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The problem with that implementation of the # operator is that some people rely on the fact that t[#t+1] is always nil to add elements to the table
> Date: Tue, 17 Apr 2012 19:29:53 -0300 > From: eduardoochs@gmail.com > To: lua-l@lists.lua.org > Subject: Re: Lua 5.2 Length Operator and tables (bug?) > > On Tue, Apr 17, 2012 at 5:41 PM, joao lobato <btnfdp.lobato@gmail.com> wrote: > > > > On 4/17/12, Coda Highland <chighland@gmail.com> wrote: > > > On Tue, Apr 17, 2012 at 3:12 PM, joao lobato <btnfdp.lobato@gmail.com> > > > wrote: > > >> On 4/17/12, Coda Highland <chighland@gmail.com> wrote: > > >>> Okay, yeah, then the only way to verify issequence is to either (1) > > >>> get the real size of the array part (which isn't exposed AFAIK) or (2) > > >>> iterate over pairs(t). > > >>> > > >>> /s/ Adam > > >>> > > >>> > > >> > > >> A sequence can have all the key-value pairs in the array-part, all in > > >> the hash-part or every case in between. > > >> > > >> Those two concepts are orthogonal. > > >> > > > > > > I meant to imply that (1) would be something implemented inside the > > > interpreter because that behavior isn't defined in the spec. (Indeed, > > > the spec doesn't require that anything is implemented array-like as > > > long as Lua scripts can treat it as a sequence.) I realized a while > > > after I sent the message that I didn't make this clear. > > > > > > /s/ Adam > > > > > > > > > > I voiced before my opinion that # could just as well be the number of > > elements in the (hash-)table. That way, > > > > seq = {1,2,3,4,5} > > #seq --> 5 > > > > #seq would still be the lenght of a sequence or the number of elements > > in a set and would be a O(1) operation. Of course one would lose the > > ability to use the array-side and the hash-side of the same table at > > the same time. That is actually quite practical and trademark Lua. > > > > Petite Abeille is right. It is best to simply master the #op as it stands. > > > > Has this idea already appeared before? I don't think it would be > concretely useful, but it could give to the people who complain about > "#" something to chew on while they think... > > Suppose that we patch a Lua interpreter slightly to add another field > to the "Table" structure - i.e., to: > > http://www.lua.org/source/5.2/lobject.h.html#Table > > Let's call this new field "npik", for the "number of positive integer > keys" in the table. It is relatively cheap to keep it up to date - we > only need to modify the code that implements "T[key] = val". Suppose > that we add a function "npik" that reads the npik field of a table; > then we can implement "issequence" as just the translation of this to > C: > > issequence = function (T) > for i=1,npik(T) do > if T[i] == nil then return false end > end > return true > end > > Cheers, > Eduardo Ochs > eduardoochs@gmail.com > http://angg.twu.net/ > |