Re: Speed of #t

[David Kastrup <dak@...>]

Enrico Colombini <erix@erix.it> writes:

> On 13/12/2010 11.14, Axel Kittenberger wrote:
>> Likely I'm a bit naive about this. But couldn't we just define for #
>> operator to simply return the size of the array part?
>>
>> Meaning counted from 1 onwards # to defined the the largest numeric
>> key where t[k] is not nil. t[#t+1] is nil.
>
> I'd be happy with that, but (apart from other people having different
> wishes) consider what will happen if you have a table such as:
>
>  t = { 1, 2, 3, nil, 5, 6, 7, ... } -- a very very large table
>
> #t would be 3.
> Now fill the hole:
>
>  t[4] = 4
>
> To update #t correctly according to your definition, all the remaining
> length of the array would probably have to be traversed sequentially.

Why? t[4] = 4 will presumably just increase the array part by one,
resulting in #t == 4.

-- 
David Kastrup