Re: [Swarm-Modelling] possible problem comparing doubles

[Katie Bentley]

-er I got it too so prob did go to the list! :)

On Wed, 2007-02-28 at 23:32 +0930, paul box wrote:
> Hi Rick
> 
> I posted this at a point where I was punchy and running around in
> circles.  I think I may have found the problem.
> 
> I had sent this message from the wrong account (not the one from which
> I had subscribed to swarm.org), so thankfully, you and I are the only
> ones who got this.  When I identified the problem, I was breathing a
> sigh of relief that the message had not gone through to the modeling
> list.
> 
> 
> On 2/28/07, Rick Riolo <address@hidden> wrote:
> >
> > hi paul,
> >
> > from a quick scan of this, i do not see the problem.
> > i assume this is a typo:
> >       bestPoool = myPool;
> >
> > its fine to do > < comparisons with doubles.
> >
> > i guess i would suggest some brute for debugging, starting with
> > changing your loop to this:
> >
> >   for (i=0; i<[[myPool getUpstreaPools] getCount]; i++){
> >     double aDen, bestDen;
> >     aPool = [[myPool getUpstreamPools] atOffset: i];
> >     aDen = [aPool getShrimpDensity];
> >     bestDen = [bestPool getShrimpDensity];
> >     fprintf( stderr, " ... print out aDen and bestDen " ... );
> >     if ( aDen < bestDen )
> >         bestPool = aPool;
> >         fprintf( stderr, " bestPool changed to ..." );
> >     }
> >   }
> >
> > if that doesn't work, you should have some clues.
> > if it does work...i dunno, i don't know why it
> > wouldn't work the way you had it.
> >
> > - r
> >
> > Rick Riolo                           address@hidden
> > Center for the Study of Complex Systems (CSCS)
> > 323 West Hall
> > University of Michigan         Ann Arbor MI 48109-1107
> > Phone: 734 763 3323                  Fax: 734 763 9267
> > http://cscs.umich.edu/~rlr
> >
> > On Wed, 28 Feb 2007, paul box wrote:
> >
> > > Date: Wed, 28 Feb 2007 15:32:07 +0930
> > > From: paul box <address@hidden>
> > > Reply-To: Agent-based modeling <address@hidden>
> > > To: Agent-based modeling <address@hidden>
> > > Subject: [Swarm-Modelling] possible problem comparing doubles
> > >
> > > Hello all
> > >
> > > I'm having a problem with a simulation, where the culprit line seems
> > > to be a comparison between doubles.
> > >
> > > I'm describing shrimp migrating upstream from pool to pool.  In this
> > > scenario, the only thing the shrimp wants is to find the least crowded
> > > pool upstream (upstream pool with the lowest shrimp density). The pool
> > > contains the following characteristics:
> > >
> > > @interface Pool {
> > >   ...
> > >  id <List> upstreamPoolList;  // any number of pools upstream
> > >  double shrimpDensity;  // number of shrimps / area
> > >  double area;               // pool surface area
> > > id <List> shrimpList;
> > >
> > >  ...
> > > }
> > >
> > >
> > >
> > > @implementation Pool {
> > >
> > >  ...
> > >  -(double) getShrimpDensity {return shrimpDensity;}
> > > - updateShrimpDensity {
> > >   shrimpDensity = (double) [shrimpList getCount] / area;
> > >   return self;
> > > }
> > >
> > > ...
> > > @end
> > >
> > > The shrimp contains the following:
> > >
> > > @interface Shrimp {
> > >
> > >   Pool * myPool;  // the pool I'm currently inhabiting
> > > }
> > >
> > > @implelmentation Shrimp {
> > >
> > > ...
> > > -step {
> > >   [self moveToPool: [self findBestPool]];
> > >   return self;
> > > }
> > >
> > > -findBestPool {
> > >   Pool * aPool;
> > >   Pool * bestPool;
> > >   int i;
> > >
> > >   bestPoool = myPool;
> > >
> > >   for (i=0; i<[[myPool getUpstreaPools] getCount]; i++){
> > >     aPool = [[myPool getUpstreamPools] atOffset: i];
> > >    if ([aPool getShrimpDensity] < [bestPool getShrimpDensity])
> > >     bestPool = aPool;
> > >    }
> > >
> > >   return bestPool;
> > > }
> > >
> > >
> > > This code was not producing results that made sense.  When I changed
> > > the [getShrimpDensity] to the following, it works fine:
> > >
> > > -(int) getShrimpDensity { return (int) (shrimpDensity * 100000);}
> > >
> > > That changed the statement in [findBestPool ] to compare integers
> > > instead of doubles.  I did not want to write the statement that way,
> > > because I don't want to be concerned in this stage as to whether
> > > 100000 is an appropriate scaling number or not.
> > >
> > > Is comparison of doubles in a logical statement a sin?  I have always
> > > avoided testing for equality in doubles, but I had thought that simply
> > > choosing the larger of two doubles a valid, if potentially sloppy,
> > > operation.
> > >
> > > The above code failed even when comparing doubles of several orders of
> > > magnitude difference (e.g., 70.334 > 0.00135, or 70.334 > 0).
> > >
> > > --
> > > //////////////////////////
> > > // Paul Box
> > > // Alice Springs, NT  Australia
> > > //
> > > _______________________________________________
> > > Modelling mailing list
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> > >
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> >
> 
>