(s)
  return(fooIter,nil,s)
end

And the harness, which appends an x or an o to the string depending
on the result of the iterator...

for p,s in foo("d") do
  if( s ) then
    p = p .. "o"
  else
    p = p .. "x"
  end
end

Note that p (the control variable) is changed in the body
of the for loop, and this works fine in pure Lua.

However, when I rewrite the iterator and factory in C, I don't
get the expected results. Changing the value of p in thebody
of the for loop does not work as expected.

(This iterator does not check for length of 10, I know...)

static int fooIter (lua_State *L) {
  size_t len;
  const char *string = luaL_optlstring(L, 2, NULL, &len);

  if( (NULL != string) ) {
    // String that we're using is already on the top of the stack!
    lua_pushboolean( L, len % 2 );
  } else {
    lua_pushnil( L );
    lua_pushnil( L );
  }

  return(2);
}

static int foo (lua_State *L) {

  lua_pushcclosure(L, fooIter, 0);
  lua_pushnil( L );
  lua_pushvalue( L, 1 );

  return(3);
}

Am I doing something really stupid that's keeping this from working
the way I want it to?

Ralph