Re: [Swarm-Support] Objects on multiple lists

[Paul Johnson]

Darold Higa wrote:
> Dear All:
>
> This is probably a really dead and beaten horse, but I couldn't find specific 
> references to it...so here it goes.  I was going about trying to improve my 
> code and in the process I decided to use collections more agressively , but 
> now I have some unusual problems.
>
> Here are my questions:
>
> 1.  When an object creates an internal collection, it seems that I can only 
> do so by creating it within the ModelSwarm zone.  Is there no way to create 
> temporary or instance collections internal to that object?  My only fear is 
> by creating them within the ModelSwarm zone, I might leave collections lying 
> about when the objects that create them are destroyed by my reaperqueue.  I 
> thought I understood zones, but now I fear that maybe I don't, so any help 
> here is appreciated.
>
> I tried to do:
>
> localAgentList = [List create: self];
>
> but had to do
>
> localAgentList = [List create: [modelSwarm getZone]];
>
> 2.  Is there some way to determine what effect having the same object in 
> multiple collections will have?  I don't remember where I read/heard it, but 
> I thought it was OK for an object to exist in multiple collections, since 
> collections were pointers to the specific instances of an object.  In one 
> case I had an object create a temporary list and assigned it the value of 
> another list from another object.  (I guess I should have used copy.)  In any 
> case, when I sent a removeAll command to the temporary list, the objects were 
> removed from the original list as well.  How does this work?  I planned to 
> have objects be on many lists at once, and I am not sure how manipulating 
> collections affect other collections.

If you create a separate zone for each agent, and create the agent in 
that zone, then each agent will have a zone. But it does not solve the 
memory leak problem. When you drop the agent, you have to override the 
drop method and then make sure you drop the list when the agent is dropped.

> To make it clearer, I have a master list of agents (for scheduling purposes), 
> then at each geographic cell in my world there is a list tracking what agents 
> are in that cell, then I also wanted each agent to maintain their own 
> internal lists of agents with similar identities, who they interacted with, 
> etc.
>
> To make a long and complicated question hopefully shorter, is the problem 
> that I had with removeAll a result of my use of :
>
> temporaryAgentList = [terrainObject getLocalAgentList];
>
> instead of
>
> temporaryAgentList = [[terrainObject getLocalAgentList] copy [modelSwarm 
> getZone]];

> ?  Or does it have to do with objets on multiple lists that I do not 
> understand.

I don't see what the problem is. If each agent has a list of others, and 
you remove all of those references, then everything gets removed. That's 
what happens on my computer, anyway.

If that's not what you see, give us a set of code we can test.


> 2A.  If a list is made of objects through addLast, what effect will removing 
> and or dropping items from this list have on any other lists?

I don't think it has been answered or discussed much.  I think the 
answer is that the "other lists" will go to hell if you drop an object 
and you don't tell those other lists about it.

It would be a perfect world if  those other lists had "nil" inserted 
where the dropped object used to be, but I was just reminded that  it is 
not so.  I had some hairy memory problems in a program that users sent 
me and the problem was that they were dropping an object from one list, 
but not setting it to nil or removing from the other lists, and so when 
processing those other lists, there was a seg fault because any old 
thing was found at the memory location where the dropped object was.

Lesson: keep careful track when dropping things, make sure they notify 
all lists in which they belong that they are gone.

pj


--
Paul E. Johnson                       email: address@hidden
Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66045                FAX: (785) 864-5700