Hello,
My question is about the fitting function. I would like to fit 5 data points to a prescribed function: A + B*x + C * log(x). The code I use is:
myfit = fittype('a + b*x + c*log(x)','dependent',{'y'},'independent',{'x'},'coefficients',{'a','b','c'});
fit1=fit(xfit,yfit,myfit)
The data points I use are:
xfit = 2.1902, 2.5940, 3.0203, 3.5356, 4.2762,
yfit = 0.5000, 1.0000, 2.0000, 4.0000, 9.0000
Now, I do get an answer, but when comparing it to the values for a,b and c I found using both Python or Wolfram Mathematica, it seems very off.
Any idea what went wrong?
The expected values are: 2.5303 + 0.0627895 x + 0.538228 Log[x]
EDIT:
clear all
close all
color='red';
xfit = [2.1902, 2.5940, 3.0203, 3.5356, 4.2762];
yfit = [0.5000, 1.0000, 2.0000, 4.0000, 9.0000];
scatter(xfit,yfit,color)
hold on
myfit = fittype('a + b*x + c*log(x)','dependent',{'y'},'independent',{'x'},'coefficients',{'a','b','c'});
fit1 = fit(xfit',yfit',myfit,'StartPoint',[2.5,0,0.5]);
plot(fit1,color)
ylim([0,10])
xlim([0,5])
This is a matlab file that should do the job. However, it still doesn't work. Adding the point [0,0] to the beginning of the list does not make any difference (it results in errors, due to log(0) = -Inf ).
