Re: C API - lua_next traversal of "array" table

[Ross Berteig <Ross@...>]

On 8/18/2016 5:48 PM, ThePhD wrote:
> My only problem is: how do I know how big the array part is?

See, this is why naming the internal implementation detail "array part" 
is a mistake. Forget there ever was such a thing.

What you have is called a "sequence". It is a table that contains 
integer keys from 1 to n with no holes. It might have other keys too, 
and generally that doesn't prevent it being a sequence.

The implementation detail that some or all of those integers are 
physically in "the array part" is not what made it a sequence. It is 
still a sequence if none of the integers are in the array part.

> I could use
> `#t` or fight with its equivalent in the Lua 5.1/2/3 code, which is
> either lua_objlen or lua_rawlength (I can't remember which).

If your code is sufficiently disciplined to never poke holes in your 
sequences, then `#t` is the right way to learn the length of the 
sequence in the table `t`. In C, the equivalent is lua_objlen() in 5.1, 
and lua_len() in 5.2 and 5.3.

> Right now I
> have a fixed size at compile-time... I could just keep iterating integer
> keys from 1 to infinity until I hit a snag, but that seems less than
> useful (and doesn't allow me to pre-allocate the size to save performance).

Since you already need to iterate, then iterating until nil saves you 
the additional O(log n) cost from `#t`.

--
Ross Berteig                               Ross@CheshireEng.com
Cheshire Engineering Corp.           http://www.CheshireEng.com/
+1 626 303 1602