Fitting data from file
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Hello,
I am trying to fit data from a file. For that I have created data with simply a sine wave function. Here is my code:
load test_fit
x = test_fit(:,1);
y = test_fit(:,2);
fo=fit(y,x,'a*sin(b*x+c)+d');
plot(fo,y,x)
But the results I got are clearly not good:
I'm just wondering if I should specify somwhere the number of itterations or the fitting constrains?
Thank you in advance for any help or comments!
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Accepted Answer
Rik
on 27 Sep 2022
Edited: Rik
on 27 Sep 2022
You can specify those parameters, but what is much more effective is providing good initial guesses (and not mixing up x and y):
test_fit=load(websave('test_fit.txt','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1137860/test_fit.txt'));
x = test_fit(:,1);
y = test_fit(:,2);
plot(x,y,'*')
fo=fit(x,y,'a*sin(b*x+c)+d','StartPoint',[1 4 0 2])
hold on,plot(fo)
10 Comments
Rik
on 30 Sep 2022
If you look at the code below, you see that c starts with a NaN. So we should remove it.
I = imread(websave('testf5.png','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1139785/testf5.png'));
x = [size(I,2)/2 size(I,2)/2];
y = [0 size(I,1)];
c = improfile(I,x,y);
c = c(:,1,1); % select red chanel
x_adapted = linspace(y(1),y(2),numel(c));
x_adapted = x_adapted.'; % transpose 1x1491 to 1491x1
subplot(2,1,1)
imshow(I)
hold on
plot(x,y,'-o')
hold off
subplot(2,1,2)
plot(x_adapted,c)
ft = fittype('m*sin(n*x+o)+p');
options=fitoptions(ft);
options.StartPoint = [125 100 0 125];
options.Weights = [1,1];
find(isnan(x_adapted)),find(isnan(c))
Here is the problem, so we'll have to remove that point.
L = isnan(x_adapted) | isnan(c); % the first part is just good habit
x_adapted(L) = [];
c(L) = [];
% Now we can safely to the fit:
fo=fit(x_adapted,c,ft,StartPoint=[125 100 0 125])
figure,plot(fo)
So the fit is not great, you will have to adjust the start point.
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