[Kst] [Bug 111574] Request for interactive fitting

Nicolas Brisset nicolas.brisset at eurocopter.com
Mon Jun 19 15:23:36 CEST 2006

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------- Additional Comments From nicolas.brisset eurocopter com  2006-06-19 15:23 -------
I have not worked on this for quite some time, but wider spread testing and help in debugging this is certainly welcome. I think it is quite close to working correctly. Below is an excerpt from a mail exchange I had with Ingo Berg, the author of muparser, regarding the problems I had observed. I have not investigated further since then, apart from trying the same fit with xmgrace (where it worked better :-().

> I don't see a problem with the formula: y=a*exp(-b*x)+c.
> In General Variables submitted to muParser must stay valid as long as 
> the parser is used. If they dont you will see an segmentation fault 
> since muParser tries to access then when evaluating the formula. There 
> is no chance of automatically validating variable pointers for 
> muParser.
> Recently I receive a mail from the author of qtiplot describing 
> problems with certain formulas. Whilst in my opinion this are problems 
> related to the M-L Algorithm based on an incorrect first choice of 
> variables I see one potential source of problem if you use the Diff 
> function of muParser.
Well, actually I have verified this and qtiplot exhibits the same behavior with y=a*exp(-b*x)+c as the kst plugin. It uses the diff method indeed, so that I will have to investigate when I find some time for that. Even if you initialize the parameters with values close to the right ones, the algorithm does not seem to converge. I will have to look at whether the diff computation is the cause of the problem of the L-M algorithm itself.
I will keep you informed...

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