Re: NSelect working? -- No (& suggested fixes).

[Rick Riolo]

Ok, here are a couple of bugs in NSelect (src/simtools/NSelect.m), 
and then a couple of suggestions:   

1. The RNG call should be:
     r = (float) [uniformDblRand getDoubleWithMin:0 withMax: 1.0] ;
  
   (Aside: Is it worth doing a cast to a float?
    I would imagine that for many/most modern machines
    just leaving it as a double would be just as fast or faster, 
    especially since the RNG already produced it for us!)

2. I believe that "put" should not be used to add things to
   the bCollection (the sample), since (if I understand it)
   put *replaces* the member at the index position.
   Not only does one not want to do a replace, it just crashes
   since for the first sample there is nothing to replace,
   so the put causes a core dump with:

*** event raised for error: InvalidIndexLoc
*** function: _i_ListIndex_linked__put_(), file: List_GEN.m, line: 380
*** execution terminating due to error

   In fact, I think the index into bCollection is not needed at all,
   and instead you can just do addFirst or addLast to it bCollection
   directly.  Thus the heart of the algorithm is:  

  while(m < n){
    r = (float) [uniformDblRand getDoubleWithMin:0 withMax: 1.0] ;    

    if( ( ((float)(N - t)) * r) >= ((float)(n - m)) ){
      [a next] ;
    } else {
      m++ ;
          [bCollection addLast: [a next]];
    }

    t++ ;
  }

3. I think there should be a way to assign the distribution
   object that Select will use (so one can have it in
   whatever RNG streams the user wants to use).

4. I think the documentation should mention that the
   sample objects will come out in the same order,
   just to make sure the user doesn't expect to 
   get sampled objects in a random order.

5. I think the documentation should mention that things are
   added to bCollection (if that is the intention),
   or bCollection should have a removeAll sent to it.
   (I prefer the former...the user should empty it if desired.)

- r

Rick Riolo                           address@hidden
Program for Study of Complex Systems (PSCS)
4068 Randall Lab                
University of Michigan         Ann Arbor MI 48109-1120
Phone: 313 763 3323                  Fax: 313 763 9267
http://www.pscs.umich.edu/PEOPLE/rlr-home.html

On 1 Feb 1998, Marcus G. Daniels wrote:

> Date: 01 Feb 1998 16:03:11 -0800
> From: Marcus G. Daniels <address@hidden>
> To: address@hidden
> Cc: Swarm-Support <address@hidden>, address@hidden
> Subject: Re: NSelect working?
> 
> >>>>> "RLR" == Rick Riolo <address@hidden> writes:
> 
> RLR> Do I need to do something else to use NSelect?  Or is there a bug
> RLR> (!) in there somewhere?  (From the message it looks like NSelect
> RLR> is maybe not using the random lib stuff correctly...)
> 
> Yeah, I agree.  It seems the call to getDoubleSample in NSelect.m needs
> to be replaced with "getDoubleWithMin:0 withMax: 1.0".
> 

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