MATLAB and Simulink resources for Arduino, LEGO, and Raspberry Pi

Learn moreOpportunities for recent engineering grads.

Apply Today**New to MATLAB?**

Asked by Alexandra
on 28 Jul 2013

Hello, Sorry for the question, im just a very beginner. I am using f=fit(Wavelength_sal_cor(517:1293),Salinity_and_Particle_corrected_spectra(517:1293),'exp2');

It calculate all 4 coefficients, but sometimes I have negative values, then fit is not good, I want to set parameters of second exp as constants 'c' as meaning(at defined x): Salinity_and_Particle_corrected_spectra(998), and d as -0.01. But I don´t understand how to do.

Could you help me please?

Answer by Shashank Prasanna
on 28 Jul 2013

You can specify a custom model with your requirements instead of using the *exp2*

model = @(z)z(1)*exp(z(2)*x)+Salinity_and_Particle_corrected_spectra(998)*exp(-0.01*x)

% Where c = Salinity_and_Particle_corrected_spectra(998) and % d = -0.01

f=fit(Wavelength_sal_cor(517:1293),Salinity_and_Particle_corrected_spectra(517:1293),model);

You could also specify 'StartPoint','Lower' and 'Upper' bounds to get better results.

There are examples in the documentation on how to fit custom models:

Shashank Prasanna
on 28 Jul 2013

Lets forget about your code for a moment. Are you able to run the example in the documentation link I provided without any errors?

If that is working fine then we may need more info about your code.

If the documentation example also shows the same error, you may have a path issue. Try `restoredefaultpath`

Jon Cherrie
on 21 Aug 2013

I think that the anonymous function, `model`, is not set up correctly for use by Curve Fitting Toolbox. Try this instead:

c = Salinity_and_Particle_corrected_spectra(998); d = -0.01; model = @(a, b, x) a*exp( b*x ) + c*exp( d*x );

f = fit( Wavelength_sal_cor(517:1293), Salinity_and_Particle_corrected_spectra(517:1293), model );

To get good results, you will probably need to supply an estimate of the start point, `StartPoint`.

Answer by Alexandra
on 28 Jul 2013

Thank you for answer! I get ??? Error using ==> subsindex Function 'subsindex' is not defined for values of class 'function_handle'. when trying to fit the model Do you know what does it mean?

Shashank Prasanna
on 28 Jul 2013

What you posted here is not an answer to your questions but rather a comment to my answer. Please see my reply on the comment to my answer.

## 0 Comments