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
> address@hidden
> http://www.swarm.org/mailman/listinfo/modelling
>
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