Re: [Swarm-Support] How to prevent sending a message to a dropped object?

[Ken Cline]

I definitely second what Paul has said.  It is problematic to iterate
thru a list with an index and then remove directly from the underlying
list.  It may work for you because you are going tip-to-tail, but it's
not recommended.[1]

Btw, if there is a chance that there's a nil member in the list, then
you should test for that in the 'if' statement that gets the birth
action, e.g. if ( agent && [agent getBirthAction] ) { ... }

Also, if 'getBirthAction' has any computational cost, eg it searches
a list of actions, _and_ if you are trying to squeeze out extra
performance, then you may wish to store the action in a local variable
instead of calling the getter twice.

Finally, another design option would be to have each agent's drop
method handle removing its birth actions.  The advantage is that
then you don't need the loop, simply call [agentList deleteAll].
Note, there's still a loop involved, it's just internal to the
deleteAll method.  A disadvantage to this design is that agents
now need pointers to either the model swarm or the schedule object.
This may be stylistically undesirable as well as increasing the memory
footprint of every agent slightly.  If you do override 'drop', you
MUST be sure to call the parent class's implementation at the end of
your implementation, otherwise the memory will never be reclaimed.

Ken
__________
[1] You are also taking a performance hit, as Paul said, because the
    [List remove: aMember] (inherited from Collection, I think)
    operation likely has to search the list for the member you've
    asked to be removed.  So (a) the list has create an internal index
    to go find this member and (b) if your list has duplicates, it
    might find the wrong copy of your member.  On the other hand,
    the [Index remove] operation involves almost no work; it just has
    to reassign a few pointers and poof, member removed.

    Note, it is probably possible to create a List data structure
    that protected its Index objects from concurrent modification,
    ie modification via other indexes or directly to the list.  But
    there would be a significant computational cost to pay and every-
    one would have pay it.

--- Paul Johnson <address@hidden> wrote:

> martijn cox wrote:
> > Well, that would indeed work, but the thing is: I don't have the pointer to
> > the object available. All I can do is loop over a List of agents like so:
> > 
> >  while( (agent = [index next]) != nil )
> >     {
> >       if ([agent getBirthAction])
> >        [lifeMutationSchedule remove: [agent getBirthAction]];
> >       [_agentList remove: agent];
> >       [agent drop];
> >       agent = nil; // ***
> >     }
> > 
> 
> remove returns a pointer to the removed object. But I don't think you 
> should remove items this way.
> 
> FIrst, don't use the while construct, use for.  If your while finds a 
> nil in your list, the iterating will stop prematurely. Second, you are 
> wrecking  the index by removing a member while iterating.  That's bad.
> 
> I can give you a much better algorithm for doiing this that fixes all of 
> these problems.  Then it will be much cleaner & faster to remove with 
> the index.
> 
> id agent, index;
> 
> index = [agentList begin: [self getZone]];
> 
> for(agent = [index next]; [index getLoc]==Member; agent = [index next])
> {
> 
>     if ([agent getBirthAction])
>          {
>             id theAgent;      
>             [lifeMutationSchedule remove: [agent getBirthAction]];
>          theAgent = [index remove];
>             [theAgent drop];
>          theAgent = nil;
>           }
> }
> 
> Actions on indexes to remove items are
> 1. safe to the index and
> 2. Much Much faster than searching through a collection to find 
> something and remove it.
> 
> 
> 
> -- 
> Paul E. Johnson                       email: address@hidden
> Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
> 1541 Lilac Lane, Rm 504
> University of Kansas                  Office: (785) 864-9086
> Lawrence, Kansas 66044-3177           FAX: (785) 864-5700
> _______________________________________________
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> 




_________________________________________________________
 Ken Cline                             W: (443) 287-2636
 address@hidden