Re: lua_getupvalue question

[Andrew Starks <andrew.starks@...>]

On Monday, December 2, 2013, Andrew Starks  wrote:

On Monday, December 2, 2013, Philipp Janda wrote:

Am 03.12.2013 02:48 schröbte Andrew Starks:

I get the upvalue. It's on the stack. It's value is 0. I want it so that
the next time I enter the c function and retrieve the value at the same
lua_upvalueindex address, it returns unto me the previous value, added
to one.

Please to illustrate?

    static int iter( lua_State* L ) {
      /* increment and push value to stack */
      lua_pushinteger( L, lua_tointeger( L, lua_upvalueindex( 1 ) )+1 );
      /* replace upvalue with incremented value at stack top */
      lua_replace( L, lua_upvalueindex( 1 ) );
      /* lua_replace popped the updated value, so push it again to
       * return it */
      lua_pushvalue( L, lua_upvalueindex( 1 ) );
      return 1;
    }

On the Lua side:

    > print( iter() )
    1
    > print( iter() )
    2
    > print( iter() )
    3

I hope that's what you wanted ...

-Andrew

Philipp

Holy crap! Thank you! I didn't mean to be so obstinate. However , I read the definition of lua_replace over and over again as part of my search. Then when it was brought up, I went to check on it again and was relieved  (sad, too) that it was described like remembered, such that the following would be equivalent to it:

lua_insert(L, -2);

lua_remove(L, -1);

That is, remove the reference to the value at the index and insert the top value in its place so that everything else stays put.  Here is the cut and paste:

lua_replace

[-1, +0, –]

void lua_replace (lua_State *L, int index);

Moves the top element into the given valid index without shifting any element (therefore replacing the value at the given index), and then pops the top element.

Am I the only one that reads that incorrectly?

Love the double post. 

So, I sat here thinking about it, and realized that:

A: This behavior is consistent with the API and should require no further explanation. 

B: Like C, the more I feel like the little Lebowski when he realizes that he's been overcomplicating things and that he shouldn't be so uptight, etc. 

So, "more drugs" is the answer. 

I think that this example makes a good use case for showing how the stack works.

-Andrew