lua-users home
lua-l archive

Re: Definition of a sequence

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index] 

On 6/11/2015 7:14 AM, Dirk Laurie wrote:

2015年06月11日 10:29 GMT+02:00 Mike <lua-l@inbox.ru>:

I cannot guess the exact rule for the length operator

....
In the Lua 5.1 manual, the following boundary property was
guaranteed:
 t[#t] is not nil, but t[#t+1] is.
The exact quote from the 5.1 manual is "The length of a table t is defined to be any integer index n such that t[n] not nil and t[n+1] is nil; moreover, if t[1] is nil, n may be zero." This allows #{} to be zero as it obviously is, and for t[#t+1] to extend any sequence, even an empty sequence. For some cases, the guarantee that t[#t+1] is nil is all that is needed; it provides a guaranteed empty slot in a table used like the registry, even if the keys do not form a sequence. That property seems useful, and I am still slightly puzzled about why it was dropped from later versions. At least it remains clear that non-numeric keys have no effect on the sequence-nature of a table, so it is acceptable to have sequences with attributes. 
This is no longer documented, perhaps because the user can now
supply a __len that does not have that property.
Obviously a user supplied __len should completely replace the entire definition of `#` for tables with that __len. 
--
Ross Berteig Ross@CheshireEng.com
Cheshire Engineering Corp. http://www.CheshireEng.com/

AltStyle によって変換されたページ (->オリジナル) /