From: "ara.t.howard" <ara.t.howard@...>
Date: 2007-10-26T05:48:05+09:00
Subject: Re: Intervals in Ruby


On Oct 25, 2007, at 4:00 AM, Steven D'Aprano wrote:

> Are there many (any?) good examples of where you need to iterate  
> over an
> integer count, rather than directly over a data structure?
>

yes.  the most obvious one is needing adjacent entries and/or self- 
modification.  something along the lines of

pixels.size.times do |i|
   pixel = pixels[i]
   left = pixels[i - 1]
   right = pixels[i + 1]

   if left.nil? and condition(right) or
      condition(left) and condition(right) or
      condition(left) and right.nil?

     pixel = transformation left, pixel, right
   end
end

granted, i personally would create my own neighborhood iterator for  
such a case - but then sometimes you need to compare adjacent  
neighborhoods...

i would go so far as to say *any* interesting algorithm can require  
random access into a data structure that's also being modified.  in  
those cases iterating by size is a smart alternative since very few  
data structures in ruby support modification + traversal.

another good use case is

min = [a.size, b.size].min

min.times do |i|
   something_with a[i] and b[i]
end

which is to say iterating datasets in parallel.

yes there is zip - fantastic if you are into duplicating both  
datasets in memory *and* are iterating over n=2 data structures but  
otherwise useless.

that said - i'll take #each or #map any day.

cheers.

a @ http://codeforpeople.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama