Re: QSort: What I've learned, what I would like to know

[Rick Riolo]

Paul,
Well, this is just a guess off the top of my head:

The example compare function as given almost looks like
it could be used to sort a Swarm Array object which
happens to contain double values:

int compare (const void *obj1, const void *obj2)
{
   double val1 = *(double *)obj1;
   double val2 = *(double *)obj2;

   return val1 < val2 ? -1 : val1 != val2;
}

For your case, I would think you would use a version that
looks like this:

int compare (const void *obj1, const void *obj2)
{
   double val1 = [obj1 getTestValue];
   double val2 = [obj2 getTestValue];

   return val1 < val2 ? -1 : val1 != val2;
}

Again, this is just a guess from a quick scan of this message
and from what I can recall of QSort.
 - r

Rick Riolo                           address@hidden
Program for Study of Complex Systems (PSCS)
4477 Randall Lab                
University of Michigan         Ann Arbor MI 48109-1120
Phone: 734 763 3323                  Fax: 734 763 9267
http://www.pscs.umich.edu/PEOPLE/rlr-home.html

On Tue, 21 Jul 1998, Paul Johnson wrote:

> Date: Tue, 21 Jul 1998 08:05:43 -0500 (CDT)
> From: Paul Johnson <address@hidden>
> To: address@hidden
> Subject: QSort: What I've learned, what I would like to know
> 
> 
> Whenever I learn how to implement some Swarm thing, I write it down
> and sometimes doublecheck to the list to fill in the gaps. Then it may
> become a FAQ entry or something else.  This is one of those times.
> 
> Here Goes:
> 
> The QSort method in Swarm implements a common, efficient method for
> sorting lists of objects.  I first read about QSort in a Pascal text
> called OH! Pascal and I was interested in using it to have agents
> rank objects in terms of their desirability.
> 
> I found the manual pages on this somewhat cryptic because they mention
> compare methods and functions but dont say much about how those are 
> constructed
> and how they work.  (I mean, they don't say much I can understand.) Last 
> Friday
> I bothered an SFI team member whose initials
> are MD several times and I found one way to make QSort work. 
> 
> In the proposal.m file, I have a double  called "proposalUtility". Scores that
> are lower are better.  There is a method "getTestNumber" which gives back
> that double.  (I know, for convenience it ought to be "getProposalUtility"
> but for some uknown reason, probably a typo, when I named the method that
> it always returned 0. Hmmm)  Inside
> the proposal.m file, there is also a "compare" method, exactly like so:
> 
> -(int) compare:  b
> {
>    double valb = [b getTestNumber];
> 
>    return proposalUtility  < valb ? -1 : proposalUtility != valb;
> }
> 
> It took me a while to understand this very elegant comparison that MD 
> sent me.  This comparison returns a -1, 0, or 1. It says "if proposalUtility <
> valb, then give back -1.  Else give back this boolean statement, which
> gets evaluated as 1 if proposalUtility and valb are not equal and 0 if they 
> are.
> Very slick, I thought.  If I had done this myself, I'd have had to write
> if-else-else or such. (Maybe this one of those clever C tricks you learn if
> you actually take a class in programming?) 
> 
> In another file "citizen.m" a citizen creates a list of objects of type
> proposal, that list is  called invitationsList.  That file includes
> simtools.h at the top, so you get some free objects, like NSelect and QSort.  
> These are objects created inside the swarm kernel and they can be told to
> carry out jobs. Very convenient! The invitationsList is QSorted by this 
> command:
> 
>          [QSort sortObjectsIn: invitationsList];
> 
> Because no additional parameters are included, the QSort method by default
> uses the compare method that is given for the object type in the
> invitationsList.  It works fine.
> 
> 
> Now here is what I wish I understood.  If an object has no compare method, one
> can make a function that compares the doubles.  It is in the manual, but I
> just don't get it! MD explained that this
> is a function, not a method, it is global and is inserted in an m file,
> typically  before the @implementation.  Here is the example:
> 
> int
> compare (const void *obj1, const void *obj2)
> {
>    double val1 = *(double *)obj1;
>    double val2 = *(double *)obj2;
> 
>    return val1 < val2 ? -1 : val1 != val2;
> }
> 
> And to QSort with this, you use this command:
> 
> [QSort sortObjectsIn: invitationsList using: compare];
> 
> I don't understand how to adapt this compare example to fit my problem. Unlike
> the other one, where I understood how proposalUtility was getting into the
> compare method, I don't understand how proposalUtility is supposed to get into
> this compare function. 
> 
> If you know, please tell me and I can close on this topic
> 
> Paul E. Johnson                      address@hidden
> Dept. of Political Science           http://lark.cc.ukans.edu/~pauljohn
> University of Kansas                 Office: (913) 864-9086
> Lawrence, Kansas 66045               FAX: (913) 864-5700
>  
> 
> 
> 
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