Re: Help on walking lists of Swarm Collection

[Ken Cline]

On Sat, 8 May 1999, Marcus G. Daniels wrote:

> >>>>> "WS" == William S Shu <address@hidden> writes:
> 
> WS> What is not clear --
> WS> not in the manual -- is whether such changes might
> WS> adversely affect the forEach: message invoking
> WS> dropDead. 
> 
> If dropDead has the side effect of touching the List, the
> Index will break, and you'll probably get a segfault. 
> 
> Conceptually, I don't think it is clean to have agents touching global
> bookkeeping data structures.  I mean, unless it is a suicide or a
> terminal illness, people don't generally cancel their utilities and
> fill out their own death certificates, etc.  Instead, a death causes a
> disruption and society makes the necessary adaptations. 
> 
> At the code level, putting colonyList in each Agent means that there
> need to be gratuitous setColonyList: calls needed in the construction
> of each agent (or some other implicit environment like a global
> variable or global environment object).

Again, I totally agree with Marcus.  You should probably
provide a funeral service in "The Almighty" (aka ModelSwarm).
Although, in a lot of cases, the agent will need to have
pointer to some global object (e.g. the ModelSwarm).

> WS> I use forEach: like so:
> WS> 
> WS> {      
> WS> ...    
> WS>   [colonyList forEach: M(dropDead)]; // destroy 
> WS>   [colonyList removeAll];
> WS>   [colonyList drop];
> WS>   colonyList = nil;   // ensure no further use
> WS> ...
> WS> }
> WS> 
> WS> Here, dropDead invokes methods that may remove entries
> WS> from colonyList. (In fact, each entry is supposed to
> WS> remove itself from colonyList, then destroy any pointers
> WS> to the object that holds the colonyList)

Some (*untried*) coding ideas...

Let me see, you are trying to empty the list by calling a
method (dropDead) that affects the list.  You need a
collection access method that doesn't use indices, e.g.
`getFirst' or `getLast'.  How about something like:

  // Drop all the members of the specified colony
  -(void) destroyColony: (id <Collection>) colony
  {
     id target = nil;

     if ( [self isEmpty: colony] ) {
       raiseEvent( WarningMessage, "Colony is empty!" );
       return;
     }

     while ( [colony getCount] > 0 ) {
       target = [ colony getLast ];
       if ( [self isColonist: target] ) {
          [ self killColonist: (id <Colonist>)target ];
       } else {
          raiseEvent( WarningMessage, "Target %p is NOT a" \
                                      " colonist.", target );
          // Do "traditional" member remove.
          // NOTE: This assumes uniqueness of target.
          [ colony remove: target ]; // <- Possible problem
          [ target drop ];
       }
     }
     // `removeAll' not needed, see `dropDead'
     [ colony drop ];
  }

  // Kills the specified colonist
  -(void) killColonist: (id <Colonist>) target {
     if ( target == nil ) {
       raiseEvent( WarningMessage, "Target is nil!" );
       return;
     }

     // `dropDead' remove target from the colony and
     // drops the object
     [ target dropDead ];
  }

  // Check if the argument is null or an empty collection
  -(BOOL) isEmpty: (id <Collection>) collection {
     if ( collection == nil ) return TRUE;
     return [ collection getCount ] <= 0;
  }

  // Check that the argument is non-null and adopts the
  // Colonist protocol
  -(BOOL) isColonist: (id) target {
     if ( target == nil ) return FALSE;
     return [ target conformsTo: @protocol(Colonist) ];
  }

  // Or, more generally...
  -(BOOL) isSelfDropping: (id) target {
     if ( target == nil ) return FALSE;
     return [ target respondsTo: M(dropDead) ];
  }

Not sure if that stuff really help...

Ken.

_________________________________________________________
Ken Cline                             address@hidden
SAIC                                 VOICE (410) 571-0413
Annapolis, MD                          FAX (301) 261-8427






                  ==================================
   Swarm-Support is for discussion of the technical details of the day
   to day usage of Swarm.  For list administration needs (esp.
   [un]subscribing), please send a message to <address@hidden>
   with "help" in the body of the message.