From: Julian Snitow <vangczung@...>
Date: 2003-03-24T13:35:23+09:00
Subject: Re: Iterate over two lists in parallel

Gavin Sinclair wrote:
> On Monday, March 24, 2003, 1:54:53 PM, Julian wrote:
> 
> 
>>>Now I'm responding to my own messages, which is dangerously close to
>>>talking to myself.  But another, more common, problem where generators
>>>have the advantage is trying to iterate over two lists in parallel.
>>>
> 
> 
>>Never understimate the amazing power of call/cc.
> 
> 
>>(Programming in a language that lacks it is like commuting to work 
>>without a personal teleporter... :-)
> 
> 
> 
> The above quote is from the thread concerned with producing slides
> comparing Ruby and Python.
> 
> I'd like someone, hopefully including Julian and Jim (Weirich), to
> demonstrate how to iterate over twop lists in parallel.  Extra marks
> for setting a realistic example problem and solving it!
> 
> Gavin
> 
>
k. :-)

With block iterators:

#!/usr/bin/ruby -w

class Array
   def splice(foo)
     ((foo.size > size) ? foo.size : size).times do |i|
       yield self[i] if self[i]
       yield foo[i] if foo[i]
     end
   end
end

animals = ["monkey", "goat", "cow", "ox", "lizard", "lion", "bat", 
"gorilla", "tiger", "frog"]
vehicles = ["car", "plane", "bicycle", "catapult", "unicycle", "hang 
glider", "bus", "pogo stick", "boat", "train", "gondola", "skateboard", 
"ski lift"]

animals.splice(vehicles) { |val|  puts val }

################################

And now a completely contrived example using continuations, since as 
we've shown above, this problem doesn't require anything so powerful.

#!/usr/bin/ruby -w

animals = ["monkey", "goat", "cow", "ox", "lizard", "lion", "bat", 
"gorilla", "tiger", "frog"]
vehicles = ["car", "plane", "bicycle", "catapult", "unicycle", "hang 
glider", "bus", "pogo stick", "boat", "train", "gondola", "skateboard", 
"ski lift"]

puts callcc { |outsideWorld|

   cc, dd = nil, nil
   curr, other = callcc { |cc|
                   cc.call callcc { |dd|
                     dd.call animals, vehicles
                   }
                 }
   # Here is where the continuation ``cc'' begins

   if(x = curr.shift)
     puts x
   end

   if curr == [] && other == []
     outsideWorld.call "Free at last!"
   else
     dd.call other, curr
   end

}
# Continuation (locally known as 'outsideWorld' in the preceding block) 
begins here.

puts "... And now the program can end."

# :-)
######################################

Or have I misinterpreted the problem?  It seems too trivial to be a 
"show me that this is possible" kind of problem...  :-(