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error when trying to fit a custom function using cftool

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Aviram Gelblum
Aviram Gelblum on 25 Nov 2013
Answered: Timothy Wiggin on 30 Aug 2017
Hi, I'm trying to fit data to a custom exponential function a*exp(-b*x)+c. No fit is generated and I get the following error in the Results window: "Dimensions of matrices being concatenated are not consistent." I've tried generating clean, simple data to fit:
aa=2;
bb=0.5
cc=0.1;
xx=0:0.1:10;
yy=aa*exp(-bb*xx)+cc;
,cleaning all other variables (by opening a new instance of Matlab), and trying other custom functions, but nothing works...
It's worth mentioning that pre-defined fits (Polynomial, Exponential, etc.) can be generated.
I am using version R2012b.

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Elizabeth Jones
Elizabeth Jones on 3 Jun 2015
I'm having the same problem now in R2014b. Did you ever find a solution?
Thanks! Elizabeth

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Answers (2)

Timothy Wiggin
Timothy Wiggin on 30 Aug 2017
I had the same problem, I'm going to document how I fixed it in case it comes up for anyone else. Skipping ahead, try restoredefaultpath. I'm using Matlab R2017a.
The root of the problem is parsing the custom equation. Running this code:
xx = 1:20;
yy = 0.5 * exp(2*xx);
fit(xx' , yy' , 'a * exp(b * x)' )
Produces this output:
Error using horzcat
Dimensions of matrices being concatenated are not consistent.
Error in symvar>findrun (line 163)
d = diff([0 x 0]);
Error in symvar (line 26)
[b,e] = findrun(s==' ');
Error in fittype>iCreateCustomFittype (line 463)
variableNames = symvar( obj.expr );
Error in fittype>iCreateFittype (line 351)
obj = iCreateCustomFittype( obj, varargin{:} );
Error in fittype (line 328)
obj = iCreateFittype( obj, varargin{:} );
Error in fit>iFit (line 157)
model = fittype( fittypeobj, 'numindep', size( xdatain, 2 ) );
Error in fit (line 108)
[fitobj, goodness, output, convmsg] = iFit( xdatain, ydatain, fittypeobj, ...
symvar crashes because the vector s is a column vector, so the s==' ' expression produces a column vector, and the concatenation expects a row vector. The problem is somehow related to the Matlab path, because running:
restoredefaultpath
Fixes the problem.

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Salaheddin Hosseinzadeh
Salaheddin Hosseinzadeh on 3 Jun 2015
Edited: Salaheddin Hosseinzadeh on 3 Jun 2015
Hi Guys,
Well done for providing the equations.
It worked fine for me, I attached an image
Or

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