On Mon, May 18, 2020 at 07:38:11PM -0400, Brett Green wrote:
> On Mon, May 18, 2020 at 7:30 PM Doron Behar <address@hidden> wrote:
>
> > Dear octaves,
> >
> > I'm a bit new to Octave and I'm trying to use it as a free software
> > alternative to Matlab which is considered a requirement for my physics
> > lab course. I strongly hope I won't have to eventually install Matlab
> > just because I can't find the equivalents of certain Matlab features
> > which seem missing from Octave. My instructors are most probably not
> > aware of GNU octave and free software and all that Jazz so I need help.
> >
> > I have an x_data and y_data for which I managed to fit a linear
> > polynomial function with:
> >
> > [fit, regression_info] = polyfit(x_data, y_data, 1);
> >
> > My instructors have asked me to (translated):
> >
> > 1) Perform a linear regression for test results.
> > 2) Add to the graph the regression curve.
> >
> > I'm mostly puzzled by the term "regression curve" which I hope I got
> > translated correctly.
> >
> > We were given the following code sample with which (If I got it
> > right) we can get the regression curve:
> >
> > ```
> > linear_regression = fitlm(x,y);
> > R_squared = linear_regression.Rsquared.Ordinary;
> > b_coefficient = linear_regression.Coefficients.Estimate(1);
> > a_coefficient = linear_regression.Coefficients.Estimate(2);
> > b_error = linear_regression.Coefficients.SE(1);
> > a_error = linear_regression.Coefficients.SE(2);
> > plot(x,y,'b.',x,linear_regression.Fitted,'k');
> > ```
> >
> > But it seems that the function `fitlm` is not available in GNU octave,
> > meaning I can't use it as is for my assignment.
> >
> > What should I do? How can I give them the plot they desire? I don't even
> > have a sample of how this should look like. I also wish to eventually
> > finish the script in a Matlab compatible manner.
> >
> > Besides the "regression curve", I think they have also asked me to
> > display on the graph the error bar according to the regression_info.
> > I've managed to learn I should use:
> >
> > errorbar(x_data, y_data, err);
> >
> > But I don't know how to compute `err` out of `regression_info`.
> >
> > It's likely that I don't have a full understanding of the whole theory
> > of curve fitting but I hope I can get the desired results never the
> > less.
> >
> > Regards.
> >
> >
> >
>
> Octave can certainly do curve fitting and has numerous functions for that
> purpose <https://octave.sourceforge.io/optim/overview.html#Data_fitting>,
> and in particular, it has a function dedicated to linear regression
> <https://octave.sourceforge.io/optim/function/LinearRegression.html>. Have
> you checked the documentation and tried adapting the examples?
Thanks for the quick reply. I haven't fully dived into these examples
yet although they look promising. It seems that these functions are not
compatible with Matlab and it's a problem for me because I need to
provide back to my instructors a `.m` file for them to run on Matlab :(.
I _will_ need to create a report eventually which wouldn't constitute a
`.m` file someone will run so I'll use these functions when the time
comes.
Hi Dohan,
Not to put words in your professor's mouth, but I believe they simply mean to graph the linear regression line that you get from the coefficients you obtained from polyfit. This can be done simply with using the coefficients, the x_data you already have, and the plot function.
Hope this helps,
James Sherman