Re: "While"-like structure in Velocity?

On 05/15/2013 12:11 PM, Christopher Schultz wrote:
> All,
> 
> I've searched the archives for "while" capabilities in Velocity, and
> mostly the answers have been things like "that's beyond the scope of
> Velocity... you want a real programming language for that".
> 
> I'd like to do something that naturally would use a while loop, but
> perhaps someone can offer an alternative suggestion.
> 
> I need to traverse a queue, but that queue needs to be able to modify
> itself during the processing. Basically, something like this:
> 
> while (!queue.empty) {
>   // do something with the head element
>   // possible enqueue some more items at the back
> }
> 
> #foreach would be the way to do it if the queue didn't have to be
> modified, but I'll get an exception if I try to traverse an iterator
> over the queue and then modify it mid-flight.
> 
> Right now, I have this written as a JSP but I'd rather have it as a
> Velocity template.
> 
> My only thought is to create an ugly class that implements Iterator and
> wraps the queue but is much less strict than a standard iterator. Any
> suggestions?
> 
> -chris
> 

While not a very clean solution, you can use a very large foreach and
manually work with the queue inside the loop, and #break($foreach) once
the queue is empty:

#foreach($i in [0..100000])
  #if ($queue.isEmpty())
    #break($foreach)
  #end
  #set ($item = $queue.pop())
  ## Do stuff with $item
  #if (should add more things)
    #set ($discard = $queue.add(new things))
  #end
#end

-- 
Sergiu Dumitriu
http://purl.org/net/sergiu