From: Joel VanderWerf <vjoel@...>
Date: 2003-04-02T09:45:05+09:00
Subject: Re: Iterate over two lists in parallel

Johan Holmberg wrote:
> Gavin Sinclair <gsinclair@soyabean.com.au> writes:
> 
>>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
>>
> 
> 
> Maybe this is off-topic,
> but I can't see anyone mentioning the 'enum' package by Joel VanderWerf.
> 
> Here is a solution to "Iterate over two lists in parallel"
> using that package:
> 
>       #------------------------------
>       require 'enum/op'
>       include EnumerableOperator
> 
>       alist = [11, 22, 33, 44]
>       blist = ["a", "b", "c", "d"]
>       clist = 1001..1004
> 
>       for a,b,c in diagonal(alist,blist,clist)
>           puts "got #{a}, #{b} and #{c}"
>       end
> 
>       #------------------------------
> 
> /Johan Holmberg
> 

It's kind of a cheat, though. Unlike the solutions using continuations, 
this diagonal thing actually converts the inputs to arrays, if they 
aren't already. So if you want to do parallel enumeration of infinite 
sequences, such as PRNGs, you never even get the first tuple. OTOH, for 
small sequences, the overhead of converting to arrays (if necessary) 
might be less than the overhead of instantiating continuations. IIRC, 
continuations are on the same order of magnitude as threads, in that 
respect.