MATLAB Answers


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aymos on 26 Jun 2018
Commented: Ameer Hamza on 27 Jun 2018
I have a semilog graph which must be fitted (in its linear region) using this equation: y = 6e17*B*log[(x+B)/B]
Can you please tell how can I obtain the value of constant B, using fun(@x) ?
Thank you so much in advance for your help !


Rik on 26 Jun 2018
You should look into the fit function.
aymos on 26 Jun 2018
Can you please be more elaborate ?

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Answers (1)

Ameer Hamza
Ameer Hamza on 26 Jun 2018
Edited: Ameer Hamza on 26 Jun 2018
If you have a vector of x and y values then you can use several functions to estimate B. The correct method to use depending on your definition of the error function. For example, if you want to estimate B by minimizing the MSE (mean square error) then use lsqcurvefit(). For example,
xdata = ...; % vector of x values
ydata = ...; % vector of y values
y = @(B, x) 6e17*B.*log((x+B)./B);
B_estmated = lsqcurvefit(y, 1, xdata, ydata);
^ initial point for the numerical optimization algorithm.
Similarly, if you have some other error function, then you can use fmincon().


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Ameer Hamza
Ameer Hamza on 27 Jun 2018
I am defining an error metric. For example, if you have a vector of predictedOutput and actualOutput, then you want your predictedOutput to match the actualOutput. Therefore you define an error metric like this
error = predictedOutput - actualOutput;
the square of error is the most commonly used error metric. Unless there is some additional information given about the error metric, this error metric is used by default. So now if we are able to minimize the value of this error metric, it will mean that difference between predictedOutput and actualOutput is very small.
aymos on 27 Jun 2018
Thanks Ameer.. and by what variable are you defining the predicted output ? (you are using y for both predicted and actual output?)
[~, y] = ode45(@(t,y) odefun(A, B, C, y), t, y0);
Ameer Hamza
Ameer Hamza on 27 Jun 2018
Yes, this equation will give the predicted output y. Also, I realize that using one y together in one statement can be a bit confusing but this syntax is correct. MATLAB does not confuse both y's with each other. You can change either one of the y to another variable name to avoid confusion.

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