Re: Does "fit" really need to use the axis structures?

[sfeam (E. Merritt) <eam...@gm...>]

On Thursday, 13 June 2013, Bastian Märkisch wrote:
> Am 12.06.2013 23:29, schrieb Ethan A Merritt:
> > In response to a request on the newsgroup, I've been looking into relaxing
> > the limit of 5 independent parameters in "fit".
> >
> > So far as I can see there are 2 reasons for this limit.
> >
> > 1)  df_readline() limits the number of fields in a 'using' clause to
> >      MAXDATACOLS, and this is currently defined as 7.
> >
> > 2)  the fit parameters can be range-limited in the "fit" command,
> >      and these ranges are stored in the axis data structures for the
> >      x, y, t, u, and v axes.
> >
> > (1) It is easy to change MAXDATACOLS, and I have confirmed that by
> > itself this doesn't break anything.
> >
> > (2) is harder to relax if it really is necessary to use the axis data
> > structures.  But is it really necessary?
> >
> > It looks to me that the only reason the axis data structures are involved
> > is that routine parse_range(), which used to be macro PARSE_NAMED_RANGE,
> > takes an axis index as input parameter and overwrites the
> > autoscale/min/max fields in that axis structure.
> > During the fit operation, these ranges are checked in the axis structure.
> >
> > Is there some other reason I am missing that requires access to the
> > axis data structures?  Can we just provide a local array containing
> > autoscale/min/max for each fit parameter and not access the axis
> > structures at all?
> >
> > 	Ethan
> 
> I am not familiar enough with the axis structures to comment on this. 
> The fitting code itself certainly does not have a restriction to the 
> number of indep. variables.  Hans-Bernhard, any comments?
> 
> Support for up to five independent variables was implemented by Jim Van 
> Zandt.  Interestingly, the discussion about the axis array was brought 
> up already in the original discussion:
> http://thread.gmane.org/gmane.comp.graphics.gnuplot.devel/8319
> 
> There was also was a proposal by SF user Hanno, where he discusses a 
> patch he was working on with support for more than 2 indep. variables. 
> Sadly, no code is attached.
> http://sourceforge.net/p/gnuplot/feature-requests/191/
> 
>    Bastian

Thanks for the feedback on understanding the current fitting code.
As I said, this is the first time I've looked at it.   

I have put a patch on SourceForge increases the number of independent
variables from 5 to 12.   In principle it could be increased much more than
that, and indeed I tested setting it as high as 99.
 
   <https://sourceforge.net/p/gnuplot/patches/625/>

First it decouples the fit variables (other than x and y) from the normal
axis structures.  That removes one cap on the maximum number of
independent variables.  The only change to current behaviour
(IMHO a benefit) is that setting a range on t/u/v does not affect
subsequent fit commands.

Having removed that limiting cap, the next one hit is MAXDATACOLS.
I've audited the code and am 99% convinced that it is OK to increase
this as needed.  The patch sets it to 14 (==MAX_VAR_NUM+2)

Finally, "fit" requires giving names ("dummy names") to the independent
variables.  The number of dummy variables is capped by MAX_VAR_NUM,
which is currently 12.    I raised that to 99 for testing, but left it at 12
in the patch.  To make it easy to set these, I've changed the code in CVS
so that "fit" will use the names (if any) previously requested by a
"set dummy x1,x2,x3,x4,...." command.  This already worked in 4.6/4.7
but only for the first two dummies (default "x" and "y").  Now it works
for as many as you like.

Hanno Hofstadt's test script was very useful in testing, so I have added
it to the CVS demo collection for both 4.6 and 4.7. 

Bastian:  I didn't try to reconcile this patch set with your patch #621 
that adds per-dimension error terms.   There may or may not be any
conflict.  At any rate this one makes it easy to read in larger numbers of
data columns, which will help if there are two input columns per dimension
needed.

	Ethan