custom logarithmic fit with three coefficients

3 visualizzazioni (ultimi 30 giorni)
Hello everyone,
I am trying to fit a custom logarithmic function
logfit2 = fittype('a + b*log(c*x)','dependent',{'y'},'independent',{'x'}, 'coefficients',{'a','b','c'});
fitulog2=fit(urot2,z,logfit2); plot(fitulog2,'magenta');
and the data are urot2 = [5.1689; 6.2631; 7.1724; 8.1874]; and z=[20;40;60;80]; I get the error:
Error using fit>iFit
Complex value computed by model function, fitting cannot continue.
Try using or tightening upper and lower bounds on coefficients.
Did anyone use the above function for fitting? How do I set lower and upper bounds on coefficients?
Thanks, Petr.

Risposta accettata

Torsten
Torsten il 6 Mar 2023
It doesn't make sense to use three fitting parameters for the function type you supply.
a + b*log(c*x) = a + b*log(c) + b*log(x)
So the fittype should be
u + v*log(x)
This way, you also circumvent the problem of a negative value for c which makes log(c*x) complex-valued (the reason for your error message).

Più risposte (0)

Prodotti


Release

R2022b

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by