From: Michael Neumann <mneumann@...>
Date: 2004-08-10T18:48:53+09:00
Subject: Re: empty? and size in Enumerable

Robert Klemme wrote:
> "Martin DeMello" <martindemello@yahoo.com> schrieb im Newsbeitrag
> news:xW%Rc.68814$gE.48438@pd7tw3no...
> 
>>Robert Klemme <bob.news@gmx.net> wrote:
>>
>>>Hm, sounds to me like it was not general enough to include it in
>>>Enumerable.  Before I see that method I'd prefer to have size and
> 
> empty?
> 
>>>in Enumerable.  Just my 0.02 EUR...
>>
>>Can empty? be reliably implemented in terms of each for every
>>enumerable? I can't think of an obvious problem with it, but that
>>doesn't mean there isn't one.
> 
> 
> Yes, that's possible IMHO.  My suggestion would be:
> 
> module Enumerable
>   # O(1)
>   def empty?
>     each { return false }
>     true
>   end
> 
>   # O(n)
>   def size
>     inject(0) {|s,| s+1}
>   end
> end
> 
> Do you see any problems?  There's only the little disadvantage that size
> is O(n) while there are most likely more efficient implementations for
> certain collections.  But classes like Array and Hash provide their own
> implementation anyway, which then overrides Enumerable#size.  So there's
> no drawback IMHO, only the advantage that you don't have to write it if
> you have a collection that can determine its size only via a complete
> iteration.

Hehe, and how long does it take to determine the size of infinite or 
cyclic Enumerable's? ;-)

   require 'enumerator'
   to_enum(:loop).each {
     ...
     break if ...
   }
   to_enum(:loop).size # ???

Regards,

   Michael