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cfit object doesn't work with 'plot' or 'differentiate'

Asked by Kai

Kai

on 26 Jun 2013

Hello,

I stored a cfit obj to hard disc. It looks like:

General model:

     ans(xd) = off+amp./(1+exp(-a.*(xd-b)))
     Coefficients (with 95% confidence bounds):
       off =      -4.047  (-4.054, -4.041)
       amp =       8.179  (8.169, 8.189)
       a =       4.053  (4.034, 4.073)
       b =       2.271  (2.27, 2.273)

In the process of creating the obj I was able to plot it and to calculate the derivatives.

Now I want to come back to these data and tried:

>>plot(cfObj)

but receive:

Error using cfit/plot (line 111)
Error evaluating CFIT function:
Error while trying to evaluate CFIT model: obj:
Error while trying to evaluate FITTYPE function obj:
Argument must contain a string or function_handle.

For:

>>differntiate(cfObj, x)

I receive:

Error using cfit/feval (line 31)
Error while trying to evaluate CFIT model: fitobj:
Error while trying to evaluate FITTYPE function obj:
Argument must contain a string or function_handle.
Error in cfit/differentiate (line 33)
    f1 = feval(fitobj,x+ms);

Can anyone please help out with that? Sure I can recreate the fit from the coefficients, but I think it is not meant to be like that.

Thanks,

Kai

2 Comments

Onno Broekmans

Onno Broekmans

on 1 Jul 2013

Your fit results are definitely not useless. You can use coeffvalues(cfObj) to get the coefficient values from the fit manually, and then plot them using:

fitCoeffs = num2cell(coeffvalues(cfObj));
fitFun = @(off,amp,a,b,xd) off+amp./(1+exp(-a.*(xd-b)));
plot(x, feval(fitFun, fitCoeffs{:}, x, '-r');

(Warning: untested code, so there might be minor syntax errors in there. I'm assuming your x data is in the variable 'x').

Note that this is what plot(cfObj,x) does internally, so the results are exactly the same :)

Kai

Kai

on 1 Jul 2013

Right, that's what I will do. I was only a bit shocked not seeing it come out in the first place. The function I will use the fits in looks like:

F(t) = kE(t)+rE(t)'+uE(t)''+c

So, I see the 'expr' statement growing a bit longer than before...

Kai

Kai

2 Answers

Answer by Onno Broekmans

Onno Broekmans

on 1 Jul 2013
Accepted answer

I can confirm this behavior (see code to reproduce at the bottom). Unfortunately, it also affects parallel processing: if you try to perform a large number of fits in parallel, using a 'parfor' loop, the resulting cfit objects are "broken" in the way described by Kai.

Unless I've missed something, I'd consider this a bug. I've filed a Service Request (ID: 1-NE8ATH) with TMW. I'll get back to this thread when I hear back.

As a workaround, you could try manually evaluting the fit's anonymous function, and using the coeffvalues() function to retrieve the coefficients from the cfit object (that does work fine).

- Onno

Code to reproduce the issue:

%% Generate some random data, and plot it
data_x = 1:50;
data_y = (2.* data_x) + randn(size(data_x)) + 5;
figure;
plot(data_x, data_y, '.b');
title('Original data');
%% Fit the data using a simple model
model = @(A, B, x) A.*x + B;
ftype = fittype(model);
fopt = fitoptions(ftype);
fopt.StartPoint = [1 1];
f = fit(data_x(:), data_y(:), ftype, fopt);
%% Plot the fit
figure;
hold on;
    plot(data_x, data_y, '.b');
    plot(f, '-r');
hold off;
title('Fitted data');
%% Store cfit object on disk, and then retrieve it again
save('fit.mat', 'f');
clear('f');
load('fit.mat');
figure;
hold on;
    plot(data_x, data_y, '.b');
    plot(f, '-r');      % <-- crash here: 
                        % Error using cfit/plot (line 113)
                        % Error evaluating CFIT function:
                        % Error while trying to evaluate CFIT model: obj: 
                        % Error while trying to evaluate FITTYPE function obj: 
                        % Argument must contain a string or function_handle.
hold off;
title('Fitted data (fit loaded from disk)');

3 Comments

Kai

Kai

on 1 Jul 2013

Hi Onno,

thanks for the code. It exactly reproduces the error. Next time I will communicate with code as well ;) Adding:

          try
             d1 = differentiate(f, data_x);
          catch err
             disp(err.message)
          end

on line 36 shows the other error s. above.

Also, thanks for filing the bug report, looking forward to the response. So long, I ll use the coefficients.

Best, Kai

Onno Broekmans

Onno Broekmans

on 1 Aug 2013

I forgot to post back, but TMW confirmed the issue (early July), and said they had their developers working on it.

Jon Cherrie

Jon Cherrie

on 21 Aug 2013

We have published a patch for this issue. See the bug report 968079 for details.

Onno Broekmans

Onno Broekmans

Answer by Andrei Bobrov

Andrei Bobrov

on 26 Jun 2013
Edited by Andrei Bobrov

Andrei Bobrov

on 26 Jun 2013
x = (0:.01:4)';                                         %
y = -4 + 8./(1+exp(4.*(x - 2))) + .5*randn(numel(x),1); % your data
ftfun = fit(x,y,fittype(@(a,b,c,d,x)a + b./(1+exp(c.*(x - d)))),...
                                                          'Startpoint',[1 1 1 1])
dft = differentiate(ftfun,x);
plot(x,[y,ftfun(x),dft])

1 Comment

Kai

Kai

on 26 Jun 2013

Hi Andrei,

thanks for your answer. As I see it, you describe how to create a fit object and then use plot and differentiate. This is not really an answer to my question, because I already have the fit objects and know how to use the mentioned functions. The point is that directly after creation I could plot and differentiate them. But now, after a few days, I receive the errors shown above. Weird, I think. Or did I misunderstand your point?

Kai

Andrei Bobrov

Andrei Bobrov

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